This manual is provided to you in electronic format.  It is 
supplied for the sole purpose of documenting and supporting 
Mult-Tech Systems' products.  Any translation, modification, 
or unauthorized copying of this manual is strictly prohib-
ited.  The information contained herein is protected by U.S. 
copyright laws and international treaties.



------------------------------------------------------------

              MULTIMODEMZDX OWNER'S MANUAL

------------------------------------------------------------


NOTE: This equipment has been tested and found to comply 
with the limits for a Class B digital device, pursuant to 
Part 15 of the FCC Rules. These limits are designed to pro-
vide reasonable protection against harmful interference in a 
residential installation. This equipment generates, uses, 
and can radiate radio frequency energy and, if not installed 
and used in accordance with the instructions, may cause 
harmful interference to radio or television reception, which 
can be determined by turning the equipment off and on, the 
user is encouraged to try to correct the interference by one 
or more of the following measures:

* Reorient or relocate the receiving antenna.

* Increase the separation between the equipment and 
  receiver.

* Plug the equipment into an outlet on a circuit different 
  from that to which the receiver is connected.

* Consult the dealer or an experienced radio/TV technician 
  for help.


WARNING: Changes or modifications to this unit not expressly 
approved by the party responsible for compliance could void 
the user's authority to operate the equipment.


                      Owner's Manual

             P/N 82030202 Revision C (3/20/95)
                      MultiModemZDX 
     (# MT1432ZDX, ZDXI, and ZDXK; MT1932ZDX, ZDXI, and     
             ZDXK; MT2834ZDX, ZDXI, and ZDXK )

This publication may not be reproduced, in whole or in part, 
without prior expressed written permission from Multi-Tech 
Systems, Inc. All rights reserved.

Copyright (C) 1995 by Multi-Tech Systems, Inc.

Multi-Tech Systems, Inc. makes no representations or warran-
ties with respect to the content hereof and specifically 
disclaims any implied warranties of merchantability or fit-
ness for any particular purpose. Furthermore, Multi-Tech 
Systems, Inc. reserves the right to revise this publication 
and to make changes from time to time in the content hereof 
without obligation of Multi-Tech Systems, Inc. to notify any 
person or organization of such revisions or changes.

RECORD OF REVISIONS

REVISION DATE	DESCRIPTION
   A	4/2/94	Manual released.
   B	6/1/94	Manual revised with 
		MT1932ZDX information.
   C	3/20/95	Manual revised with MT2834ZDX and firmware 
		version 1.11/1.12 information.


TRADEMARKS

Trademarks of Multi-Tech Systems, Inc. are as follows: 

MultiModem, MultiModemII, MultiModemZDX, MultiExpress for 
Windows, MultiExpressFax for Windows, Multi-Tech, and the 
Multi-Tech logo.
MNP and Microcom Network Protocol are trademarks of 
Microcom, Inc.
MS-DOS and Windows are trademarks of the Microsoft 
Corporation.
PKZIP is a trademark of PKWARE, Inc.
StuffIt is a trademark of Aladdin Systems, Inc.
UNIX is a trademark of X/Open Co., Ltd.
Apple, Macintosh, and System 7 are trademarks of Apple 
Computer, Inc.


Multi-Tech Systems, Inc.
2205 Woodale Drive, Mounds View, Minnesota 55112
Phone (612) 785-3500 or (800) 328-9717
Fax (612)785-9874
Technical Support (800) 972-2439
BBS (800) 392-2432



------------------------------------------------------------

CHAPTER 1  
INTRODUCTION

------------------------------------------------------------

Congratulations on your purchase of the MultiModemZDX modem, 
one of the finest fax modems available today from one of 
America's oldest and most respected modem manufacturers. The 
MT1432ZDX has a top speed of 14,400 bps for modem transmis-
sions, while the MT1932ZDX has a top speed of 19,200 bps. 
The MT2834ZDX has a top speed of 28,800 bps. All are full-
duplex intelligent modems with V.42 error correction and 
V.42bis data compression, and all have 14,400 bps V.17 and 
9600 bps Group 3, Class 2 fax capabilities. 


WHAT CAN I DO WITH MY MULTIMODEMZDX?
------------------------------------

Your MultiModemZDX is the gateway to the exciting world of 
telecomputing.

You can use the ZDX to access commercial on-line information 
services such as CompuServe, America On-Line, Genie, and 
Prodigy. These services provide access to databases, ency-
clopedias, stock reports, news, weather, and shopping. They 
provide electronic mail (E-mail) links to subscribers of the 
same and other services. Public message areas allow sub-
scribers to trade information and opinions on any of a vast 
array of topics from A to Z, while vendor forums provide 
hardware and software support from Multi-Tech and other 
manufacturers. On-line services also provide access to a 
vast array of programs and data files -- from the mundane, 
such as software updates, to the celestial, such as high-
resolution pictures from the Hubble Space Telescope.

The ZDX can also connect you with the Internet, an interna-
tional computer network of universities, libraries, busi-
nesses, and government agencies. Like the commercial on-line 
services, the Internet provides E-mail services, public mes-
sage areas, and access to information and software.

You can also use the ZDX to dial into bulletin board serv-
ices (BBSes). BBSes offer a subset of the services provided 
by on-line information services -- usually E-mail, public 
message areas, and file transfers. Most are run as hobbies 
by individuals, but some, including the Multi-Tech BBS, pro-
vide extensive, up-to-date information, and even flash-PROM 
files for quick upgrading of firmware.

You can use your ZDX for telecommuting -- do your work at 
home and send it to the office over the information highway. 
And its small size makes it easy to take along on business 
trips. Other uses include direct links to friends or col-
leagues with modems, and to banks and service bureaus.

And of course, since the ZDX is a fax modem, you can use it 
to send and receive faxes anywhere in the world. More and 
more, companies rely on faxes instead of regular mail for 
faster communications. Since fax machines are commonplace in 
the business world, you can use the fax capabilities of the 
ZDX at home to order directly from catalogs. The ZDX opens a 
wider world to you than a modem without fax capabilities. 
You do not need a fax machine to send and receive faxes any-
where in the world; your ZDX modem, fax software, and a 
regular phone line are all you need.


FEATURES
--------

The MultiModemZDX automatically adjusts to line conditions 
and the capabilities of the modem it connects to, resulting 
in the highest possible speed, the most accurate error cor-
rection, and the most efficient data compression possible 
for each connection. The MT1432ZDX is designed according to 
the international ITU V.32bis specification for data rates 
as high as 14,400 bps in full-duplex mode over public tele-
phone lines. The MT1932ZDX is designed according to the AT&T 
V.32 terbo specification for data rates up to 19,200 bps. 
The MT2834ZDX meets ITU V.34 specifications for data rates 
up to 28,800 bps. 

ZDX features include:

* Support of data rates of 14,400 bps, 12,000 bps, 9600 bps, 
  4800 bps, 2400 bps, 1200 bps, and 300 bps for communi-
  cating with older modems as well as with other V.34, 
  V.32bis or V.32ter modems.

* Automatic fallback to slower speeds in noisy line condi-
  tions, and fall-forward to faster speeds as conditions 
  improve.

* ITU V.42 LAP-M and MNP Classes 2-4 error correction.

* Data transfer rates up to 115,200 bps, 76,800 bps or 
  57,600 bps with V.42bis 4-to-1 data compression.

* Automatic disabling of compression when transferring 
  already-compressed files.

* Serial port data rates adjustable to 115.2K bps.

* Autodial, redial, pulse (rotary) and touch-tone dial.

* Dial tone and busy signal detection for reliable call-
  progress detection.

* Compatibility with the standard AT command set used by 
  most communication programs.

* On-screen help menus.

* Non-volatile memory for storage of customized modem 
  parameters and two telephone numbers.

* Sends and receives faxes from your computer at 
  14,400 bps, 9600 bps, 4800 bps, or 2400 bps.

* Responds to V.17 and EIA TR.29 Class 2 fax commands.

* Limited warranty: Ten years (USA, UK, Canada, and
                    Mexico)
                    Five years (all other countries)


WHAT IS IN YOUR MODEM PACKAGE?
------------------------------

Your MultiModemZDX has many components. Make sure you have 
them all before trying to operate your modem. Your package 
should include:

* MultiModemZDX data/fax modem

* DC power supply

* One set of four plastic feet

* Two sets of Velcro fasteners

* Telephone cord

* The MultiModemZDX Quick Reference Guide Manual 

* MultiExpress for Windows (MEW) diskette

* MultiExpress Fax for Windows (MEWFAX) diskette

* Warranty Registration Card

If any of these items are missing, please contact Multi-Tech 
Systems or your dealer/distributor.


PLEASE REGISTER
---------------

Please take the time to fill out and mail your Warranty 
Registration Card. As a registered user, you are eligible 
for technical support via the telephone, the Multi-Tech bul-
letin board system (BBS), and CompuServe. See Chapter 7, 
"Service, Warranty, and Technical Support." Sending in your 
Warranty Registration Card now will also speed up any serv-
ice you might require under your warranty. 


HOW TO USE THIS MANUAL
----------------------

SUMMARY OF EACH CHAPTER

This manual is divided into eight chapters. There are also 
several appendices, providing summary information for refer-
ence use. The information contained in each chapter and ap-
pendix is as follows:

CHAPTER 1.  INTRODUCTION

This chapter begins with a short introduction, followed by a 
brief description of a fax modem's applications. The fea-
tures of the ZDX are summarized, and the contents of the ZDX 
package are listed. You are reminded to send in your 
Warranty Registration Card. The chapter concludes with a 
guide to the use of the manual, which you are now reading.

CHAPTER 2.  MULTIMODEMZDX INSTALLATION

This chapter describes the equipment you must supply, then 
leads you step-by-step through mounting and connecting the 
ZDX.

CHAPTER 3.  FOR THE NOVICE: WHAT IS A FAX MODEM?

This chapter explains the basics of modem communications and 
how a fax modem works. It lays a foundation for understand-
ing the AT commands in Chapters 4 and 6. Included is a brief 
description of each of the protocols implemented in the ZDX. 
Experienced users may skip this chapter.

CHAPTER 4. SERIAL PORTS AND SOFTWARE CONFIGURATION

This chapter describes how to configure data communications 
(datacomm) software for a MultiModemZDX. Since your com-
puter's serial port is critical to the perceived performance 
of your modem, this chapter explains how to determine 
whether your serial port's speed is limited, before you con-
figure your communications software. It begins by describing 
speed limitations of some serial ports and how to determine 
whether yours is one of the affected ones. It then guides 
you through typical configuration menus, and recommends 
choices appropriate for a ZDX. It also tells you how and why 
to turn off call waiting, data compression, and error cor-
rection. Sample initialization strings are included for da-
tacomm programs that do not directly support the ZDX.

CHAPTER 5.  USING THE MODEM

This chapter tells you how to read your front panel indica-
tors when using your modem, then guides you through a typi-
cal call and connection. You are told how to test whether 
the modem is operating correctly. A troubleshooting section 
discusses typical problems, their probable causes, and 
solutions. 

CHAPTER 6.  AT COMMANDS AND S-REGISTERS

This chapter describes in detail the AT command set and S-
Registers.

CHAPTER 7.  SERVICE, WARRANTY & TECHNICAL SUPPORT

Chapter 7 provides instructions for obtaining service at our 
factory, statements on your warranty, and information about 
our user bulletin board service.

APPENDIX A.  MULTIMODEMZDX SPECIFICATIONS

APPENDIX B.  MULTIMODEMZDX DEFAULTS

APPENDIX C.  AT COMMAND SUMMARY

APPENDIX D.  S-REGISTER SUMMARY

APPENDIX E.  RESULT CODE SUMMARY

APPENDIX F.  ESCAPE METHODS USED BY MULTI-TECH SYSTEMS

APPENDIX G.  ASCII CONVERSION CHART

APPENDIX H.  RS-232C SPECIFICATIONS

APPENDIX I.  FCC AND DOC REGULATIONS

APPENDIX J.  LICENSE AGREEMENT

APPENDIX K.  DIAL PULSE AND TONE DIAL FREQUENCIES

GLOSSARY


TYPOGRAPHIC CONVENTIONS
-----------------------

Certain typographic conventions have been adopted throughout 
the text of this manual.

Most function-specific keys are shown as uppercase charac-
ters enclosed by angular brackets. Example: <TAB>.

Certain function-specific keys are abbreviated as follows:

<BS>	Means press the backspace key, which can be 
	represented on the keyboard by a left-pointing 
	arrow or the words Backspace or Delete (on a 
	Macintosh keyboard).

<CR>	Means press the carriage return key, which can be 
	represented on the keyboard by a broken left-
	pointing arrow or the words Enter or Return. In 
	datacomm software strings, ^M, the ASCII code 
	normally generated by the carriage return key, is 
	used instead of <CR>.

<ESC>	Means that you should press the escape key.

ALT+X	Means you should hold down the Alt key while you 
	press the key represented by X.

CTRL+X	Means you should hold down the Control key while 
	you press the key represented by X.

^	In a data string, a caret (^) indicates a control 
	character . For example, ^M is the control 
	character for carriage return.

Please note the difference between a zero and the letter O. 
This is a zero: 0. This is a capital O: O.


------------------------------------------------------------

CHAPTER 2
MULTIMODEMZDX INSTALLATION

------------------------------------------------------------


REQUIRED EQUIPMENT
------------------

In addition to the contents of the MultiModemZDX package, 
you will need the following equipment.


COMPUTER

The ZDX can be connected to any computer with an RS-232C 
serial port, such as the IBM PC, XT, AT, and PS/2 computers, 
and most IBM compatibles. It can also be connected to the 
RS-422 serial ports on Apple Macintosh computers.


SERIAL CABLE

You must provide a serial cable, obtainable at computer 
stores and many office supply stores, to connect the ZDX to 
your computer. The cable must have a DB-25P connector at the 
modem end. For IBM and compatible computers, the other end 
may have a DB-25P connector or a DB-9S connector, depending 
on your particular computer and whether you are using the 
COM1 or the COM2 serial port. Most Macintosh computers re-
quire a round, mini-DIN 8-pin connector. The Macintosh se-
rial cable must be wired correctly to use hardware flow con-
trol. See Appendix H, "RS-232C Interface," for the wiring 
diagram. To reduce electrical interference, the FCC requires 
that the cable be shielded.


TELEPHONE LINE

You must have a telephone line with a conveniently located 
connector (jack) to accept the cable that comes with the 
ZDX. If you do not have a telephone jack near your computer, 
you should install an extension before proceeding. 

In North America, telephone extension kits and accessories 
are available at electronics stores and wherever telephones 
are sold. You may also hire an independent contractor or 
your local telephone company to do the work. If you want a 
dedicated line for your ZDX fax modem, you must contact your 
telephone company.


COMMUNICATIONS SOFTWARE

You will need data communications (datacomm) and fax commu-
nications software to operate the ZDX. The ZDX comes with 
MultiExpress for Windows (MEW), a datacomm program, and 
MultiExpressFax for Windows (MEWFAX), a fax communications 
program. You must have Microsoft Windows 3.1 or later to run 
these programs. If you require software for DOS or for the 
Macintosh operating systems, please see Chapter 7, "Service, 
Warranty, and Technical Support". The ZDX is also compatible 
with your favorite datacomm or fax program.


ASSEMBLY
--------

The ZDX is fully assembled except for mounting. You may fin-
ish it off with non-marring feet, if you expect to move it 
from time to time, or you may use Velcro to attach it to a 
convenient surface, such as the side of your computer. The 
ZDX has no special placement restrictions, but we recommend 
that you place it where you can see the LEDs on the front 
panel.

A strip of four self-adhesive plastic feet is provided with 
the ZDX. To mount them, simply peel each foot off the strip, 
center it in one of the four recesses on the bottom of the 
modem, and press it firmly into place.

The Velcro strips are also self-adhesive. Mount them across 
the bottom of the ZDX near the front and back edges, so they 
do not obscure the labels. With the mating pieces fastened 
together, remove one protective strip from each set and 
press the Velcro into place on the bottom of the modem. Then 
remove the other protective strips and, positioning the mo-
dem carefully over the place where you want to mount it, 
press it firmly against the surface.


CONNECTIONS
-----------

The ZDXK and ZDXI models connect to your computer, to a 
telephone line ("LINE"), and to power (and, optionally, to 
surge suppressers or other protection equipment). The ZDX 
model may also be connected directly to a telephone set 
("PHONE").


CONNECTING TO THE COMPUTER

Decide which of the serial ports on your computer you want 
to connect the MultiModemZDX to. On an IBM Personal Computer 
or compatible, there are usually two serial ports named 
"COM1" and "COM2." COM1 typically uses a DB-9 connector, 
whereas COM2 typically uses a DB-25 connector. Most Apple 
Macintosh computers use round mini-DIN-8 connectors for the 
serial ports. Choose the one marked with the icon of a tele-
phone; the Macintosh gives priority to that port and will 
not interrupt your communications link. Using a matching se-
rial cable, connect the serial port to the RS-232 connector 
on the ZDX. Be sure to tighten the mounting screws on the DB 
connectors.


CONNECTING TO THE TELEPHONE LINE ("LINE")

Plug one end of the cable provided with the ZDX into the 
telephone jack in your home or office. Plug the other end 
into the LINE jack on the ZDX. 

NOTE: The LINE jack is not interchangeable with the PHONE 
jack on ZDX models. Do not plug the telephone into the LINE 
jack or the line cable into the PHONE jack.

NOTE: The USA Federal Communications Commission (FCC) and 
the Canadian Department of Communi- cations (DOC) impose 
certain restrictions on equipment connected to public tele-
phone systems. See Appendix I, "FCC and DOC Regulations and 
BABT Information," for more information. 


CONNECTING A TELEPHONE SET ("PHONE")

If you wish to have a telephone connected to the same line 
as the ZDX for voice communications, you may plug it into 
the PHONE jack on the ZDX model. This connector is provided 
as a convenience; you may also connect your telephone to a 
duplex jack inserted into your wall jack. The ZDXK and ZDXI 
models do not have a PHONE jack. 

NOTE: The PHONE jack is not interchangeable with the LINE 
jack; do not plug the telephone into the LINE jack or vice-
versa.


CONNECTING POWER 

Low voltage DC power is supplied to the ZDX through a modu-
lar power supply included with the modem. Plug the power 
supply module directly into a convenient AC power outlet. 
Plug the connector on the other end of the power cord into 
the POWER jack on the modem. A power on/off switch is lo-
cated on the right side of the modem. 

NOTE:	Use only the power supply supplied with the ZDX. 
Use of any other power supply will void the warranty and 
could damage the modem.

As soon as you connect power to the modem, it will perform a 
diagnostic self-test, indicated by the speed LEDs flashing 
in sequence for approximately two seconds; then the "14", 
"19" or "28" LED should light. If this does not happen, 
check that the power supply is solidly connected and that 
the AC outlet is live.


SURGE PROTECTORS AND LIGHTNING

Power surges and other transient voltages on power lines, 
such as those caused by lightning strikes, can damage or de-
stroy your modem. Damaging voltages can even enter your mo-
dem through the telephone line, especially during an elec-
trical storm. Therefore, we recommend that you plug the ZDX 
into a surge protector rather than directly into the wall. 
Some surge protectors provide protection against electrical 
spikes on the telephone line as well as the power line. Note 
that not even a surge protector can guard against damage 
from a nearby lightning strike. During an electrical storm, 
your safest course is to unplug all your computer equipment 
from the telephone line as well as the power outlets.


------------------------------------------------------------

CHAPTER 3
FOR THE NOVICE: WHAT IS A FAX MODEM?

------------------------------------------------------------

You are the new owner of one of the world's finest fax mo-
dems, but if you are new to computers or to telecommunica-
tions, you may be uncertain what a fax modem is or how it 
works. If you're a novice, this chapter is for you. By the 
time you have finished it, you should understand not only 
what a fax modem does, but how it does it. If you are al-
ready experienced with fax modems, on the other hand, you 
are welcome to skip ahead to the next chapter. 


WHAT IS A MODEM, AND HOW DOES IT WORK?
--------------------------------------

A fax modem is a modem that can send and receives faxes. But 
what is a modem? Very simply, a modem is a device that lets 
computers communicate with each other over the telephone 
system. It does this, through a process called modulation, 
by converting the square waves that a computer uses for in-
ternal communications into special audio-frequency waves 
that can pass unhindered over telephone wires. A modem also 
turns audio frequencies back into square waves through a 
process called demodulation. So a modem is a MOdulator-
DEModulator, which is where the name comes from.


THE BINARY NUMBER SYSTEM

All information in a computer can be reduced to numbers. The 
numbering system we normally use is the decimal system, so 
called because it uses ten digits, 0 through 9. (Decimal 
comes from the Latin word decem, which means ten.) The num-
bering system used by computers, however, is the binary sys-
tem, so called because it uses only two digits, 0 and 1. 
(Bi- means two.) The word bit, which you may be familiar 
with, is simply the shortened form of binary digit. 

Bits are typically grouped into larger units called bytes. 
Though bytes can vary in size, there are typically eight 
bits to a byte. A typical byte looks like this: 01110100. 
This binary number is the same as the decimal number 116, 
which happens to be the ASCII code for the character t. When 
information is sent over telephone lines, it is the byte, 
not the bit, that is the basic unit of exchange.


CHARACTER SETS

THE ASCII CHARACTER SET

If computers work only with binary numbers, how do they 
store and transmit text? They do it by using a unique nu-
meric code to represent each character -- not just the let-
ters of the alphabet, but all the characters on a computer 
keyboard, including punctuation marks and the digits 0 
through 9, as well as several non-printing characters, in-
cluding delete, escape, backspace, tab, line feed, and car-
riage return. Although, historically, there have been sev-
eral different sets of codes, the most common one today is 
the American Standard Code for Information Interchange, or 
ASCII. (See Appendix G, "ASCII Conversion Chart," for the 
ASCII codes.) The ASCII set has 128 unique codes, each con-
sisting of seven bits (the first bit in an ASCII byte is al-
ways a 0). Although it has some limitations, the ASCII set 
is so widespread that it has become a common denominator in 
personal computer communications. If you want to make sure 
anyone can read your text file, put it in ASCII.

THE ANSI CHARACTER SET

The ASCII character set uses only seven bits; if you use an 
eight-bit byte, you can extend the number of character codes 
to 256. Unfortunately, there are several different sets of 
extended codes in use. Perhaps the most common is the 
American National Standards Institute, or ANSI, character 
set. The first half of the ANSI set is identical to the 
ASCII set. The second half includes special characters from 
European languages, and typesetting characters, such as 
curly quotes and em dashes, not found on the computer key-
board. The ANSI character set is the standard character set 
used in Windows. Be careful using ANSI extended characters 
when leaving messages on BBSes; if the BBS doesn't use the 
ANSI set, it may interpret the ANSI code wrongly, or it may 
ignore it altogether. Remember, when you send text over a 
modem, the receiving computer sees only numeric codes, not 
the characters you see on your screen. How the codes are in-
terpreted depends on the other computer.


WHY WE NEED MODEMS

Computers cannot communicate directly with each other over 
phone lines because physical limitations in the phone system 
would garble the data. We need modems to translate the na-
tive speech of computers into a form that can pass undis-
torted over the wires. Without modems, the information su-
perhighway would be a path limited to voice only.

It is the high speed of computers that makes modems neces-
sary. In a computer, binary numbers are represented by se-
quences of high and low voltages. Computer circuits switch 
between those two voltages very rapidly. Because of the 
rapid transition, the waves, or pulses, look square. The ex-
act voltages are not critical so long as the high voltage is 
above a certain threshold and the low voltage is below an-
other threshold. That the computer need only detect "on" or 
"off" is one of the things that make digital circuitry as 
reliable as it is. Mathematically, the sharp corners on 
square waves are made up of very high frequency sine waves 
added together. In a circuit that cannot transmit high fre-
quencies, such as the telephone system, square waves become 
severely rounded, causing the values of bits to change or 
become uncertain, and the information to become corrupted.


PARALLEL AND SERIAL TRANSMISSION

PARALLEL TRANSMISSION

Because a data byte consists of several bits, there are two 
ways you can transmit it. You can send it all at once, one 
bit per wire over multiple wires, which is called parallel 
transmission, or you can send it one bit at a time over a 
single wire, which is called serial transmission. Obviously, 
parallel transmission is faster. Think of a 16-lane super-
highway compared to a two-lane country road. The main disad-
vantage is cost. Parallel transmission requires a separate 
wire for each bit, so parallel cables tend to be bulky and 
expensive. Use of parallel transmission typically is limited 
to inside the computer and to connections to nearby periph-
erals such as printers.

SERIAL TRANSMISSION

In serial transmission, bits are sent one after another over 
a single wire. Because telephone cables do not have enough 
wires for parallel communications, serial transmission is 
the only practical way to communicate with a remote com-
puter.


THE RS-232C SERIAL PORT

Your modem must be connected to an RS-232C serial port. A 
serial port consists of the connector and circuits in your 
computer that allow it to send and receive data one bit at a 
time. It has several functions: It converts parallel data 
bytes in the computer into serial data bytes for transmis-
sion to the modem, and vice-versa. It inserts and removes 
start and stop bits, which signal the beginning and end of a 
byte. It inserts and removes parity bits, which test for 
corrupted data. And it trims ASCII text bytes from eight 
bits to seven. The serial port can be located on the com-
puter's motherboard, on an expansion card, or in an internal 
modem. It can be labeled "RS-232," "SERIAL," "COM," "MODEM," 
or with a drawing of a telephone. Or it may not be labeled 
at all. The standard connector on an RS-232C serial port is 
a 25-pin connector shaped like an elongated, angular D. It 
is called a DB-25S connector if it is a socket, and a DB-25P 
connector if it is a plug. Some computers use a 9-pin DB-9P 
connector instead of, or in addition to, a 25-pin connector. 
When the latter is the case, the 9-pin connector is usually 
COM1 and the 25-pin connector is COM2. On a Macintosh com-
puter, the serial port has a round, 8-pin, mini-DIN 
connector 

THE UART

The serial port's integrated circuit (IC) that converts par-
allel data bytes to serial data bytes and back is called a 
Universal Asynchronous Receiver/Transmitter, or UART. (In a 
Macintosh it is called a Serial Communications Controller, 
or SCC.) The important thing to remember about UARTs is that 
older ones, such as the 8250 and the 16450 in PC-compatible 
computers, may not be reliable at modem speeds faster than 
19,200 bps. If your computer has one of those UARTs, you 
should consider installing a newer serial card with a 16550 
UART or equivalent, which can reliably handle speeds up to 
115,200 bps.

THE RS-232C STANDARD

The EIA RS-232C standard, defined in 1969 by the Electronics 
Industry Association, specifies how a serial interface 
should work. It is so universally accepted that today 
"serial interface" and "RS-232C" are practically synonymous. 
The things you are most likely to need to know about it are 
how the RS-232C cable should be wired and what the RS-232C 
signals do.

THE RS-232C CABLE

If your ZDX did not come with a serial cable, you must pro-
vide one that is compatible with your computer. The easiest 
way is to buy a ready-made serial cable from a computer or 
office supply store. The connectors should match the connec-
tors on your modem and on your computer's serial port. Nor-
mally, you do not need to know more than this to hook up 
your modem and dial out. However, if you would like to make 
a serial cable or to test one for compatibility, you will 
find contains cable wiring information in Appendix H.

THE RS-232C SIGNALS

                     9-pin      25-pin    25-pin
                      DTE        DTE        DCE
                  (computer)  (computer)  (modem)
                   connecter  connecter  connecter
                    +-----+    +------+   +------+
      Transmit Data |TD  3|--> |TD   2|-->|2   TD|
       Receive Data |RD  2|<-- |RD   3|<--|3   RD|
    Request to Send |RTS 7|--> |RTS  4|-->|4  RTS|
      Clear to Send |CTS 8|<-- |CTS  5|<--|5  CTS|
     Data Set Ready |DSR 6|<-- |DSR  6|<--|6  DSR|
      Signal Ground |SG  5|--- |SG   7|---|7   SG|
     Carrier Detect |CD  1|<-- |CD   8|<--|8   CD|
Data Terminal Ready |DTR 4|--> |DTR 20|-->|20 DTR|
     Ring Indicator |RI  9|<-- |RI  22|<--|22  RI|
                    +-----+    +------+   +------+

Fig. 3-1. RS-232C signals, signal directions, and pin 
numbers


Of the nine RS-232C signals used by the ZDX, only two are 
used for data. The computer, called the data terminal equip-
ment, or DTE, in the EISA standard, transmits data on pin 2 
(TD) and receives it on pin 3 (RD) The modem, called the 
data communications equipment, or DCE, receives data on pin 
2 and transmits it on pin 3. Pin 7 is Signal Ground (SG). 
This arrangement allows the computer and modem to be con-
nected with straight-through cables, that is, cables in 
which each wire connects to identically-numbered pins on ei-
ther end. Note that the signals are named with reference to 
the DTE, so the signal name for pin 3 on the DCE is Receive 
Data even though the DCE uses that pin to transmit data.

The other signals are used for control. The computer tells 
the modem it is ready to transmit data by raising, or turn-
ing on, the Request to Send (RTS) signal on pin 4. The modem 
then tells the computer it is ready to receive data by rais-
ing the Clear to Send (CTS) signal on pin 5, starting the 
flow of data. Under the RS-232C standard, CTS would drop, or 
turn off, when RTS did, but many applications require CTS to 
operate independently. (You can control how the modem re-
sponds to RTS by issuing the command &RF through your modem 
software.

The modem cannot answer or originate a call unless the com-
puter is turned on and ready, which the latter signals by 
raising the Data Terminal Ready (DTR) signal on pin 20. (For 
Macintosh computers, which do not supply a DTR signal, the 
modem must be set to ignore the state of the DTR signal. See 
"Macintosh Initialization." When the modem detects a carrier 
over the communications link, it raises the Carrier Detect 
(CD) signal on pin 8. Data Set Ready (DSR), on pin 6, sig-
nals that the modem is turned on and ready. Under the RS-
232C protocol, it normally doesn't change, but in typical 
applications, it follows the CD signal (the factory default 
on the ZDX). Finally, the modem raises pin 22, Ring 
Indicator (RI), to signal the computer that something on the 
communication link is trying to contact it.


TRANSMISSION SPEED -- BAUD AND BPS

Two units are used to measure transmission speed: baud and 
bits per second (bps). Often, a modem's baud rate is the 
same as its speed in bps, so the units are often confused, 
but in fact they measure different things. 

The baud rate of a transmission is the number of electrical 
signals transmitted in one second. It might be helpful to 
think of it as the raw speed of a transmission. 

Bps is the number of bits transmitted in one second. Since a 
bit is the smallest unit of data, bps is a measurement of 
data transmission. When each electrical signal that is 
transmitted represents a bit, baud and bps are equal; this 
is especially true at lower rates such as 300 and 1200 bps. 
At higher transmission rates, however, electrical signals 
are normally modulated to carry four or more bits per sig-
nal. A 2400 baud transmission, therefore, can actually carry 
data at 9600 bps. Since data rates are more important to us 
than signal rates, modem speeds are typically measured in 
bps.


MAXIMUM EFFECTIVE THROUGHPUT AND CHARACTERS PER SECOND

There are two other ways to measure data rates: Maximum ef-
fective throughput and characters per second (cps). Because 
data can be compressed before transmission and decompressed 
afterward, the potential rate of transmission can be much 
higher than the nominal speed of transmission. This poten-
tial rate is called maximum effective throughput, and it is 
measured in bps. If a compression ratio of 4-to-1 is used, 
the maximum effective throughput of a 19,200 bps modem will 
be 76,800 bps.

Maximum effective throughput is the fastest rate at which a 
modem can transmit data. In practice, however, poor line 
conditions can slow transmission by causing the modem to 
resend garbled blocks of data. Likewise, compression ratios 
depend on the type of data being sent. So the actual 
throughput may vary from day to day or file to file and be 
much lower than the maximum effective throughput. Data com-
munications programs that track actual throughput during a 
transmission usually display it in characters per second. 
Because a modem normally transmits ten bits for every char-
acter code, you can find the speed of a transmission in bps 
by multiplying the character rate by ten.


DIAL-UP OR LEASED LINES

Most modem communications are over public switch telephone 
network (PSTN) lines -- i.e., the regular phone system. Many 
Multi-Tech modems can communicate over private, two-wire 
leased lines. With leased-line operation, there is no dial-
ing, since the modems are always connected to each other. 
The ZDX, however, is designed only for dial-up operations.


SYNCHRONOUS OR ASYNCHRONOUS TRANSMISSION

Serial communications can be synchronous or asynchronous. In 
synchronous communications, data bytes are sent down the 
wire at regular intervals timed by a clocking signal -- the 
word synchronous literally means "timed." Synchronous commu-
nication is used mostly by IBM mainframe computers and mini-
computers.

In asynchronous communications, data bytes can be sent at 
irregular intervals -- the word asynchronous means "not 
timed." Asynchronous bytes must have start and stop bits 
added to them to signal the receiving computer that a data 
byte is enclosed. Because these extra bits must be transmit-
ted along with the data bits, the maximum effective through-
put is less than with synchronous communications; however, 
asynchronous communications is simpler and less expensive, 
so it is used by personal computers and most non-IBM main-
frames and minicomputers. Although many Multi-Tech modems 
can be used either way, the ZDX is an asynchronous modem 
only.


ASYNCHRONOUS DATA

Data bits. In asynchronous communications, data is sent a 
byte at a time. A byte typically has 7 or 8 data bits. If a 
7-bit byte is sent, it is padded out with an extra 0 to make 
a full 8-bit byte, or a parity bit is added. (See drawing.) 

	 +---+---+---+---+---+---+---+---+---+---+
	 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 1 |
	 +---+---+---+---+---+---+---+---+---+---+
	   |                               |   |
	 Start           Data          Parity Stop
	  Bit            Bits             Bit Bit

	    Fig. 3-2. An asynchronous data byte

The parity bit. Parity is a simple form of error-checking 
used only with 7-bit data bytes. Before a character is sent, 
the hardware adds up all the 1s in the number. If their num-
ber is even, the binary number has even parity; if their 
number is odd, it has odd parity. The hardware then adds a 0 
or a 1 parity bit, depending on the parity of the binary 
number and on the type of parity selected. For instance, if 
even parity is selected, the hardware will add a 1 parity 
bit to a binary number with odd parity to make the overall 
parity even. The receiving hardware compares the parity bit 
to the parity of the data byte, and if they do not agree, it 
knows the data has become corrupted.

Stop and start bits. Because asynchronous data bytes are un-
clocked, there must be some way to tell the serial port on 
the receiving end when to expect the first bit in a byte. To 
do this, the transmitting hardware adds a start bit at the 
beginning of each byte. The start bit is always a 0. The 
hardware also adds a stop bit at the end of each byte to let 
the receiving hardware know it has received all of the data 
byte. The stop bit is always a 1. Some special applications 
require two stop bits, but that is unusual.

Except for rare cases, a transmitted asynchronous data byte 
is always 10 bits long. That's why you multiply characters 
per second by 10 to get the bps transmission rate. The added 
bits are stripped off at the receiving end. Mainframe com-
puters normally use 7-bit character transmissions, while PC-
based systems use 8-bit. Thus the two most common communica-
tions settings are 7-E-1 (7 data bits-even parity-1 stop 
bit) for mainframe-based BBSes, and 8-N-1 (8 data bits-no 
parity-1 stop bit) for PC-based BBSes. The important thing 
to remember is that you must match your modem settings with 
the communications parameters of the system you are calling 
for communications to take place. 


FLOW CONTROL AND PACING

When the modem regulates the flow of data from computer to 
modem -- i.e., by turning off the data flow and restarting 
it as needed, it is called flow control. When the computer 
regulates the flow of data from modem to computer, it is 
called pacing.

There are two basic ways to control the flow of data: hard-
ware and software. Hardware flow control uses pins in the 
RS-232C connection -- CTS for flow control and RTS for pac-
ing. Software flow control inserts special control charac-
ters -- such as Xon and Xoff -- into the flow of data be-
tween computer and modem.

You can set the ZDX for either hardware or software flow 
control. Since software flow control does not work for some 
types of file transfer operations, your communications soft-
ware must likewise be able to use either hardware and soft-
ware flow control. Also note that hardware flow control may 
not be possible on some older Macintosh computers. 


CONTROLLING THE MODEM

Because they must be able to work with a variety of communi-
cations standards and computer types, and deal with special 
circumstances such as line noise, modems must have flexible 
sets of controls. The ZDX is controlled by AT commands, by 
S-Registers, and by software.

AT COMMANDS

The ZDX is controlled primarily by commands issued to it 
through your computer. The commands can be sent directly 
from your computer keyboard, or they can be stored in soft-
ware for automated control. To let the modem know that the 
following data are to be interpreted as commands and not 
transmitted, each command or string of commands is preceded 
by the "attention" letters AT. For this reason the commands 
are known as AT commands. The AT command set is an industry-
wide de facto standard, although it includes many special-
ized commands that are not used by all modems. The AT com-
mand set and how to use it is described further in Chapter 
6. 

S-REGISTERS

A register is an electronic circuit where a number is 
stored. The S-Registers store user-definable values such as 
the line feed character (S-Register S4), the number of rings 
until the modem answers (S-Register S0), and the duration of 
the break character (S-Register S17). The S-Registers are 
described more fully in Chapter 6.

DATACOMM SOFTWARE

You must have data communications (datacomm) software to 
communicate with the modem; you cannot issue commands to the 
modem from the DOS prompt or the Macintosh Finder. Datacomm 
software simplifies control of the modem by guiding you 
through the process of selecting your serial port, your port 
speed, and other variables, and then storing your settings, 
including frequently-called phone numbers, so they can be 
recalled with the stroke of a key or the click of a mouse 
when you want to dial out. The ZDX comes with the datacomm 
program MultiExpress for Windows (MEW). Chapter 4 describes 
typical configurations for your datacomm program.


ESCAPE SEQUENCES

TRANSPARENCY

Ideally, 8-bit data flow is transparent. In other words, the 
contents of the data flow should not trigger any activity by 
the communications hardware. The contents of the data are 
invisible to the hardware, and each byte is passed along 
exactly as received.

ESCAPE SEQUENCES

The hardware is not completely blind to the data, however. 
Sometimes it is necessary to get the computer's attention 
and interrupt the flow of data. A signal that does this is 
called an escape sequence, and there are two. The ZDX's de-
fault escape sequence is a string of three plus characters: 
"+++." The command to hang up, for instance, is "+++ATH." 
The other escape sequence is a long string of 0 bits called 
a break signal. The break signal must be issued by your com-
munications software; it is not generated by the Control-
Break key on your keyboard. In MultiExpress for Windows, you 
can send the break signal by choosing it from a menu or by 
pressing ALT+B. You can set the ZDX to respond to either 
escape sequence, to both, or to neither.


HALF-DUPLEX AND FULL-DUPLEX MODES

Half duplex means that data can be sent in only one direc-
tion at a time (only one carrier frequency is used). Think 
of a one-lane bridge that cars must take turns to cross. 
Full duplex means that data can be sent in both directions 
at the same time (two different carrier frequencies are 
used). Like a two-lane bridge, it keeps the traffic flowing. 
In half-duplex mode, the characters you type are normally 
sent directly to your screen. In full-duplex mode, they are 
sent to the remote computer, which then echoes the charac-
ters back to your screen. When you choose the duplex setting 
in your software, you are normally setting the echo parame-
ter, not the transmission mode.


THE CARRIER

The carrier wave is a constant tone of a given frequency and 
amplitude that is transmitted over the phone line. When it 
is altered in a systematic way, it is said to be modulated. 
The modulations carry not only the data that are sent over 
the line, but signals that establish the parameters of the 
connection and control data flow. When the ZDX detects the 
carrier, it turns on the Carrier Detect (CD) signal on pin 2 
of the RS-232C interface and, at the same time, turns on the 
CD LED on the front panel. When the carrier is lost, no data 
can be exchanged, and the ZDX drops the connection and the 
CD signal.


MAKING THE CONNECTION ("HANDSHAKING")

If your modem speaker is turned on when you dial another mo-
dem, you will hear a series of harsh tones that sounds like 
a cat fight. What's happening here?

When you call another computer and modem, you begin by turn-
ing on your own computer and modem. The LED speed indicators 
on the modem flash briefly as it performs a diagnostic self-
test, then the LED for your default speed lights. Data 
Terminal Ready (DTR) goes high, indicating that the computer 
is ready to communicate. To dial, you enter ATDT followed by 
the phone number. Entering AT sets the modem's speed to 
match the speed of the computer, and also sets the modem's 
parity. Your modem (the originating modem), dials the number 
of the remote modem. The remote modem responds to the ring-
ing by switching into the answer mode and turning on its 
Ring Indicator (RI). When the called modem answers, its Off-
Hook (OH) circuit comes on, lighting the "OH" LED, and the 
ringing stops. It now waits two seconds of billing time be-
fore it sends an answer tone. This pinging answer tone is 
used to disable phone-line echo suppress equipment so the 
modem can do two-way simultaneous communication. Then the 
modem shifts to a second tone. The originating modem detects 
the first or second tone from the answering modem, then 
sends its own tone, which identifies the protocol it will 
use.

Both modems now do a sophisticated procedure to "train" 
their adaptive equalizer and echo canceler circuits in order 
to set them up for optimal performance over the specific 
phone conditions encountered on this call. (If line condi-
tions change later during a call, the modems might "retrain" 
the circuits.)

The modems now exchange tones, negotiating speed, error cor-
rection, and compression. Then both modems exchange an E se-
quence, which ends the rate sequence. The modem carriers are 
now connected. After a one-second delay, Carrier Detect (CD) 
and Clear to Send (CTS) come on. Data Set Ready (DSR), fol-
lowing CD, now goes high, permitting the exchange of data 
between the computer and modem. This entire exchange, called 
handshaking, takes place in 12 to 15 seconds.


DISCONNECTING

When a ZDX modem is dialing or connected to another modem, 
it is said to be "off-hook." This status is indicated visu-
ally by the "OH" LED on the front panel. When a ZDX hangs 
up, it is said to be "on-hook." There are several ways to 
accomplish this. 

COMMAND MODE CONTROL

To terminate a call by command mode, enter +++ATH <CR>; or 
send a break signal followed by ATH <CR>. 

DTR CONTROL

If DTR is turned off for 50 milliseconds or more, a discon-
nect occurs. This is probably the most common method used by 
computer systems at the automatic answer end of the line to 
cause the answering modem to disconnect after logging-off 
procedures.

LOSS OF CARRIER

After a data connection has been established, the modem dis-
connects if the carrier is lost for the time set by S-
Register S10. 

ABORT TIMER, ANSWER MODE

When OH comes on, the called modem starts a 45-second timer 
and waits for a carrier signal from the originating modem. 
If it does not detect it, the modem hangs up.

ABORT TIMER, ORIGINATE MODE

After the originating modem has finished dialing, it starts 
a 45-second timer and waits for a carrier signal from the 
answering modem. If it does not detect it, the modem hangs 
up. The abort time for both originate and answer modes can 
be changed by reconfiguring S-Register S7.

INACTIVITY TIMER

Causes the modem to disconnect if no data is being transmit-
ted or received for the time specified by S-Register S30. 
The timer is restarted any time a character passes through 
the serial port in either direction.

USING DATACOMM SOFTWARE

The most common way to terminate a call from the originating 
modem is through communications software such as Multi-
Express for Windows. In the MultiExpress for Windows Modem 
Setup window, you specify the command string the software 
will send to the modem when you click on the Disconnect 
icon. The default hang-up string is +++ATH <CR>. You can 
also disconnect from within a script by using the HANGUP or 
DISCONNECT script commands.


INTERPRETING MODEM SPEED AND CONTROL STANDARDS

When you read on the box cover that the ZDX has ITU V.42, 
V.42bis, V.32, V.32bis, AT&T V.32 terbo, V.34, LAP-M, MNP 2, 
3, and 4, and so forth, did you understand what those terms 
meant? It's really quite simple; they are a list of standard 
protocols that modems must follow to communicate with each 
other. A protocol is a set of rules. A protocol can be de 
facto -- that is, an unofficial standard that manufacturers 
follow to stay compatible with the pioneers in the industry 
-- or it can be de jure -- that is, an official standard es-
tablished after lengthy wrangling and debate by a national 
or international organization such as the International 
Consultative Committee on Telephone and Telegraph (CCITT), 
now called the Telecommunication Standardization Bureau of 
the International Telecommunication Union (ITU-TSB). 

ITU standards such as "V.32" and "V.42" are known as "V dot" 
standards. When a standard is enhanced or upgraded, it is 
labeled "bis," which is Latin for "twice." So V.42bis is the 
second version of the V.42 standard. A standard labeled 
"ter," which is Latin for "thrice," would be the third 
version.

TRANSMISSION SPEED STANDARDS

Transmission speed standards are also known as modulation 
standards. They define how modems should communicate with 
each other -- with what frequencies, timings, and types of 
modulation -- at different speeds. The important thing to 
remember is that each protocol is associated with a differ-
ent speed, so the statement "The ZDX can fall back from V.32 
terbo to V.32bis to V.32 to V.22bis to V.22..." is much the 
same as saying, "The ZDX can fall back from 19,200 bps to 
14,400 bps to 9600 bps to 2400 bps to 1200 bps...."

ITU V.34: 28,800 bps, 26,400, 21,600, 19,200, 16,800, 14,400 
and 12,000 bps. The fastest de jure standard presently 
available. A V.34 modem can adjust its speed up and down as 
line conditions improve or worsen.

AT&T V.32terbo: 19,200 bps and 16,800 bps. A de facto stan-
dard from AT&T. A V.32 terbo modem can adjust its speed up 
and down as line conditions improve or worsen.

ITU V.32bis: 14,400 bps and 12,000 bps. A V.32bis modem can 
adjust its speed up and down as line conditions improve or 
worsen.

ITU V.32: 9600 bps. A V.32 modem can adjust its speed down 
as line conditions worsen, but cannot adjust its speed back 
up when line conditions improve.

ITU V.22bis: 2400 bps. The standard followed by most -- but 
not all -- 2400 bps modems. V.22bis and earlier protocols do 
not adjust speed for line conditions.

ITU V.22: 1200 bps. International standard for 1200 bps 
modems.

Bell 212A: 1200 bps. De facto standard for modems used in 
the United States. Equivalent to the V.22 international 
standard.

ITU V.21: 300 bps. International standard for 300 bps modems 
(ZDXI models only).

Bell 103/113: 300 bps. This US de facto standard was intro-
duced in the 1950s for access to mainframe computers. Early 
PC communications technology also used it. Equivalent to the 
V.21 international standard (ZDXI models only).

ERROR CORRECTION STANDARDS

You've probably had the experience of trying to understand 
someone over a noisy telephone line. Likewise, noise, static 
or distortion can cause transmitted computer data to become 
corrupted and hard to "understand." 

Error-correcting modems use the "checksum" technique to 
identify corrupted data. The sending modem examines outgoing 
data a block at a time, applying a mathematical algorithm to 
calculate a checksum number for each block. Then the modem 
transmits the block of data, followed by the checksum for 
that block.

The receiving modem examines each block of data in the same 
way and compares the checksums it arrives at to the check-
sums sent with the data. If they agree, fine. If they disa-
gree, the modem asks the sending modem to retransmit the 
data. Though error correction can slow transmission over a 
noisy line because of the retransmissions, it ensures one 
hundred percent accurate communications 

ITU V.42 (LAP-M or MNP 3 & 4): The most efficient and accu-
rate standard presently available for error correction. 
Based on the ITU's LAP-M (Link Access Procedure for Modems). 
V.42 includes MNP 3 and MNP 4 as alternates to LAP-M.

MNP 4, MNP 3, MNP 2: Error correction protocols developed by 
Microcom, a modem manufacturer. Used by a large base of in-
stalled modems.

DATA COMPRESSION STANDARDS 

Data compression lets modems increase effective transmission 
speed by "squeezing" or "compressing" data into a smaller 
size for transmission, then restoring the compressed data at 
the other end. Modems do this by using a variety of ways to 
examine the data stream for repeated characters and common 
letter combinations, and then substituting shorter codes for 
them. When the receiving modem sees the codes, it translates 
them back into the original data. 

How far the data can be compressed depends on the nature of 
the data; some data files can be compressed more than oth-
ers. Precompressed files, such as ".ZIP" and ".SIT" files, 
cannot be compressed any further, and trying to compress 
them is a waste of time.

When data that has been compressed to half its original size 
and sent over the line at, say, 14,400 bps is expanded at 
the other end, the receiving modem must send twice as much 
data to its computer as it receives. In this example, the 
serial port rate must therefore be at least 28,800 bps to 
keep from losing data. For V.42bis, the serial port rate 
must be at least four times the phone line transmission rate 
when compression is enabled.

ITU V.42bis (4-to-1): Can achieve 4-to-1 compression. Auto-
matically shuts off if data cannot be compressed. V.42bis 
modems also include V.42 error correction.

MNP 5 (2-to-1): Can achieve 2-to-1 compression. Does not 
shut off if data is already compressed. Used by a large base 
of installed modems. Relies on MNP 2 through MNP 4 for error 
correction. MNP 5 is not compatible with V.42bis, but many 
modems that use V.42bis also include MNP 5.


WHAT DOES A FAX DO?
-------------------


FAXING AND FAX MODEMS

"Fax" is derived from "facsimile," which means an exact copy 
or reproduction. A fax machine scans a document, makes an 
electronic copy of it, and transmits it to another fax ma-
chine, which turns the electronic image back into a paper 
document. In recent years fax machines have become as indis-
pensable as copiers.

The fax modem allows you to transmit and receive documents 
without scanning or printing, without waiting on line for a 
fax machine, and without worrying about privacy. Most docu-
ments today are created on computers and stored as computer 
files. There is no point in printing out a document just to 
run it through a fax scanner if you can send it directly 
from the computer. With a fax modem, you can send documents 
directly to business associates' computers, or fax them to 
their fax machines.


INTERPRETING FAX STANDARDS

Whether you are using modems or fax machines, they must con-
form to certain standards to be able to communicate with 
each other. The standards define two categories of operation 
referred to as "group designations" and "class designa-
tions." Group designations are a family of standards that 
specify how a fax modem communicates and exchanges fax data 
over the phone line with other fax modems. Class designa-
tions are defined and controlled by the Electronic Institute 
Association (EIA). By standardizing the commands used to 
configure and control the fax modem software, they specify 
how your computer talks to your fax modem.

EIA FAX CLASS DESIGNATIONS

Class 1: It provides a basic command set of only six com-
mands, which leaves much of the processing for the communi-
cations link to the computer. Class 1 is EIA-approved.

Class 2: This extended command set of 40 commands frees up 
the computer by putting more functions in the modem. EIA 
approval is pending.

Class 3: Adds file conversion to the modem for even more 
efficient operation, but EIA approval is years away.

ITU FAX GROUP DESIGNATIONS

Group 1: Over 20 years old; analog encoding; one page every 
6 minutes; obsolete.

Group 2: 1976: Analog encoding; one page every 3 minutes; 
practically obsolete.

Group 3: 1980: Digital encoding; one page in less than a mi-
nute. The most popular group today, Group 3 calls for a T.4 
coding and compression scheme and basic black-and-white 
scanning with optional gray-scale and color. The transmis-
sion speed is specified by using V.21 for 300 bps, V.27ter 
for 4800 bps, V.29 for 9600 bps, or V.17 for 14.4K bps op-
eration. The ZDX uses Group 3 technology.

Group 4: 1984: Digital transmission (56K or 64K bps). Use of 
Group 4 is limited by the narrow bandwidth of public tele-
phone lines. It may become more important in the future as a 
new generation of digital links called the Integrated 
Service Digital Network (ISDN) gradually replaces the pres-
ent telephone system.

FAX TRANSMISSION SPEED STANDARDS

ITU V.17: 14,400 bps.

ITU V.29: 9600 bps

ITU V.27ter: 4800 bps

ITU V.21: 300 bps 

OTHER FAX PROTOCOLS

ITU T.4: Defines a standard page image data format for 
transmission of fax images. Image data is compressed for 
transmission.

ITU T.30: Defines how fax machines talk over telephone 
lines. Includes modulation and data format standards.

EIA TR29.2: Defines the standard fax command set for Class 2 
fax machines. These commands are an extension of the AT com-
mand set and can be entered line by line from any communica-
tions program. However, use of the fax command set is com-
plex. Furthermore, knowledge of it is not needed by users, 
since software such as MultiExpressFax for Windows controls 
the fax process automatically. Therefore, we will not de-
scribe the fax commands in this manual.



------------------------------------------------------------

CHAPTER 4
SERIAL PORTS AND SOFTWARE CONFIGURATION

------------------------------------------------------------

The ZDX comes with MultiExpress for Windows (MEW), a data-
comm program. The quickest way to get started with your ZDX 
modem is to install and use this program, which was designed 
especially for Multi-Tech modems. 

Since your datacomm configuration is affected by the capa-
bilities of your computer, this chapter begins with a dis-
cussion of the limitations of some serial ports and how to 
identify them. It then discusses datacomm configuration in 
general and recommends settings specifically for the ZDX.


SERIAL PORT LIMITATIONS
-----------------------

Datacomm software allows you to centralize a lot of deci-
sions regarding modem communications, but in order for you 
to make those decisions, which you will need to do as soon 
as you've installed the software, you need to think about 
how the hardware on both ends of the connection would best 
handle the data. Some serial ports, particularly those in 
older PC-compatible computers, may limit the performance of 
the ZDX. Before you configure your software, you should know 
if yours is one of them. 

The limiting factor is an integrated circuit called a 
Universal Asynchronous Receiver/Transmitter, or UART. All 
data from your modem flows through it. The UARTs typically 
used in PC-compatible computers are types 8250, 8250A, 
16450, and 16550AFN. The 8250 is unreliable above 9,600 bps, 
and the 8250A and 16450 are unreliable above 19,200 bps. If 
the modem sends data to the UARTs above those speeds, the 
UARTs may not be able to process the data fast enough to 
keep from losing some of it. The 16550AFN, however, can 
safely handle data to 115,200 bps. 

When a modem communicates with V.42bis 4-to-1 compression 
enabled, it sends up to four times as much data to the se-
rial port as it receives in compressed form over the tele-
phone line. Therefore, a modem communicating at 14,400 bps 
may require a serial port that can reliably transfer data at 
57,600 bps, and at 19,200 bps it may require one that works 
reliably to 76,800 bps. If your serial port cannot handle 
these speeds, we recommend that you replace your present se-
rial card with one that has a 16550AFN UART or equivalent.

Macintosh computers do not use UARTs. The Macintosh SE 
through IIfx models use a Zilog Z8530 chip called a Serial 
Communications Controller, or SCC, that has a maximum speed 
of 57,600 bps. This speed can be compromised by other serial 
communications, including printer transmissions and Apple-
talk, the networking software that allows Macintoshes to 
share files. When Appletalk is active it controls all serial 
communications on the Macintosh. Because it gives priority 
to network communications, it may lose modem data at higher 
transmission speeds. Therefore, when Appletalk is active you 
risk losing data on serial port communications over 2400 
bps, though most users can work up to 9600 bps without prob-
lems. Other activities that could cause the serial driver to 
drop bits at high speeds include the floppy disk driver for-
matting a disk, the CPU paging in or out in virtual memory 
mode, and the Mac IIci or IIsi running the on-board video in 
8-bit mode. Therefore, for maximum communications speed on 
the Macintosh, we recommend as few concurrent activities as 
possible. To use V.42bis compression at 19,200 bps, we rec-
ommend that you install a high speed serial port card. Newer 
Macintosh computers, such as the Quadra and Centris models, 
support serial port speeds up to 115,200 bps.


WHAT IF YOU DON'T HAVE A 16550 UART?

Unless you have a computer that predates the IBM AT model, 
your serial port is probably good to 19,200 bps. That means 
you can communicate reliably at top speed if compression is 
disabled. 

Disabling compression will not necessarily slow your trans-
missions. High speed transmissions are most useful for down-
loading long files. Since most files from commercial infor-
mation services and BBSes are already compressed, and since 
V.42bis modems cannot compress them further, there is no 
speed gain from using hardware compression on software-
compressed files. In fact, there is a slight speed penalty. 
A fast UART is useful mainly for uploading or downloading 
long, uncompressed files with hardware compression enabled, 
and you may discover that that is only a small part of your 
modem use.


HOW CAN YOU IDENTIFY YOUR UART TYPE?

If you have MS-DOS 6.0 or later, you can find your UART type 
from a diagnostic program called MSD. To use it, type MSD at 
the DOS prompt. After the opening screen, select COM 
Ports.... The last line of the report tells you what type 
UART you have for each COM port. MSD does not distinguish 
between the 8250 and the 8250A. However, if you have an IBM 
AT or newer computer, you likely have an 8250A or 16450 UART 
installed, both of which are reliable to 19,200 bps. There 
are also shareware programs available on CompuServe and some 
BBSes that can identify your UART more precisely than MSD.


THE MULTI-TECH ISI CARD

Consider replacing your slow serial card with a Multi-Tech 
high speed Intelligent Serial Interface (ISI) card. It comes 
in two-port and eight-port versions for DOS/Windows and UNIX 
systems, and includes a 50K data buffer that eliminates data 
loss even at high speeds. See your dealer for further 
details.


CONFIGURING YOUR SOFTWARE
-------------------------

Datacomm software must be configured to work with your mo-
dem, your computer, and the remote system it is calling. 
Fortunately, most datacomm programs make the process easy by 
providing a default initialization string for your modem as 
well as defaults for most of the other required parameters.


CONFIGURING DATACOMM SOFTWARE FOR YOUR MODEM

Because remote computers may have different connection re-
quirements such as speed, number of bits, parity, log-on 
sequences, etc., datacomm software is typically configured 
by sessions, each session having a unique configuration for 
a given connection (e.g., to a BBS or commercial on-line 
service). Most datacomm programs, however, have a separate 
modem configuration menu because modem configurations rarely 
change from session to session.

The most important configuration is the modem initialization 
string. This is a sequence of commands the software uses to 
configure the modem when the datacomm software is loaded or 
when a session begins. Always begin the initialization 
string with the attention command AT, then follow it with 
the modem reset command, &F. Issuing a reset command before 
other commands ensures that you are starting with a known 
state. 

The rest of the commands in the initialization string depend 
on the capabilities of the modem and what you want it to do. 
Some older datacomm programs require you to create the ini-
tialization string by yourself. Most modern datacomm pro-
grams, however, provide you with a ready-made initialization 
string that is automatically selected when you choose your 
modem model from a list. It is a poor idea to use an ini-
tialization string intended for another modem, especially 
one from another manufacturer, because modem capabilities 
and command implementations vary from modem to modem. How-
ever, if your ZDX does not appear on a modem list, you may 
use the MultiModemII initialization string.

PC INITIALIZATION STRINGS

We recommend the following initialization string for a ZDX 
connected to a PC-compatible computer and sharing a line 
with a telephone:

	AT &F X4 S0=0 ^M

This string resets the ZDX to the factory default settings, 
selects extended result codes with NO DIAL TONE and BUSY, 
and turns off auto-answer. ^M must end every string sent to 
the modem from software. It is the ASCII code for the RETURN 
key on most keyboards, and the default code for the carriage 
return character in the ZDX and most datacomm programs. The 
carriage return character is defined in the ZDX in S-
register S3; if you change it, you must also change the 
carriage return character code used in your datacomm soft-
ware. If you send a command directly to the modem rather 
than indirectly through datacomm software, you must end the 
command string by pressing the RETURN key (<CR>) instead of 
adding ^M to the string.

The following initialization string is for a ZDX on a dedi-
cated telephone line:

	AT &F X4 S0=1 ^M

In this string, S0=1 instructs the modem to answer after one 
ring. Since this is a factory default, the string can be 
simplified to:

	AT &F X4 ^M


CHANGING DEFAULT PARAMETERS

By default, the ZDX will answer after the first ring and try 
to communicate with a modem on the other end of the line. If 
you have one telephone line for voice, fax, and modem commu-
nications, the ZDX may attempt to answer all incoming calls, 
voice as well as data. To change auto-answer to default off, 
open your communications program and type the following 
string in the terminal window (in MEW you might first have 
to click the Connect/Disconnect button to establish a con-
nection with the modem's COM port):

	AT &F S0=0 &W0 &F9 <CR>

This string selects the factory default parameters, then 
turns auto-answer off and stores that setting, along with 
all other current parameters, in nonvolatile memory. The &F9 
command causes the modem to load the values from nonvolatile 
memory the next time it sees the &F command. You will use 
the same initialization string as before:

	AT &F X4 ^M

But now the ZDX will load the values stored in nonvolatile 
memory when you turn on the modem and when you issue the ATZ 
reset command. (Note: Because it clears the command buffer, 
ATZ must be used on a line by itself; it cannot be part of 
an initialization string.)

MACINTOSH INITIALIZATION 

Macintosh computers cannot use RTS/CTS hardware flow control 
without the correct serial cable (see Appendix H, "RS-232C 
Interface"). The Macintosh 128 and 512 models cannot use 
RTS/CTS flow control at all. For those Macintoshes turn off 
the default RTS/CTS hardware flow control, turn on Xon/Xoff 
flow control and pacing, and ignore DTR:

	AT &F X4 &E5 &E13 &D0 ^M

For hardware flow control, use the following initialization 
string:

	AT &F X4 &E13 &D0 ^M

Add S0=0 to both strings to disable auto-answer if the ZDX 
is on a voice line. 

You can store the initialization string in nonvolatile mem-
ory. With your datacomm software open and connected to the 
modem's COM port, type the initialization string in the ter-
minal window, substituting a carriage return for ^M. To 
store the string, enter

	AT &W0 <CR>.

Now you can initialize your modem with the following simple 
string:

	AT Z ^M


RESULT CODES 

The ZDX has two sets of result codes that provide you with 
on-going information during dialing and connection. The de-
fault Multi-Tech result code set gives the most information 
when error correction and data compression are on, and we 
recommend it. However, some datacomm programs can use only 
the standard AT result codes. If that is the case with your 
software, you should add &Q1 to your initialization string 
to enable the standard result codes. The following table 
compares the two sets of result codes:

        Table 4-1. Result Codes

&Q0 Multi-Tech 			&Q1 Standard
    Result Codes 		    Result Codes
------------------------------------------------------------
Terse	  Verbose		Terse	Verbose
0	  OK			0	OK
1	  CONNECT		1	CONNECT
2	  RING			2	RING
3	  NO CARRIER		3	NO CARRIER
4	  ERROR			4	ERROR
5	* CONNECT 1200		5	CONNECT 1200
6	  NO DIALTONE		6	NO DIALTONE
7	  BUSY			7	BUSY
8	  NO ANSWER		8	NO ANSWER
9	  CONNECT 2400		
				10	CONNECT 2400
11	* CONNECT 4800		11	CONNECT 4800
12	* CONNECT 9600		12	CONNECT 9600
13	* CONNECT 14400		13	CONNECT 14400
19	* CONNECT 19200		19	CONNECT 19200
21	* CONNECT 21600		21	CONNECT 21600
24	* CONNECT 24000		24	CONNECT 24000
26	* CONNECT 26400		26	CONNECT 26400
28	* CONNECT 28800		28	CONNECT 28800
------------------------------------------------------------
* With error correction on, RELIABLE (or R) is added to 
  these result codes. With data compression on, COMPRESSED 
  (or C#) is added. The Reliable/Compressed modifiers are 
  not displayed with the standard result codes.



OTHER PARAMETERS. 

The default values for the other parameters in modem con-
figuration menus rarely need changing. They typically in-
clude the dialing prefix (ATDT for touch-tone service and 
ATDP for rotary service), the dialing suffix (^M), the hang-
up string (+++ATH0^M), and response messages (RING, NO CAR-
RIER, BUSY, etc.). Datacomm software with a host mode might 
also include an auto-answer string (AT S0=1^M).


CONFIGURING DATACOMM SOFTWARE FOR YOUR COMPUTER

You must configure your datacomm software to match your com-
puter's configuration. If the ZDX is connected to the COM2 
serial port, you must tell the software you are using COM2. 
Another important parameter is the serial port baud rate. 
This is the speed at which your modem communicates with your 
computer, not the speed at which your modem communicates 
with another modem. When V.42bis data compression is en-
abled, you must have a serial port baud rate four times the 
transmission speed of the modem to prevent data from being 
lost. So if your UART is fast enough, you should set the se-
rial port baud rate to a minimum of four times the top speed 
of your modem. For an MT1432ZDX, which has a top speed of 
14,400 bps, set it at 57,600 bps; for an MT1932ZDX, which 
has a top speed of 19,200 bps, set it at 115,200 bps; for an 
MT2834ZDX, which has a top speed of 28,800 bps, set it at 
115,200 bps. If you have an 8250 UART, your most reliable 
serial port speed will be 9200 bps; if you have an 8250A or 
a 16450 UART, try 19,200 bps.


CONFIGURING DATACOMM SOFTWARE FOR THE REMOTE SYSTEM

You must meet the requirements of the remote system for suc-
cessful communications. Though the ZDX can automatically 
synchronize with the speed of the other modem, you must 
specify parameters such as number of data bits, number of 
stop bits, and parity. This information is often listed with 
the name and number of the remote system. If you do not know 
it, try 7 bits, even parity, 1 stop bit (7-E-1) if the host 
is a mainframe computer, and 8 bits, no parity, 1 stop bit 
(8-N-1) if the host is a personal computer. If you guess 
wrong, you will get gibberish on your screen. Change the 
settings and try again. 

TERMINAL MODE

If you are accessing the remote computer as if from an on-
site terminal, you are in terminal mode. The keyboard codes 
expected by your computer may not match the ones used by the 
remote computer. To be compatible with the remote computer, 
your software must be able to substitute the appropriate 
codes in what is known as terminal emulation. Most datacomm 
programs can emulate the most common mainframe terminals, 
including the DEC VT100, VT 102, and VT52 terminals, and the 
basic TTY mode. If in doubt about which to choose for a BBS, 
try ANSI or VT100 first.


FILE TRANSFER PROTOCOLS

When you upload or download files with your modem, the host 
computer will ask which file transfer protocol you want to 
use. Most datacomm programs allow you to choose a default 
protocol. The most common choices are:

ASCII: For uploading and downloading text files only. Use 
only if other protocols fail.

Kermit: Used mostly by mainframe computers, rarely by per-
sonal computers. An ACK (acknowledge) is sent after every 
data packet.

Xmodem: The oldest and slowest PC file transfer protocol, it 
transmits files in blocks of 128 characters. It uses a one-
byte checksum.

Xmodem/CRC: A variation of the Xmodem protocol, with either 
128- or 1024-byte blocks and a two-byte cyclic redundancy 
check (CRC).

Xmodem/1K: A variation of the Xmodem protocol, with 1024-
byte blocks and either a one-byte checksum or a two-byte 
CRC.

Ymodem: Includes a header record in the downloaded data. The 
record contains file-dependent information, such as name, 
size, and date/time stamps. As a result, file names need not 
be specified prior to starting downloads. Ymodem protocols 
assume the modem is performing error correction.

Ymodem/G: A Ymodem batch protocol that dumps 1024-byte 
blocks once it receives the response character G. You can 
use this protocol for transferring either single or multiple 
files.

Zmodem: Developed from Ymodem, Zmodem uses a variable-sized 
block to transfer data over even the noisiest telephone 
lines. Should errors occur, Zmodem can tell the sending mo-
dem exactly where to restart. Zmodem can also restart an 
aborted file at a later time. Zmodem is the fastest of these 
protocols for standard files, and slightly slower than 
Ymodem/G for compressed files. 


SPECIAL PROBLEMS
----------------


THE 16550 UART AND WINDOWS 3.1

Windows 3.1 may cause a loss of data when communicating at 
high speed, even with a 16550 UART installed. Because 
Windows is multitasking -- it switches between several pro-
grams running at the same time -- it can make the serial 
port wait briefly while Windows performs other tasks. Mean-
while, incoming data can be lost. The older 8250A and 16450 
UARTs can store data in a one-byte first in-first out (FIFO) 
buffer (a memory area in the UART in which the first bit in 
is the first out), but once the buffer is full, the data is 
lost. The 16550 has a 16-byte FIFO buffer, which accounts 
for much of its high speed. However, in Windows 3.1, the 
16550's FIFO buffer is disabled by default. To enable the 
buffer, use Notepad to open the SYSTEM.INI file in your 
Windows directory, and add the following line to the 
[386Enh] section:

	COMnFIFO=1

where n is the number of the COM port the 16550 is installed 
on. For example, if the 16550  is installed on COM2, the 
line should be:

	COM2FIFO=1

After you save SYSTEM.INI you must restart Windows for the 
change to take effect.

However, there is another problem when the FIFO buffer is 
enabled: Windows cannot recognize the COM port because of a 
bug in the Windows 3.1 serial port driver, COMM.DRV. The 
only way around this problem is to replace the COMM.DRV file 
in the WINDOWS\SYSTEM directory with another driver. Several 
drivers are available commercially or on BBSes that will 
solve this problem. A freeware driver called CyberCom is 
available as CYBERC.ZIP on CompuServe in the IBM Communica-
tions Forum, Library 0. CyberCom will enable Windows commu-
nications at speeds up to 115,200 bps if you have a 16550 
UART installed.


CALL WAITING

If you have call waiting service on the telephone line with 
which you use your ZDX, and someone tries to call you while 
you are using your ZDX, call waiting will interrupt your 
data connection. Most telephone companies in North America 
allow you to temporarily disable call waiting by preceding 
the telephone number with *70. Check with your telephone 
company to see what method, if any, is used in your area. 
You can program your datacomm software to automatically dis-
able call waiting by including the disabling code in the di-
aling string for each directory entry, as in the following 
example.

	*70,555-1212

Note the comma, which inserts a pause before the number is 
dialed. Some datacomm programs have a dialog box for a dial-
ing prefix where the disabling code can be entered.


WHEN TO DISABLE DATA COMPRESSION

The ZDX uses MNP 5 2-to-1 data compression or V.42bis 4-to-1 
data compression when it connects to a modem with similar 
capabilities. V.42bis compression can quadruple your maximum 
effective throughput on uncompressed files to as much as 
57,600 bps with the MT1432ZDX and 76,800 bps with the 
MT1932ZDX. Why would anyone want to turn off such a feature?

If your serial port cannot keep up with the above speeds be-
cause it has an older UART, you may lose data when using 
data compression. Also, the speed advantage hardware com-
pression gives you is entirely dependent on how much the 
data being transmitted can be compressed. If the data is al-
ready in compressed form -- a .ZIP or a .SIT file, for exam-
ple -- trying to compress it more will actually slow the 
transmission slightly compared to transmitting the same file 
with compression disabled. This effect will be most notice-
able if your modem negotiates MNP 5 compression with the 
other modem. V.42bis will not try to further compress a com-
pressed file, but MNP 5 will.

The command to disable compression is AT &E14 <CR>. If you 
have an older UART or if you use your modem mostly for down-
loading long, compressed files from BBSes, you may want to 
include the &E14 command in your initialization string as 
follows:

	AT &F S0=0 X4 &E14 ^M

As a general rule, you should try to transmit files in 
already-compressed form rather than relying on V.42bis hard-
ware compression. Because software compression is more effi-
cient than hardware compression, you will have a higher 
throughput with the former. Of course, this efficiency does 
not include the time spent compressing and decompressing 
.ZIP or .SIT files, but it will save on phone bills. And 
hardware compression will still be there for those occasions 
when it is inconvenient to compress a file with software. 
Note also that when you download files with compression dis-
abled, you can use a slower serial port if you have an older 
UART.


DISABLING ERROR CORRECTION

By default, the ZDX is set to Auto-Reliable mode. In this 
mode the ZDX determines during the handshake whether the 
other modem is using V.42 error correction. If it is, the 
ZDX then switches itself to Reliable mode and enables error 
correction. If it is not, the ZDX remains in Non-Error 
Correction mode.

Normally, we recommend that you leave the ZDX set to Auto-
Reliable mode (&E1). However, you may encounter some circum-
stances in which the ZDX will work better with error correc-
tion turned off. For example, it has been reported that on 
CompuServe error correction will slow file transfers at mo-
dem speeds of 9600 bps and under. If this is a problem for 
you, you can turn off error correction with the command AT 
&E0, or you can include the command in your CompuServe ini-
tialization string as follows:

	AT &F S0=0 X4 &E0 ^M

However, if you connect with CompuServe at 14,400 bps, you 
may leave the ZDX in Auto-Reliable mode without a speed 
penalty. 


------------------------------------------------------------

CHAPTER 5
USING THE MODEM

------------------------------------------------------------

Your ZDX is hooked up and your software is installed and 
configured. Now it's time to make your first call, check out 
the operation of the ZDX, and, if necessary, do some 
troubleshooting.


      +--------------------------------------------+
      |                                            |
      |                                            |
      |   TD  RD  CD  28  14  96  OH  TR  EC  FX   |
      +--------------------------------------------+

             Fig. 5-1. MT2834ZDX front panel


THE FRONT PANEL
---------------

Before you make your first call, you should know how to in-
terpret the LEDs on the front panel. The ZDX has 10 LEDs 
that indicate status, configuration, and activity, as de-
scribed below:

TD (Transmit Data). This LED blinks when data is being 
transmitted. The state of the TD LED matches the TD circuit 
on pin 2 of the RS-232C/V.24 interface.

RD (Receive Data). This LED blinks when data is being re-
ceive. The state of the RD LED matches that of the RD cir-
cuit on pin 3 of the RS-232C/V.24 interface. 

CD (Carrier Detect). This LED lights when a valid carrier 
signal from another modem is detected.

28 (28,800 bps). This LED lights when the modem is set for 
28,800 bps operation. Unless another default baud rate is 
selected and stored, this LED will light when the modem is 
turned on (MT2834ZDX only). Note: Both the 28 and 14 LEDs 
light when the MT2834ZDX operates at 26,400, 24,000, 21,000, 
19,200, or 16,800 bps.

19 (19,200 bps). This LED lights when the modem is set for 
19,200 bps operation. Unless another default baud rate is 
selected and stored, this LED will light when the modem is 
turned on (MT1932ZDX only). Note: Both the 19 and 14 LEDs 
light when the MT1932ZDX falls back to 16,800 bps while in 
V.32 terbo mode.

14 (14,400 bps). This LED lights when the modem is set for 
14,400 bps operation. Unless another default baud rate is 
selected and stored, this LED will light when the modem is 
turned on (MT1432ZDX only). Note: Both the 14 and 96 LEDs 
light when the MT1432ZDX falls back to 12,000 bps while in 
V.32bis mode.

96 (9600 bps). This LED lights when the modem is set for 
9600 bps operation.

24 (2400 bps). This LED lights when the modem is set for 
2400 bps operation (MT1432ZDX only).

OH (Off-Hook). This LED lights when the phone line is off-
hook, which occurs when the modem is dialing, on-line, or 
answering a call. This LED will also flash when the modem is 
pulse dialing. 

TR (Terminal Ready). This LED lights when a datacomm program 
initializes the ZDX. It means the modem is ready for an out-
going or incoming call. It goes out when the datacomm pro-
gram disconnects the COM port. When it goes out, a connected 
modem will disconnect. The state of the TR LED matches that 
of the DTR circuit on pin 20 of the RS-232C/V.24 interface.

EC (V.42). This LED is continuously lit when the modem is in 
error correction mode, and flashes on and off when compres-
sion is activated.

FX (Fax). This LED is lit when the modem is in Fax mode.

NOTE: When you turn on the ZDX, the speed lights will flash 
briefly as the modem does a self-test, then the LED for the 
default baud rate will light. Normally, this would be 28,800 
for the MT2834ZDX, 19,200 bps for the MT1932ZDX and 14,400 
bps for the MT1432ZDX. After a call, the LED for that con-
nection's baud rate will remain lit until another call is 
made or the modem is reset. If you make a call at a rate for 
which there is no LED, all LEDs will be dark after the data-
comm program is terminated, even though the modem is still 
turned on. 


MAKING THE CONNECTION
---------------------


INTRODUCTION 

We'll assume that you are using the ZDX to dial up a remote 
computer. The MT1932ZDX modem is factory preset to originate 
a call to another 19,200 bps modem that is also set for V.32 
terbo operation. The MT1432ZDX modem is factory preset to 
originate a call to another 14,400 bps modem that is also 
set for V.32bis operation. Both modems are factory set for 
V.42 error correction, V.42bis data compression, and CTS/RTS 
operation. If the answering modem is not set up similarly, 
the modems will negotiate a common protocol.


KEYBOARD DIALING/ON-LINE/ANSWERING

There are several basic steps for originating or answering 
calls in data communications mode:

Turn on the ZDX modem if it is not already on. The speed 
LEDs will flash briefly and then the fastest one will light.

Load communication software, such as MEW. The Terminal Ready 
(TR) LED will light.

Dial manually or using the software. The Off-Hook (OH) LED 
will light, then the Carrier Detect (CD) LED will light.

Establish on-line connection. The LED corresponding to the 
speed negotiated by the modems will light. The Error 
Correction (EC) LED will light if the modems are using error 
correction, and it will blink if they are using hardware 
data compression.

Exchange data with the remote computer. The Transmit Data 
(TD) and Receive Data (RD) LEDs will flash as the modems 
exchange data and flow control messages.

Terminate Call. All but the speed LED and TR LED will go 
dark.

A simple way to dial is from the keyboard of your computer 
or terminal. You enter a command, such as ATDT1234567<CR>, 
on the keyboard to tell the ZDX to dial telephone number 
123-4567. You must, however, do this from within a data com-
munications program; you cannot dial from the DOS command 
line.

If you use one of the more sophisticated communication soft-
ware packages, such as MEW, you can store phone numbers and 
other dialing information in the software. By storing your 
dialing commands and other choices, the software simplifies 
the communications process for you. Moreover, once you have 
configured it properly, it cannot introduce errors, so you 
get consistent results.

Whether or not you use datacomm software in terminal emula-
tion mode, or have automated AT sequences through your data-
comm software, each modem command must begin with the atten-
tion characters AT. AT characters may be entered in upper or 
lower case (i.e., they are not case sensitive).Entering AT 
automatically sets the modem's speed to match the speed of 
the computer or terminal, and also sets the modem's parity. 

The AT characters alert the modem that a command follows. 
The AT characters can also be used to clear the command 
buffer by typing AT and hitting RETURN.

The letter D in a command causes the modem to dial the num-
bers immediately following it (e.g., ATD6127853500). You can 
choose either pulse (ATDP) or tone (ATDT) dialing methods.

After the number has been dialed and a connection signal has 
been detected, the ZDX modem responds on your video display 
with CONNECT (since the ZDX defaults to 
V.32bis/V.42/V.42bis, the actual response may be CONNECT 
14400 RELIABLE COMPRESSED if the ZDX modem connects with a 
similar modem). The modem is now in on-line mode, and is 
ready to communicate with a host site. 

If no connection signal is detected within 45 seconds (this 
time period can be adjusted by S-Register S7), the ZDX goes 
on-hook (hangs up) and returns to command mode. At this 
point, your terminal displays NO CARRIER.

Enter ATH to hang up the on-line connection (bring modem on-
hook), and terminate the modem's on-line mode. At this point 
your terminal display displays OK. 

In addition to its call-originating capabilities, the ZDX 
can automatically answer incoming calls. You need not be 
present. You can, however, control the situation by config-
uring the modem to either answer or to not answer, or to 
answer after a specified number of rings. This is done by 
setting the value of S-Register S0. Refer to Chapter 6 for 
modem commands and S-Registers. 


HANDSHAKING

This section briefly explains what happens between two mo-
dems in a typical call. We are assuming that there are ZDXs 
at both the originating and the answering ends of the tele-
phone line. (If other brands of modems are used with our 
modems, they will still communicate, but each manufacturer 
tends to use slightly different delay timings and sequences, 
and many of the brands vary in their LED designations.)

When a call is dialed from the originating modem, the called 
modem responds to the ringing by switching to answer mode 
and by turning on its RI signal (Ring Indicator, RS-
232C/V.24 Pin 22) as the rings are detected.

Note that for the called modem to answer the incoming call, 
it must have a high DTR (Data Terminal Ready). The DTR sig-
nal normally would come from the computer or terminal to 
which the modem is attached, on RS-232C Pin 20. If the modem 
is connected to a computer that does not supply a DTR sig-
nal, such as an Apple Macintosh, the modem should be config-
ured to ignore DTR.

When the called modem answers the call, its OH (Off-Hook) 
circuit comes on and the ringing stops. Two seconds after 
the call is answered, the called modem begins transmitting 
its answerback tone. The originating modem, which has been 
off-hook and waiting for the answerback tone, turns on its 
transmitter when it hears the tone. Then, after a one second 
delay, the CD (Carrier Detect) signal comes on.

The called modem then responds to the originating modem by 
turning on its CD (Carrier Detect) signal, and handshaking 
is completed. 

NOTE: If you are using the Reliable or Auto-Reliable mode, 
some additional handshaking takes place.


CALL TERMINATION

There are several ways you can terminate a call or, simply 
stated, hang up:

Command Mode Control. To terminate a call by command mode, 
type +++ATH, then hit RETURN; or send a break signal fol-
lowed by ATH and a RETURN.

DTR Control. If DTR (Data Terminal Ready) is turned off for 
50 milliseconds or more, a disconnect occurs. This is proba-
bly the most common method used by computer systems at the 
auto-answer end of the line to cause the originating modem 
to disconnect after log-off procedures.

Loss of Carrier. After a data connection is established, the 
modem disconnects if a loss of carrier occurs for 700 milli-
seconds (0.7 seconds) or more. (This time can be set to any 
time from 0 to 25.5 seconds, in 100 ms increments, with S-
Register S10.) Note that one cause of carrier loss would be 
if one of the modems were to disconnect normally.

Abort Timer, Answer Mode. When OH (Off-Hook) comes on, the 
called modem will start a forty-five-second timer and wait 
for a carrier signal from the originating modem. If a car-
rier is not detected within this period, the modem will dis-
connect (hang up) and be ready for another call. In origi-
nate mode the abort timer functions the same as in answer 
mode, except that the timer begins after the modem has com-
pleted dialing, instead of when it first goes off-hook. You 
can change this 45-second wait period to any value from 0 
seconds up to 255 seconds, or disable the timer completely 
by changing S-Register S7.

Inactivity Timer. Causes the modem to disconnect if no data 
is being transmitted or received for a specified time. The 
timer is restored any time a character passes through the 
serial port in either a send or receive state. This timer 
runs in both Non-Error Correction and Reliable con-nections 
and is controlled by S-Register S30. The Inactivity Timer 
can be disabled by setting S30 to 0, which is the factory 
default setting.


TESTING YOUR MODEM
------------------

Your first tests of your modem should be functional tests. 
Try dialing a number, even your own. Listen for a dial tone, 
the dialing pulses or tones for the number you selected, and 
the ringing or busy signal from the telephone you have 
called. If this test does not succeed, refer to "Solving 
Problems" for possible solutions. 

Your next step should be to call a BBS. Your local computer 
paper probably has a BBS list. If you don't have a local 
computer paper, ask your dealer for the number of a local 
BBS, or call the Multi-Tech BBS (see Chapter 7, "Service, 
Warranty, and Technical Support"). You should hear your mo-
dem dial the number. When the other modem answers, you 
should hear the two modems exchange tones. When they finally 
go on-line together the speaker will shut off. You may see 
on your screen a result code indicating how the modems are 
connected, such as "CONNECT 14400 RELIABLE COMPRESSED." Wait 
a couple of seconds after you see the result code, then tap 
the RETURN key on your keyboard once or twice to signal the 
other computer. Most BBSes will respond with an opening mes-
sage and instructions on how to proceed. Follow the instruc-
tions and have fun. If you have problems connecting, refer 
to "Solving Problems."


SELF-TESTS

The ZDX has four self-test features. The first is an auto-
matic self-test done each time the modem is powered up. This 
test checks the modem's processor timing and memory chips. 
Operation of the automatic self-test is described in 
"Automatic Self-Test." The three other self-tests are local 
analog loopback, digital loopback (remote/automatic) and 
digital loopback (local/manual).

A loopback test involves entering data from your PC and 
looping that data through the circuits of your modem and/or 
a remote modem. When the loop has been completed, the origi-
nal data entered should match the data received back on your 
PC's monitor after the test.

The local analog loopback test allows you to verify that the 
modem's transmitter and receiver circuits are functioning 
properly.

The digital loopback test (local/manual) allows you to ver-
ify that the remote computer or terminal, the remote modem, 
serial ports, the telephone line, and the local modem are 
functioning properly. 

The digital loopback test (remote/automatic) allows you to 
verify that the local computer or terminal, the two modems 
and the transmission line between them are functioning prop-
erly. Upon completion of testing, enter either escape 
sequence: +++AT<CR> or <BREAK>AT<CR>.

NOTE: All loopback tests will operate at all speeds except 
300 bps.


AUTOMATIC SELF-TEST

Each time you power up the ZDX, it checks to see if the com-
puter or terminal has turned DTR on, which is indicated by 
the TR LED. If the TR LED is not lit, the modem performs an 
automatic self-test to insure proper operation. During this 
test, the speed LEDs blink for a few seconds, indicating 
that the automatic self-test is proceeding. At the success-
ful end of the test, the LEDs stop blinking. If they do not 
stop, or if they never start, there is a problem with your 
modem. If the automatic self-test fails, call Tech Support 
(see Chapter 7, "Service, Warranty, and Technical Support").


LOCAL ANALOG LOOPBACK TEST

In this test, data from your computer or terminal is sent to 
your modem's transmitter, converted into analog form, looped 
back to the receiver, converted into digital form, and then 
received back at your monitor for verification. No connec-
tion to the phone line is required. The test procedure is as 
follows:

1. Connect the modem to your computer. With your communi-
   cation software, set the desired baud rate. 

2. Type ATU or ATU0 and hit RETURN. This places your modem 
   in analog loopback mode in the originate mode. A CONNECT 
   message should appear on your display. The modem is now 
   out of the command mode and in a pseudo-on-line mode.

3. Note that the CD LED is lit. If you're set for 14,400 
   bps, the 14 LED should be lit. If you're set for 9600 
   bps, the 96 LED should be lit. If the CD is not lit, 
   there is a defect in your modem.

4. Now enter characters from your keyboard. For this test, 
   typing multiple upper case U characters is a good way to 
   send an alternating test pattern of ones and zeros. The 
   characters entered should be displayed on your monitor. 
   The TD and RD LEDs should flash when a character is 
   entered.

5. For a more complete test, you should also test the modem 
   in answer mode. To do this, you must first terminate the 
   originate mode by entering an escape sequence (+++AT<CR> 
   or <BREAK>AT<CR>). An OK should appear on your display. 
   Then type ATU1 and hit RETURN to place the modem in ana-
   log loopback in answer mode. 

6. Note that the CD LED is lit. If you're set for 28,800 
   bps, the 28 LED should be lit. If you're set for 14,400 
   bps, the 14 LED should be lit. If you're set for 9600 
   bps, the 96 LED should be lit. If the CD is not lit, 
   there is a defect in your modem.

7. Enter characters from your keyboard. Type multiple upper 
   case U characters to send an alternating test pattern of 
   ones and zeros. The characters entered should be dis-
   played on your monitor. The TD and RD LEDs should flash 
   when a character is entered.

8. When testing is completed, you may exit answer mode 
   analog loopback test by entering an escape sequence 
   (+++AT<CR> or <BREAK>AT<CR>), which returns the modem to 
   command mode. 

9. Your modem passes this test if the data entered from your 
   keyboard are the same as the data received on your moni-
   tor. If different data appears on your monitor, your mo-
   dem is probably causing the problem, although it could 
   also be your computer. If your modem passes this test, 
   but you are receiving errors while on-line, the remote 
   modem or the phone line could be at fault.


REMOTE DIGITAL LOOPBACK TEST

In this test, your modem must be on-line with another modem, 
such as another ZDX, that is set up to respond to a request 
for remote digital loopback. With the ZDX, this ability to 
respond is controlled by the &T command. AT&T4 enables the 
response to remote digital loopback. AT&T5 disables the re-
sponse. The modem defaults to &T5 on power-up, so this must 
be changed on the remote modem before initiating remote 
digital loopback.

Initiate remote digital loopback with the ATU2 command, 
which places the remote modem in digital loopback mode. Data 
from your computer or terminal are transmitted through your 
modem and over the phone line to the remote modem, where 
they are then looped back to your modem. 

The test procedure is as follows:

1. Arrange to have &T4 set on the remote test modem.

2. Go into terminal mode by opening a session in your data-
   comm program. (In MEW, choose Connect from the Perform 
   menu.) Type AT and hit RETURN; you should get an OK mes-
   sage. Enter AT&E0<CR> to disable error correction.

3. Dial the remote modem and establish your on-line 
   connection.

4. Type the escape sequence (+++AT<CR> or <BREAK>AT<CR>) to 
   bring your modem into command mode while maintaining the 
   connection with the remote modem. 

5. Type ATU2 and hit RETURN. The local modem responds to 
   this command by transmitting an unscrambled marking sig-
   nal, which causes the remote modem to place itself in 
   digital loopback mode. Then the local modem exits command 
   mode and enters on-line mode.

6. Enter data from your keyboard. For this test, typing 
   multiple upper case U characters is a good way to send an 
   alternating test pattern of ones and zeroes. Data 
   received by the remote modem will enter its analog 
   receiver, be converted to digital data, be reconverted 
   into analog, and then be transmitted back to your modem. 
   Your modem passes this test if the data entered from your 
   keyboard is the same as the data received on your 
   monitor. 

7. When testing is complete, you may end the test by typing 
   an escape sequence (+++AT<CR> or <BREAK>AT<CR>) to bring 
   your modem into command mode. The modem should respond 
   with an OK message. If you wish to stay on-line with the 
   remote modem for normal data transmission, type ATO and 
   hit RETURN. If you wish to terminate the call, type ATH 
   and hit RETURN to hang up.


LOCAL DIGITAL LOOPBACK TEST

This test is identical to the remote digital loopback test 
with one exception. Instead of using your modem to signal a 
remote modem to place itself in digital loopback mode, your 
modem is placed in digital loopback mode while the remote 
modem is not. Data is entered and transmitted from the re-
mote modem, sent across the phone line to the local modem, 
and looped back to the remote modem. 

The test procedure is as follows: 

1. Go into terminal mode in your datacomm program if you are 
   not already in it. (In MEW, choose Connect from the 
   Perform menu to take over the modem COM port.) Type AT 
   and hit RETURN; you should get an OK message. Enter 
   AT&E0<CR> to disable error correction.

2. Dial the remote modem and establish your on-line 
   connection.

3. Type an escape sequence (+++AT<CR> or <BREAK>AT<CR>) to 
   bring your modem into command mode while still main-
   taining the connection with the remote modem. 

4. Type ATU3 and hit RETURN. Once you receive an OK message 
   from your modem (if responses are enabled), your modem is 
   placed in digital loopback mode.

5. Have someone enter data from the remote keyboard. For 
   this test, typing multiple uppercase "U" characters is a 
   good way to send an alternating test pattern of ones and 
   zeros. The data received by your modem will enter its 
   analog receiver, be converted to digital data, be recon-
   verted into analog, and then transmitted back to the re-
   mote modem. Your modem passes this test if the data 
   entered from the remote keyboard is the same as the data 
   received on the remote monitor. 

6. When testing is complete, you may end the test by typing 
   an escape sequence (+++AT<CR> or <BREAK>AT<CR>) to bring 
   your modem into command mode. The modem should respond 
   with an OK message. If you wish to stay on-line with the 
   remote modem for normal data transmission, type ATO and 
   hit RETURN. If you wish to terminate the call, type ATH 
   and hit RETURN to hang up.


SOLVING PROBLEMS
----------------

Your ZDX was thoroughly tested at the factory before it was 
shipped. If you are unable to make a successful connection 
or if you experience data loss or garbled characters during 
your connection, it is possible that the modem is defective. 
However, it is more likely that the source of your problem 
lies elsewhere. If you experience problems, please check out 
the following possibilities before calling Tech Support.


NONE OF THE LEDS LIGHT WHEN THE MODEM IS TURNED ON.

When you turn on the ZDX, the LEDs on the front panel should 
flash briefly as the modem runs a self-test. If the LEDs re-
main dark, try the following procedures:

Make sure the power supply module is firmly connected to the 
modem and to the wall outlet. If the power supply is plugged 
into a power strip or surge protector, make sure the power 
strip or surge protector is plugged in and the power switch 
is on. If you normally turn on the modem by turning on the 
power strip or surge protector, make sure the modem power 
switch is on. If the power strip or surge protector is on 
and the modem switch is on, try moving the modem power sup-
ply to another outlet on the power strip or surge protector. 
Finally, test that the outlet is live by plugging a lamp 
into it. 

Check that your data line is good; plug in a telephone and 
check for dial tone. Check that your cable is good; replace 
the cable with a known working cable. 


THE MODEM DOES NOT RESPOND TO COMMANDS.

First make sure the modem is plugged in and turned on. (See 
previous section.) 

Are you issuing the modem commands from the datacomm soft-
ware, either manually in terminal mode or automatically? 
(You cannot send commands to the modem from the DOS prompt.) 

Make sure you are in your datacomm program in terminal mode, 
then type AT<CR>. If you get an OK response, your connec-
tions are good and the problem likely is in your phone book 
entry or session settings. 

Make sure there is a reset command (&F) in your initializa-
tion string. Without it, your modem may not have initialized 
correctly. Enter ATZ<CR> from terminal mode, or turn the 
modem off and on again, and try again.

If you don't get an OK, the problem still may be in the 
software. Make sure you have done whatever is necessary in 
your datacomm software to make a port connection. Not all 
datacomm programs use the same procedure to connect to the 
COM port. Some connect when the software loads and remain 
that way. Others, like MEW, can be disconnected without ex-
iting the program. (Make sure the Connect icon looks plugged 
in and the red light is on.) The TR LED on the modem's front 
panel shows that the modem is ready to accept commands.

Also, your datacomm settings may not match the physical port 
the modem is connected to. The serial cable may not be 
plugged into the correct computer port -- check your com-
puter documentation to make sure. Or you may have selected a 
COM port in your datacomm software other than the one the 
modem is physically connected to -- check the settings in 
your software.

If the modem is on, the cable is plugged into the correct 
port, the datacomm software is configured correctly, and you 
still don't get an OK, you may have a faulty serial cable. 
Make sure it is firmly connected at both ends. If the con-
nectors are not screwed down, they can be accidentally 
pulled loose. 

Is this the first time you have used the cable? If so, it 
may not be wired correctly. Check the cable description on 
the packaging to make sure the cable is the right one for 
your computer. If you have access to an ohmmeter and know 
how to use it, check the continuity against the relevant 
drawing in Appendix H.

Some peripheral expansion cards, such as bus mouse and sound 
cards, may come with an extra serial port preconfigured as 
COM1 or COM2. If you have more than two serial ports, it is 
possible another port or device uses the same COM port, DMA 
address, or interrupt (IRQ) number as your communications 
port. Use the Microsoft System Diagnostic program (MSD.EXE) 
to check the settings for your peripherals. If you need to 
disable or otherwise change the COM ports on an expansion 
card, refer to the card's documentation.

Last, the serial port itself may be defective. If you have 
another serial port, install the modem on it, change the COM 
port setting in your software, and try again. 


THE MODEM DIALS BUT IS UNABLE TO MAKE A CONNECTION.

There can be several reasons the ZDX fails to make a connec-
tion. Possibilities include lack of a physical connection to 
the telephone line; a busy signal; a wrong number; no modem 
at the other end; a faulty modem, computer, or software at 
the other end; and incompatibility between modems.

You can narrow the list of possibilities by using extended 
result codes. To enable them, enter AT V1 X4 <CR> while in 
terminal mode, or include V1 X4 in the modem's initializa-
tion string. Then dial again; the modem will report the 
call's progress.

If the modem reports NO DIAL TONE, check that the modem 
telephone cord is connected to both the modem and to the 
telephone wall jack. If the telephone cord looks secure, the 
problem may be in your building's telephone installation. To 
test the building installation, plug a telephone into your 
modem's telephone wall jack and listen for a dial tone. (If 
you are in North America, you can plug the telephone into 
the ZDX's PHONE jack instead.) 

If the modem reports BUSY, the other number is busy. Try 
again later.

If the modem reports NO ANSWER, the other system has failed 
to go off-hook, or you might have dialed a wrong number. 
Check the number.

If the modem reports NO CARRIER, the phone was answered at 
the other end, but no connection was made. You might have 
dialed a wrong number, and a person answered instead of a 
computer, or you might have dialed the correct number but 
the other computer or software was turned off or faulty. 
Check the number and try again, or try calling another 
system to make sure your modem is working. 


THE MODEM DISCONNECTS DURING A TRANSMISSION.

If you have call waiting on the same phone line as your mo-
dem, it may interrupt your transmission when someone tries 
to call you. If you have call waiting, disable it before 
each call. 

If you have extension phones on the same line as your modem, 
you or someone else could interrupt the transmission by 
picking up another phone. If possible, disconnect the exten-
sion phones before using the modem, or install another phone 
line especially for the modem.


FILE TRANSFER IS SLOWER THAN IT SHOULD BE.

If you are running under Windows 3.1 and have a 16550 UART, 
you may need to turn on the 16550's data buffers and/or 
replace the Windows serial driver, COMM.DRV. 

Or you may be using a slow transfer protocol, such as 
Xmodem. Try Zmodem or Ymodem/G instead. 

Is your line noisy? If there is static on your line, the mo-
dem will have to resend many blocks of data to insure accu-
racy. You must have a clean line for maximum speed. 

Or you might be downloading a compressed file with MNP 5 
hardware compression enabled. Since hardware data compres-
sion cannot further compress a file already compressed by a 
software program such as PKZIP or StuffIt, the transfer can 
actually be slower with data compression enabled than with 
it disabled.

Try entering the L8 (List On-Line Diagnostics) command in 
on-line mode; make a screen print of the diagnostics listing 
and check for parameters that may be unacceptable (number of 
retrains, round trip delay).


I AM LOSING DATA.

Your UART may not be reliable at serial port speeds over 
9600 bps or 19,200 bps. Turn off data compression, reset 
your serial port speed to a lower rate, or replace your 
serial port with a faster one. 

Also, make sure the flow control method you selected in 
software matches the method selected in the modem. If you 
have a Macintosh, you may have the wrong cable for hardware 
flow control. 

If you are running under Windows 3.1 and have a 16550 UART, 
you may need to turn on the 16550's data buffers and/or 
replace the Windows serial driver, COMM.DRV.

Try entering the L8 (List On-Line Diagnostics) command in 
on-line mode; make a screen print of the diagnostics listing 
and check for parameters that may be unacceptable (line 
quality, signal strength).


I AM GETTING GARBAGE CHARACTERS ON THE SCREEN.

Your computer and the remote computer may be set to differ-
ent word lengths, stop bits, or parities. If you have con-
nected at 8-N-1, try changing to 7-E-1, or vice-versa. 

At speeds above 2400 bps, the remote modem might not use the 
same transmission or error correction standards as your mo-
dem. Try connecting at a slower speed, or disabling error 
correction.

Try entering the L8 (List On-Line Diagnostics) command in 
on-line mode; make a screen print of the diagnostics listing 
and check for parameters that may be unacceptable (data for-
mat, line quality, signal strength). 


I CAN'T RUN MY FAX AND COMMUNICATIONS SOFTWARE AT THE SAME 
TIME.

Communications devices can be accessed by only one applica-
tion at a time. You can run either your fax software or your 
datacomm software, but not both at the same time, unless you 
have a special communications device management application.



-----------------------------------------------------------

CHAPTER 6
AT COMMANDS AND S-REGISTERS

-----------------------------------------------------------


THE AT COMMAND SET
------------------

You control ZDX modems by issuing instructions called AT 
commands, so called because the attention characters AT pre-
cede each command or command string (a sequence of commands 
on one line). You may send commands directly to the modem 
from your computer's keyboard, or you may use your communi-
cations software to issue these commands. 

Your modem is in command mode when it is not dialing or on-
line with another modem. When your modem is in command mode, 
you have access to a complete communications system that al-
lows you to use a number of features, including the basic AT 
command set described in this chapter. The basic AT command 
set lets you enter phone numbers for automatic dialing with-
out a telephone set, configure various modem options, and 
monitor telephone activity. In addition, you can command 
your modem to perform advanced features such as error cor-
rection, data compression, speed conversion, and more.

This chapter explains the command mode and shows you how to 
use each of the basic AT commands. It also explains the ad-
ditional commands used for error correction, data compres-
sion, and speed conversion.

The ZDX commands and responses are compatible with all 
systems and software programmed for automatic, software-
controlled autodialing using the AT command set.

The ZDX has a help command that provides you with short, on-
screen explanations on how to use the modem commands. The 
help command is explained later in this chapter.


COMMAND EDITING

The BACKSPACE key on your keyboard edits characters in the 
AT command line. An AT command is not executed until the 
RETURN key is pressed. The BACKSPACE key erases the previous 
command character. It will not erase the AT characters once 
they are typed. If your keyboard has no BACKSPACE key, use 
CTRL+H. You may change the character recognized by the modem 
as BACKSPACE to any other ASCII character with S-Register 
S5.

To cancel an entire command that has been typed but not yet 
executed, enter a CTRL+X. This also clears the command 
buffer. The effect is the same as backspacing the entire 
command, only quicker.

The ZDX stores characters entered in a command in the ZDX's 
command buffer (memory) until they are executed by hitting 
RETURN. The command buffer's capacity is 60 characters. The 
attention characters (AT) do not count in the 60 allowed 
command characters. You may use spaces for increased read-
ability when typing a command; they are neither stored in 
the command buffer nor counted in the sixty allowed charac-
ters. Hyphens, parentheses, etc. are not allowed.

If you exceed the 60-character limit or type invalid charac-
ters, the command buffer is automatically erased, and an 
ERROR message is displayed. You must then retype the command 
within the 60-character limit, using only the allowed 
characters.


MODES OF OPERATION

The ZDX operates in one of two functional modes: "command 
mode" and "on-line mode." (There is also an in-between 
state, "wait-for-carrier," where the modem is out of command 
mode, but not yet on-line.) When you power up the modem, it 
is in command mode, and is ready to accept and respond to 
commands from your keyboard or software.

The ZDX enters the on-line mode after it dials and makes a 
connection with another modem, and then detects a valid car-
rier signal. If it does not detect a carrier signal within 
the time frame controlled by S-Register S7, the modem aban-
dons the call and re-enters command mode.

The modem will exit on-line mode if the carrier signal is 
lost or intentionally dropped. When this happens, the modem 
hangs up and re-enters command mode.

By sending certain "escape" characters to the ZDX while on-
line, you can make it enter command mode from the on-line 
mode without losing the carrier signal.

Also, you can make the ZDX enter on-line mode without dial-
ing, by typing either the D command or the A command. All of 
the ZDX's commands are explained later in this chapter.


AT COMMANDS
-----------

You can control a wide variety of modem operations and op-
tions when the ZDX modem is in command mode. Commands can 
tell the modem to dial a number, to answer a call, to oper-
ate at a certain speed, to use a certain compression tech-
nique, and many other functions. AT commands are one or two 
letters which may be preceded by an ampersand (&), a dollar 
sign ($), a pound sign (#), or a percent sign (%). For exam-
ple, the Q command determines whether the modem returns re-
sult codes, and the &D command controls the modem's use of 
the DTR signal.

The parameter (0, 1, 2, etc.) follows a command to tell the 
modem which option to use. If you do not specify a parame-
ter, the modem assumes the 0 (zero) option. For example, E 
is the same as E0. You can issue several commands on a sin-
gle line (a command string) as long as the line does not 
exceed 255 characters. Although Q1 is one command, it counts 
as two characters in the command line. 

Each command has a valid range of parameters. For instance, 
&W can have parameters of 0 or 1. Valid commands generate an 
OK result code. A few generate an additional response, such 
as a list of parameters. An invalid command, such as &W3, 
which has a parameter outside the valid range, generates an 
ERROR result code. Most commands have a default parameter, 
one that is enabled when the modem is powered up or reset 
with ATZ or AT&F. Factory defaults are stored in read-only 
memory (ROM) and cannot be changed. User-defined defaults 
can be stored in nonvolatile random-access memory (NVRAM). 
They can be changed or deleted at will.

The commands in this chapter are organized into several 
groups based on function. A brief summary follows. 
(Remember, nearly all commands begin with AT.)


           TABLE 6-1.  AT COMMANDS BY FUNCTION

TOPIC	      COMMAND	DESCRIPTION
------------------------------------------------------------
Dialing Action	D	Dial
		$D	DTR dialing
		A:	Continuous redial
		H	On-hook/off-hook

Dial		P	Pulse dial
Modifiers	T	Tone dial
		W	Wait for new dial tone
		R	Reverse originate/answer mode
		,	Dialing pause
		:	Continuous redial
		;	Return to command mode after 
			dialing
		!	Flash on-hook
		@	Quiet answer
		$	Call card tone detect

Phone Number	D...N	Store phone number
Memory		N	Dial a stored number
		N...N	Number linking
		L	List stored telephone numbers

Configuration	&W	Store configuration
& Default	&F	Load default config.
Storage		Z	Modem reset

Modem		E	Echo command mode characters
Response	Q	Result codes enable/disable
Commands	&Q	Result codes (Multi- Tech or 
			standard)
		V	Result codes (verbose/terse)
		X	Result codes and call progress
		&A	Answerback caller ID

Phone Line	M	Modem speaker control
Conditioning	#T	Trellis coded modulation
		#F	Fallback modes when on- line
		Y	Long space disconnect
		&G	Guard tones
		&P	Set pulse dial ratios
		B	Answer tone
		#A	Auto speed detection

RS-232c		&C	Carrier Detect control
Interface	&D	Data Terminal Ready control
Controls	&R	Clear to Send control
		&S	Data Set Ready control
		&RF	CTS/RTS interaction control
		&SF	DSR/CD interaction control

Error		&E0	Non-Error Correction mode
Correction	&E1	Auto-Reliable mode 
and Data	&E2	Reliable mode 
Compression	&E14	Data compression disabled 
		&E15	Data compression enabled
		#L	V.42 error correction modes
		$A	Auto-Reliable buffering
		$F	Auto-Reliable fallback character 
			enable/disable
		$R	Retransmit count
		$E	V.42 error correction at 300 bps

Speed 		$BA	Baud adjust on/off
Conversion	$MB	Modem baud rate
		$SB	Serial port baud rate

Immediate	$H	Help screens
Action		I	Inquire product code
Commands	L5	List current operating parameters
		L6	List S-Register values
		L8	List on-line diagnostics
		A/	Repeat last command

Flow Control	&BS	Maximum Reliable block size
		$EB	Asynchronous word length 
			(10/11-bit)
		&E3	Flow control disabled
		&E4	Hardware flow control
		&E5	Xon/Xoff flow control
		&E6	Xon/Xoff no pass-thru
		&E7	Xon/Xoff pass-through
		&E8	Hewlett Packard ENQ/ACK pacing off
		&E9	Hewlett Packard ENQ/ACK pacing on
		&E10	Non-Error Correction mode flow 
			control off
		&E11	Non-Error Correction mode flow 
			control on
		&E12	Pacing off
		&E13	Pacing on
		#X	Number of Xoff characters sent

Escape	      +++AT<CR> Default in-band escape code
Sequences <BREAK>AT<CR> Alternate out-of-band escape code
		%E	Escape sequence options
		A	Force answer mode
		O	Go back on-line

Diagnostics	&T	Respond to remote digital loopback 
			signal
		U	Loopback test modes
------------------------------------------------------------


DIALING COMMANDS

Dialing commands are used for dialing and hanging up.


Dial			Ds
s = phone #
Default: none

The letter D in a command causes the ZDX to dial the tele-
phone number immediately following it. For example, if you 
type ATD5551212<CR>, the ZDX will dial the number 555-1212.

The ZDX supports several dialing methods. With the ATD com-
mand, you can specify either pulse (ATDP) or tone (ATDT) di-
aling. You can also modify the dialing command with several 
characters which are explained in "Dial Modifier Commands."

The ZDX lets you select either smart (wait-for-dial-tone) 
dialing or blind dialing. With smart dialing, the modem 
waits for and detects dial tones and busy signals. With 
blind dialing, the modem works with timed pauses (determined 
by the value of S-Register S6), not dial tone and busy sig-
nal detection. Refer to the X command for smart/blind dial-
ing information. 

Another method of dialing, called DTR dialing, causes the 
modem to automatically dial a number stored in modem memory. 
Refer to the $D command for DTR dialing information.


Continuous Redial	A:

If you select smart dialing through the X3 or X4 command, 
the A: command lets you continuously redial a busy number 
until your call is answered (a maximum of 10 redials for DOC 
units).

Use this command only if you have reached a busy number af-
ter executing a normal dial command. Type A: (you need not 
type AT, nor do you need to hit RETURN), and the modem redi-
als the number. If you again reach a busy signal, the modem 
continues to redial until it gets through. You can stop the 
ZDX from redialing by pressing any key.

To save a step when redialing a phone number, enter a colon 
(:) at the end of the phone number. The result is the same 
as if you had entered A:


DTR Dialing		$Dn
n = 0 or 1
Default: $D0

$D is an alternate way to cause the modem to automatically 
dial a telephone number. When DTR dialing is enabled, the 
modem dials the number stored in memory location N0 when it 
receives a high DTR signal from the computer on pin 20 of 
the RS-232C/V.24 interface. The signal must remain high for 
the duration of the call, or the connection is dropped. DTR 
dialing is most useful in synchronous dial-up environments.

$D0 disables DTR dialing (factory default).
$D1 enables DTR dialing.


On-Hook/Off-Hook 	Hn
n = 0 or 1
Default: none

You can make the ZDX go on-hook (hang up) or go off-hook 
with the H command. 

H0 (or ATH) hangs up the modem
H1 brings the line off-hook, just as if you had picked up 
the telephone handset. 

It is not necessary to use the H1 command to bring the line 
off-hook when using the D command. The modem automatically 
goes off-hook when you hit RETURN at the end of the dial 
command.


DIAL MODIFIER COMMANDS

The dial string can include the digits 0 through 9, the 
pound sign (#), the asterisk (*), and the letters A, B, C, 
or D. The latter are used by some PBXs; the exact function 
will depend on the PBX manufacturer's feature set and imple-
mentation. There are also several command characters, called 
"dial modifiers," that can be included within a dialing com-
mand after the letter D. Their functions include pulse or 
tone dialing, pauses in the dial sequence, automatic redials 
if a number is busy, and reverting to command mode or 
switching to answer mode after dialing.


Pulse or Tone Dialing	P, T
Default: P

The ZDX can dial numbers by using either pulse or tone dial-
ing, or a combination of both methods. Pulse dialing, used 
by rotary-dial telephones, uses the timed opening and clos-
ing of a relay to encode the numbers. Tone dialing, used by 
push-button (touch tone) telephones, uses DTMF, or Dual Tone 
Multi-Frequency, dialing.

P selects pulse dialing.

T selects tone dialing.

Insert the P or D in the dialing command just before the 
digits you want to have dialed in that manner.

For example, to pulse-dial the number 555-1212, type 
ATDP5551212 and hit RETURN. To tone-dial the same number, 
enter ATDT5551212 and hit RETURN. If neither pulse nor tone 
dialing is specified in the dial command, the ZDX uses what-
ever method was used last.

If the modem has been reset or has just powered up, it uses 
pulse dialing, even if you do not include the P in your dial 
command.

As an example of combining pulse and tone dialing, assume 
you are calling out of a PBX (switchboard) system where a 
"9" has to be pulse-dialed first, then the rest of the num-
ber tone-dialed after pausing for a second dial tone. This 
example is dialed by typing ATDP9,T5551212 and hitting 
RETURN. (The comma causes a pause.)


Wait for New Dial Tone		W

A W inserted in the dialing command causes the ZDX to wait 
for another dial tone before resuming dialing.

It is not necessary to enter a W at the beginning of the 
dialing command, because the modem pauses automatically.

Because the ZDX must be able to detect the dial tone for 
this command to work, you also must select wait-for-dial-
tone dialing with the X2 or X4 command.


Reverse Originate/Answer Mode	Rn
n = 0 or 1
Default: R0

The R command lets you reverse the modem's mode of operation 
from originate to answer, or from answer to originate; for 
example, if you need the modem to answer the phone but then 
go into originate mode.

R (with no number) at the end of the dialing string reverses 
in originate mode.

R0 disables mode reversing. 

R1 enables mode reversing.


Dialing Pause		,

You can make the ZDX pause while dialing by entering a comma 
in the dialing string where the pause is desired. This pause 
will last two seconds (ZDX units) or four seconds (ZDXK and 
ZDXI units) for each comma entered. Longer pauses can be 
forced by entering multiple commas, or you can change the 
length of the pause caused by a comma by setting S-Register 
S8 to any value from 0 to 255 seconds (ZDX units), 4 to 7 
seconds (ZDXK units) or 4 to 255 seconds (ZDXI units).

Each comma in a dialing command counts as one of the sixty 
allowed characters.


Continuous Redial	:

A colon (:) entered as the last character of a dialing com-
mand causes the ZDX to continuously redial the number, if it 
detects a busy signal, until the call is answered. ZDX DOC 
models are limited to 10 redials. Smart dialing (X3 or X4) 
must be selected for this to work.


Return to Command Mode After Dialing	;

A semicolon (;) entered as the last character of a dialing 
command causes the ZDX to return to command mode immediately 
after executing the command, instead of waiting for a car-
rier signal and going on line. 

For example, enter ATDT5551212; to tone-dial the number and 
immediately go back into command mode. This is useful in di-
aling applications where modem data transfer is not desired, 
such as voice communications, or in applications using touch 
tones as a data entry method, such as bank-by-phone.


Flash On-Hook		!

Some switchboard systems react to a momentary on-hook. An 
exclamation mark inserted in the dialing command causes the 
modem to "flash" on-hook for half a second, just as if you 
had pressed the switch hook on a telephone set for half a 
second. (For the ZDXK models, the exclamation mark causes 
the modem to flash on-hook for 90 milliseconds.)

For example, to flash on-hook to transfer to extension 5678 
after dialing the number 555-1234, type ATDT5551234,,!5678. 
The commas cause a 4-second pause.


Quiet Answer		@

The @ command is used for accessing a system that does not 
provide a dial tone. The @ command causes the ZDX to wait 
before processing the next character in the dialing string. 
The wait is for one or more rings back followed by 5 seconds 
of silence.

For example, ATDT5551212@6313550 causes the ZDX to dial the 
first number (555-1212) and wait for the time specified in 
S-Register S7 for at least one ringback and 5 seconds of 
silence. If a busy signal is detected, the ZDX hangs up and 
generates a BUSY result code. If it does not detect 5 sec-
onds of silence, a NO ANSWER result code is generated after 
hanging up. If 5 seconds of silence is detected, the second 
number (631-3550) is then dialed.


PHONE NUMBER MEMORY COMMANDS

The ZDX can store up to two telephone numbers in nonvolatile 
memory. You can store the numbers with the D...N... command, 
dial them with the N command, link them so that one will be 
automatically dialed after the other with the N...N... com-
mand, or list them with the L command.


Store Phone Number	DsNd
s = phone #  d = 0 or 1
Default: none

A telephone number and command string of up to 60 characters 
can be stored in the ZDX's phone number memory. You can 
store two of these 60-character strings using the D...N... 
command. The memory locations are labeled N0 and N1.

Store a command line by typing it as you would any other 
command, appending the appropriate Nx memory location.

For example, the tone-dialed number 1-612-555-1212 is stored 
at memory location N0 by entering ATDT16125551212N0 and 
RETURN. The number is not dialed with this store command.

After storing a number, check to see that it has been stored 
correctly by typing ATL and hitting RETURN.

When phone numbers are stored, the entire command is also 
stored, so you can effectively create a macro for each num-
ber. For example, if you know a particular number needs to 
have extended result codes, detect busy or dial tone, error 
correction, Xon/Xoff flow control, pacing, and data compres-
sion enabled, the command string is: 
ATX4&E1&E5&E13&E15DT16125551212N0. This stores the entire 
command string at location N0.


Dial a Stored Number	Nd
d = 0 or 1
Default: none

A telephone number that is stored in the ZDX's number memory 
may be automatically dialed by entering ATNn where n = 0 or 
1. For example, a number stored at N0 is dialed by entering 
ATN0 and RETURN. 

Warning: Do not include the letter D in this command, or the 
stored number will be erased.


Number Linking		NdNe
d = 0 or 1  e = 1 or 0
Default: none

Number linking establishes an alternate phone number for the 
ZDX to dial using the stored phone number memory locations 
if the first number is busy. (See the DsNd command.) This is 
useful when a computer can be accessed through more than one 
phone number.

To link the number in memory location N0 to the number in 
N1, enter ATN0N1 and hit RETURN. You can cycle back and 
forth between the two numbers by linking them several times 
in one command: ATN0N1N0N1N0N1<CR>.

The only limit on the number of strings you can link is the 
60-character maximum allowed in a command line. Number link-
ing cannot be used with blind dialing, since busy signals 
would not be detected.


List Stored Telephone Numbers	L

Use the L command to display telephone numbers (or full 
dialing commands) stored in the ZDX's nonvolatile memory. 
Typing ATL<CR> displays the stored N numbers in the follow-
ing format:

	0  T14082345678

	1  P9,T14089876543

All digits and command letters are displayed. The number's 
"name" is shown first, followed by the complete dialing com-
mand and telephone number as originally entered.


CONFIGURATION AND DEFAULT STORAGE COMMANDS

The ZDX has two places where parameters can be stored: fac-
tory default read-only memory (ROM) and non-volatile random 
access memory (NVRAM). You cannot change the default values 
in ROM. However, you can alter them and then store them in 
NVRAM as customized settings. You can then treat the custom-
ized settings as if they were default settings.


Store Configuration	&Wn
n = 0 or 1
Default: &W1

The &W command stores AT command settings and S-Register 
values in memory, so you won't lose your settings if you 
power down the ZDX, or reset it with the Z command. 

&W0 (or &W)causes the ZDX to store your current AT command 
settings and S-Register values in its nonvolatile random 
access memory (NVRAM). &W0 also sets the modem so that it 
reads your customized settings stored in NVRAM when powered 
down or reset. (The &W0 command only changes the settings 
stored in NVRAM that you specifically intend to alter. All 
other default parameters are applicable.)

&W1 causes the ZDX not to store your customized settings to 
NVRAM so that, after powering down or resetting the modem, 
it reads the factory default settings stored in ROM.


Load Default Configuration	&Fn
n = 0, 8, or 9
Default: &F8

Your modem stores the factory default AT command settings 
and S-Register values in read only memory (ROM); it stores 
your customized AT command settings and S-Register values in 
its nonvolatile random access memory (NVRAM). 

The &F command lets you choose between your customized set-
tings in NVRAM and the factory defaults in ROM. The &F8 and 
&F9 commands define the function of the &F command. 

When the ZDX is set with the default &F8 command and an &F 
command is issued, the modem always reads the factory de-
fault settings, regardless of what is stored in NVRAM via 
the &W0 command. 

When &F9 is set and an &F command is issued, the &W setting 
defines whether the ZDX reads settings stored in NVRAM or 
ROM. If the modem is set with &W0, it reads your customized 
settings stored in NVRAM. If the modem is set with &W1, it 
reads the factory default settings stored in ROM. 

Once you issue the &F9 command, it is in effect until you 
issue &F8, and vice-versa.

Following is a summary of how the &F and &W command settings 
interact:


 Command   Previously	
issued to   selected	Where your modem reads
  modem	    settings 	configuration parameters
------------------------------------------------------------
  &F	    &F8, &W0	Factory default settings in ROM
  &F	    &F8, &W1	Factory default settings in ROM
  &F	    &F9, &W0	Customized settings in NVRAM
  &F	    &F9, &W1	Factory default settings in ROM
------------------------------------------------------------


Modem Reset		Z

The Z command resets all configurations to their default 
values and clears the entire command mode buffer. The result 
is the same as disconnecting and then reconnecting power to 
the modem. When ATZ is typed, the state of the &W command 
determines where the default values originate. &W0 defaults 
come from NVRAM and &W1 defaults come from ROM. Because ATZ 
clears the command buffer, it must be on a line by itself; 
it cannot be part of a longer string.


MODEM RESPONSE (RESULT CODE) COMMANDS

The ZDX can give responses to commands. The most common one 
is OK, but the modem also can alert you or your software to 
dial tones, busy signals, connection speeds, and whether the 
connection is made with error correction or compression en-
abled. These responses are called result codes, and they can 
be terse or verbose, Multi-Tech or standard, basic or 
extended.


Echo Command Mode Characters	En
n = 0 or 1
Default: E1

If the ZDX is connected to a full-duplex computer, you may 
need to configure the modem to echo back characters entered 
while in the command mode in order for them to be displayed. 
Use the E command to configure the command mode echo. 

E0 disables the echo.

E1 enables the echo (default).


Result Codes Enable/Disable	Qn
n = 0, 1, or 2
Default: Q0 

Use the Q command to enable or disable result codes for ap-
plications such as computer-controlled auto-dialing. You can 
also use it to disable responses and echo in answer mode 
while leaving originate mode still intelligent; this is 
called the no response answer mode of operation.

Q0 (or Q) enables result codes (factory default).

Q1 disables result codes for applications such as computer-
controlled auto-dialing.

Q2 enables the no response answer mode, which leaves origi-
nate mode intelligent while turning off answer mode re-
sponses and echo.


Result Codes (Multi-Tech or Standard AT) 	&Qn
n = 0 or 1
Default: &Q0

The ZDX gives you a choice between the Multi-Tech result 
codes and result codes that more closely match the standard 
AT command set responses.

&Q0 selects Multi-Tech responses with Reliable/Compressed 
modifiers. With &Q0, the verbose result code for 14,400 bps 
is CONNECT 14400 LAPM COMPRESSED.

&Q1 selects AT responses with no Reliable/Compressed modifi-
ers. With &Q1, the verbose result code for 14,400 bps is 
CONNECT 14400.


Result Codes (Verbose/Terse)	Vn
n = 0 or 1
Default: V1

The V command controls whether the ZDX's result codes dis-
play as word ("verbose") or single digit ("terse") messages. 
For example, if after dialing, no carrier signal is de-
tected, the result can be displayed either as NO CARRIER, or 
as the digit 3.

V0 (or V) displays the ZDX's result codes as digits. 

V1 displays result codes as words (default).


Result Codes and Call Progress Selection 	Xn
n = 0-4
Default: X0

The X command is used to select the result codes the modem 
provides in the command mode and whether the ZDX uses "smart 
dialing" or "blind dialing". When it smart dials, the ZDX 
listens for dial tones and busy signals, and responds to 
them. When it blind dials, the ZDX ignores the signals and 
relies instead on timed pulses.

X0 causes the ZDX to blind dial. Instead of looking for a 
dial tone, it pauses for the time set in S-Register S6 and 
then dials regardless. Once a connection has been made, it 
sends the Bell 103 basic code CONNECT to the terminal. It 
ignores any busy signals. X0 is the factory default.

X1 causes the ZDX to blind dial, but in addition to the ba-
sic CONNECT code it provides extended codes consisting of 
the word CONNECT and the speed of the connection: CONNECT 
1200, CONNECT 2400, CONNECT 4800, CONNECT 9600, CONNECT 
14400, and CONNECT 19200. In this mode, the ZDX does not 
recognize or respond to dial tones or busy signals.

X2 causes the ZDX to wait for a dial tone before dialing. If 
it does not detect a dial tone within the time set by S6, 
the ZDX sends a NO DIALTONE result code to the terminal. In 
this mode, the ZDX provides extended result codes, but does 
not respond to busy signals.

X3 causes the ZDX to blind dial, but also it looks for a 
busy signal, and if it detects one, it sends a BUSY result 
code to the terminal. In this mode, the ZDX provides ex-
tended result codes, but it does not respond to dial tones.

X4 causes the ZDX to look for a dial tone and a busy signal, 
and respond with NO DIALTONE or BUSY, as appropriate. It 
also provides extended result codes.

The default setting of X0 selects basic result codes and 
blind dialing, but X4 is most useful for most datacomm 
programs.


Answerback Caller ID	&An
n = 0 or 1
Default: &A0

The &A command controls the ZDX's answerback feature. An-
swerbacks are used in some on-line realty applications, and 
elsewhere, as a security measure. The &A command enables the 
ZDX to return a message to an identification request from 
another computer. The default identification is "MESSAGE NOT 
PROGRAMMED."

&A0 disables the answerback feature. It is the factory 
default.

&A1 enables the modem to return a message to an inquiry.


PHONE LINE CONDITIONING COMMANDS

These commands define the conditions of the on-line 
connection. 


Modem Speaker Control	Mn
n = 0-3
Default: M1

To allow you to monitor the phone line, the ZDX has an in-
ternal speaker, which functions like the speaker in a tele-
phone handset. The M command controls if and when the 
speaker is operational.

M0 disables the speaker completely. 

M1 causes the speaker to be on only until a carrier signal 
is detected (factory default). 

M2 causes the speaker to remain on at all times. 

M3 causes the speaker to be on during dialing, and off 
during handshake.


Trellis Coded Modulation 	#Tn
n = 0 or 1
Default: #T1

The #T command enables or disables Trellis Coded Modulation 
(TCM) on the ZDX. There is usually no need to disable TCM, 
except under an unusual line condition called impulse noise. 

#T0 disables TCM.

#T1 enables TCM (factory default).


Fallback Modes When On-Line	#Fn
n = 0, 1, or 2
Default: #F2

If line conditions deteriorate, the ZDX has the ability to 
"fall back" from its maximum speed in V.34/V.32 
terbo/V.32bis/V.32 operation. The #F command controls 
whether the ZDX will fall back to a lower speed because of 
an unacceptable error rate. If the error rate becomes too 
great, the modem performs a retrain at the next lower speed. 
If, after the retrain, the error rate is still too high for 
operation at maximum speed, the modem performs a retrain at 
the next lower speed, down to 4800 bps.

If the modem returns on line at the maximum speed after the 
first retrain, the modem starts start a counter and a timer. 
If three retrains occur within a two minute period, the mo-
dem will fall back to 4800 bps.

#F0 (or #F) disables fallback when on-line. 

#F1 lets the ZDX fall back (based on the error rate, or if 
three retrains occur within two minutes) from 28.8K to 24.6K 
to 21.6K to 19.2K to 14.4K to 12K to 9.6K to 4.8K bps. 

#F2 enables fallback from 28.8K to 24.6K to 21.6K to 19.2K 
to 14.4K to 12K to 9.6K to 4.8K, but also enables fall for-
ward from 4.8K to 9.6K to 12K to 14.4K to 19.2K to 21.6K to 
24.6K to 28.8K if the phone line improves (factory default).

NOTE: The above speed ranges are for the MT2834ZDX; other 
ZDX models have lower maximum speeds.


Long Space Disconnect	Yn
n = 0 or 1
Default: Y0

When two modems are connected in Non-Error Correction mode, 
there is no "polite" way to request a disconnect. A link 
disconnect request packet is sent under reliable connec-
tions. As a result, some "garbage" may be received when a 
hang-up command is issued.

Y0 disables the modem's use of the break signal (factory 
default). 

Y1 enables the modem's use of the break signal, which shuts 
off the modem's receiver and disconnects (both modems must 
have Y1 enabled).


Guard Tones		&Gn
n = 0, 1, or 2
Default: &G0 (ZDX and ZDXI) or &G2 (ZDXK)

The &G command is used to control the presence or absence of 
guard tones from the transmitter when in answer mode at ei-
ther 1200 or 2400 bps. Guard tones are used in Europe and 
other areas to allow the modem to function in the telephone 
systems. Guard tones are not used in the United States. The 
UK model (ZDXK) is locked at &G2 (1800 Hz guard tone).

&G0 disables ITU guard tones (ZDX and ZDXI factory default).

&G1 enables ITU 550 Hz guard tone.

&G2 enables ITU 1800 Hz guard tone (ZDXK factory default).


Set Pulse Dial Ratios	&Pn
n = 0 or 1
Default: &P0 (ZDX and ZDXI) or &P1 (ZDXK)

&P sets the time ratios between the open and closed posi-
tions of the pulse-dialing frequencies. The UK model (ZDXK) 
is locked to &P1 (67-33 ratio).

&P0 sets a pulse dial ratio of 60 ms to 40 ms. (ZDX and ZDXI 
factory default.)

&P1 sets a pulse dial ratio of 67 ms to 33 ms. (ZDXK factory 
default.)

Refer to Appendix K for more dial pulse information. 


Answer Tone		Bn
n = 0 or 1
Default: B0

The B command selects the frequency that the modem uses for 
its answer tone. (The answer tone is the tone transmitted by 
a modem receiving a call to the modem that called it, which 
initiates the handshaking between the two modems.) At higher 
speeds (2400 to 28,800 bps) there is no conflict, because 
all use the Bell frequency of 2225 Hz. However, the ITU 
specification for V.22 has an answer tone frequency of 2100 
Hz. This command applies to International models only. 

B0 enables ITU frequencies including V.21 (300 bps) and V.23 
(1200/75 bps). This is the factory default setting.

B1 enables Bell frequencies including Bell 103 (300 bps). 


Auto Speed Detection	#An
n = 0-3
Default: #A0

The ZDX can operate as a 28,800 bps ITU V.34 modem, a 19,200 
bps AT&T V.32 terbo modem, a 14,400 bps ITU V.32bis modem, a 
9600/4800 bps ITU V.32 modem, a 2400 bps ITU V.22bis/Bell 
2400 modem, a 1200 bps ITU V.22/Bell 212A modem, or a 300 
bps Bell 103/113 modem. The #A command is used to detect and 
select the operational data rate ("starting" speed) that the 
ZDX uses for initial handshake and speed selection. The #A 
command does not control the originating bps rate of the mo-
dem (that is done by the modem baud rate command $MB), but 
only answer mode starting speed.

#A0 (or #A) causes the modem to start to operate at maximum 
speed, with fallback to 14,400 to 12,000 to 9600 to 4800 to 
2400 to 1200 to 300 bps. #A0 is the factory default setting.

#A1 causes the modem to operate at maximum speed only. 

#A2 causes the modem to start at maximum speed, with incre-
mental fallback to 4800 bps. 

#A3 causes the modem to start at 2400 bps in V.22bis mode, 
with fallback to 1200 to 300 bps. 


RS-232C INTERFACE CONTROL COMMANDS

These commands define how the ZDX will use and respond to 
the standard RS-232C signals.


Carrier Detect Control	&Cn 
n = 0, 1, 2, or 4
Default: &C1

The &C command lets you control the status of the Carrier 
Detect signal (CD -- Pin 8) on the RS-232C interface. Nor-
mally, CD goes high when the modem detects a carrier on the 
communications link and drops when it loses the carrier. You 
can control CD in four ways. You can force the signal high, 
allow it to act normally, or set it to stay high until the 
modem disconnects, go low momentarily, and then go high 
again. (This option is useful with some CBX phone systems 
and mainframe front ends, which require CD to act in this 
manner.) You can also cause the modem to reset when the CD 
signal drops. 

&C1 allows CD to act normally. This is the factory default 
setting. 

&C0 forces CD on. 

&C2 causes CD to drop for one second on disconnect and then 
come up again. (If you want to set the drop time to some-
thing other than one second, use S-Register S24.)

&C4 resets the modem to its current parameters when CD 
drops.


Data Terminal Ready Control	&Dn
n = 0, 1, 2, or 3
Default: &D2

Data Terminal Ready (DTR) on pin 20 of the RS-232C interface 
is required in order for the ZDX to operate. A high DTR sig-
nal tells the modem that the device to which it is connected 
is active, or "ready" to communicate through the modem.

DTR can also be used to trigger a dialing sequence, called 
DTR dialing. The condition of DTR can also be used to cause 
the modem to reset to its default parameters, just as if you 
had given the modem an ATZ command. 

&D0 (or &D) causes the modem to ignore DTR. 

&D1 causes the modem to enter command mode when DTR drops 
from on to off. Auto-answer is disabled while DTR is low. 

&D2 causes the modem to go on-hook and enter command mode 
when DTR goes from on to off. This is the factory default.

&D3 causes the modem to reset itself, as if you had given it 
an ATZ command, whenever DTR drops from on to off. It will 
also go on-hook if it is on-line.


Clear to Send Control	&Rn
n = 0, 1, or 2
Default: &R1

The &R command lets you control the status of the Clear to 
Send signal (CTS -- Pin 5) on the RS-232C interface. Nor-
mally, CTS follows RTS when the ZDX is on-line. You have 
three choices. You can force the signal high, allow it to 
act normally, or set it to stay high until the modem discon-
nects, go low momentarily, and then go high again. The last 
option is useful with some CBX phone systems and mainframe 
front ends, which require CTS to act in this manner. 

&R0 allows CTS to act normally, that is, to follow RTS. 

&R1 forces CTS on (the factory default setting). When the 
modem goes on-line, CTS still provides flow control.

&R2 sets up CTS so that it drops for one second on discon-
nect and then comes up again. (If you want the drop time to 
be something other than one second, use S-Register S24 to 
change this value.


Data Set Ready Control	&Sn
n = 0, 1, or 2
Default: &S1

The &S command lets you control the status of the Data Set 
Ready signal (DSR -- Pin 6) on the RS-232C interface. Nor-
mally, DSR follows CD. You have three choices. You can force 
the signal high, allow it to act normally, or set it to stay 
high until the modem disconnects, go low momentarily, and 
then go high again. The last option is useful with some CBX 
phone systems and mainframe front ends, which require DSR to 
act in this manner.

&S0 forces DSR on. 

&S1 allows DSR to act normally, that is, to follow CD. This 
is the factory default setting. 

&S2 sets up DSR so that it drops for one second on discon-
nect and then comes up again,. (If you want to set the drop 
time to something other than one second, use S-Register S24. 


CTS/RTS Interaction Control	&RFn
n = 0 or 1
Default: &RF1

In normal operation, Clear to Send (CTS) follows Request to 
Send (RTS) when the modem is on-line. In other words, if RTS 
goes off, CTS goes off. In some applications, however, it is 
necessary for CTS to operate independent of RTS.

&RF0 enables CTS to follow RTS. 

&RF1 enables CTS to operate independent of the state of RTS. 
If this is the case, refer to the &R command for control of 
CTS. This is the factory default.


DSR/CD Interaction Control	&SFn
n = 0 or 1
Default: &SF0

Normally Data Set Ready (DSR) follows Carrier Detect (CD). 

&SF0 enables DSR to follow CD (factory default). 

&SF1 enables DSR to operate independent of CD. If this is 
the case, refer to the &S command for control of DSR.


ERROR CORRECTION AND DATA COMPRESSION COMMANDS

You can configure your ZDX in one of three different V.42 
modes of operation (each mode can be with or without com-
pression). These are the Non-Error Correction, Auto-Reliable 
and Reliable modes. 


Non-Error Correction Mode	&E0

In Non-Error Correction mode, the ZDX's V.42 error correc-
tion capabilities are disabled, and the modem functions as a 
Non-Error Correction modem. 


Auto-Reliable Mode		&E1 
Default

In Auto-Reliable mode, during the handshaking procedures at 
the start of the on-line connection, the ZDX automatically 
determines whether the modem with which it is communicating 
is using V.42 error correction. If the ZDX determines that 
the other modem is using V.42, it switches itself into 
Reliable (V.42) mode and enables error correction. If it 
determines that the other modem is not using V.42, the ZDX 
remains in Non-Error Correction mode.

The ZDX uses the "Link Request" to determine if the modem it 
calls is using V.42 error correction.

When the ZDX is in Auto-Reliable mode and originates a call, 
it goes through the same handshaking procedures as any dial-
up modem. After establishing the on-line connection, the ZDX 
transmits a "Link Request" message to the answering modem. 
If the answering modem replies with an appropriate V.42 ac-
knowledgment, the ZDX switches to the Reliable mode. Other-
wise it will stay in Non-Error Correction mode. This V.42 
Link Request handshaking generally takes about five seconds.

When operating in V.42 Reliable (error correcting) mode, the 
ZDX uses its memory, or buffer, to store data as it is re-
ceived. During periods of error-caused retransmissions or 
compression slowdowns, this buffer may fill up. To prevent 
buffer overflow and subsequent loss of data, the modem uses 
flow control to signal the computer attached to its RS-232C 
port that the modem buffer is almost full. This causes the 
computer to pause in its data transmission until the modem 
is able to empty its buffer sufficiently to accept more 
data, at which time the modem signals the computer that it 
may resume transmission. 


Reliable Mode		&E2

In Reliable mode, the ZDX uses its V.42 error correction ca-
pabilities for all transmissions. In Reliable mode, the ZDX 
must be connected to a modem with a similar protocol (MNP or 
LAP-M).

The V.42 standard includes MNP Class 3 & 4 and LAP-M error 
correction methods. The V.42 mode select command (#L) se-
lects which type of error correction your ZDX uses for 
transmissions.


Data Compression 	&En
n = 14 or 15
Default: &E15

Data compression is normally enabled, providing &E1 (Auto-
Reliable mode) or &E2 (Reliable mode) has also been se-
lected. However, you may wish to turn it off if you are 
transferring files previously compressed by a software com-
pression utility, or if your UART is incapable of the high 
serial port speeds required by V.42bis data compression. 

&E14 disables data compression.

&E15 enables data compression (factory default).


V.42 Error Correction Modes	#Ln 
n = 0-3
Default: #L0

The #L command selects the V.42 error correction method (MNP 
or LAP-M) the modem will use when originating a call. In an-
swer mode, the modem ignores the #L command and accepts 
whichever V.42 mode, MNP or LAP-M, the originating modem 
offers.

#L0 causes the modem to negotiate the V.42 error correction 
method with the answering modem, and to begin negotiation 
with LAP-M. If both modems have LAP-M capability, the modems 
will use LAP-M mode. If one or both modems do not have LAP-M 
capability and both have MNP, the modems will use MNP 
(factory default).

#L1 enables MNP error correction and disables LAP-M.

#L2 enables LAP-M error correction and disables MNP.

#L3 enables LAP-M error correction and disables MNP, but 
skips the V.42 detection phase used by the other commands. 
Both modems must have #L3 in effect.


Auto-Reliable Buffering		$An 
n = 0 or 1
Default: $A0

In Auto-Reliable mode, the modem has four seconds to estab-
lish a Reliable connection. After this four seconds, the mo-
dem drops to Non-Error Correction mode. Any data received 
during this period will normally be discarded. The $A com-
mand causes the modem to buffer (save) data that is received 
during the Auto-Reliable time-out period. This data is then 
output by the modem after the CONNECT message.

$A0 discards the data received during the Auto-Reliable 
time-out period. This is the factory default.

$A1 buffers data received during the Auto-Reliable time-out 
period. 


Auto-Reliable Fallback Character Enable/Disable		$Fn
n = 0 or 1
Default: $F1

In Auto-Reliable mode, the modem is given four seconds to 
establish a Reliable connection. If a single carriage return 
is received from the remote modem during this four second 
period, the Auto-Reliable modem assumes that the remote mo-
dem is not in Reliable mode and drops to Non-Error Correc-
tion mode. The carriage return is the only character that 
causes the modem to drop to Non-Error Correction mode. Any 
other data is either buffered or discarded.

The $F command is used to disable this fallback-due-to-
carriage-return feature.

The Auto-Reliable fallback character ($F) and Auto-Reliable 
buffering ($A) commands can be used together to cause the 
modem to buffer all data received up until the carriage re-
turn, and then drop to Non-Error Correction mode. All data 
received is then output following the CONNECT message.

$F0 causes the modem not to fall back to Non-Error 
Correction connect if a carriage return is received.

$F1 causes the modem to fall back to Non-Error Correction 
connect if a carriage return is received. This is the 
factory default.


Retransmit Count 	$Rn 
n = 0 or 1
Default: $R0

If errors are received during a Reliable connection, the mo-
dem re-sends the block of data that contained an error. If 
another error occurs, it re-sends the block again. The modem 
counts the number of times that a data block is re-sent. 

$R0 causes the modem to disconnect (hang up) if the same 
block of data is re-sent 12 times and still has not been 
received properly. (After 12 retransmit attempts, the modem 
assumes that the line is unsuitable for transmission and 
aborts the connection.) This is the factory default.

$R1 causes the modem to disable the retransmit counter. When 
the retransmit counter is disabled, the modem keeps trying 
to send data and will not abort, no matter how many times 
the same block is resent. Type AT$R0 to disconnect the modem 
if all attempts to re-send the block fail.


V.42 Error Correction at 300 bps 	$En
n = 0 or 1
Default: $E0

At 300 bps, error correction is not normally used.

$E0 disables 300 bps/V.42 error correction altogether. This 
is the factory default.

$E1 enables the ZDX to function at 300 bps in either Non-
Error Correction (&E0), Auto-Reliable (&E1) or Reliable 
(&E2) mode.


SPEED CONVERSION COMMANDS

Speed conversion is a necessary part of data compression, 
since for data compression to be effective, data must be 
sent from the computer to the modem faster than the latter's 
transmission speed.

Speed conversion allows the ZDX to communicate at one speed 
over the phone line and at another speed at the RS-232C in-
terface. The baud rate at the RS232 interface can be fixed 
independently of the baud rate of the on-line transmissions. 
Besides data compression, another application for speed con-
version involves an auto-answer ZDX connected to a computer 
that does not have autobaud capability. This means that the 
computer must be set at a fixed baud rate, regardless of 
whether the modem communicates over the phone line at 300, 
1200, 2400, 9600, 14,400, 19,200, or 28,800 bps. In this ap-
plication, speed conversion allows the modem to match its 
speed to that of a calling modem, while at the same time 
communicating with the attached computer through its RS-232C 
port at a fixed baud rate. This fixed baud rate can be pre-
selected at 300, 1200, 2400, 4800, 9600, 19,200, 38,400, 
57,600, or 115,200 bps.


Baud Adjust		$BAn
n = 0 or 1
Default: $BA0

When using speed conversion, you must set the modem to not 
adjust its speed at the RS-232C serial port, even if the mo-
dem does adjust its transmission speed.

$BA0 turns baud adjust off and speed conversion on. Even 
though the term "baud adjust" sounds like it should mean the 
same thing as "speed conversion," it actually means the op-
posite. Turning baud adjust off turns speed conversion on; 
turning baud adjust on turns speed conversion off. 

$BA1 turns baud adjust on and speed conversion off. The mo-
dem will now match its RS-232C speed to that of the com-
puter, and will adjust its speed to any changes in the com-
puter's speed in originate mode, or to the speed of the 
originating modem when it's in answer mode. With baud adjust 
on, the speed at which the modem communicates over the phone 
line is always the same as the speed at which it communi-
cates via its RS-232C serial port.


Modem Baud Rate		$MBn
n = speed
Default: $MB14400, $MB19200, or $MB28800

The $MB command presets the ZDX's transmission (baud) rate 
for originate mode, (i.e., the speed of the modem's trans-
missions over the telephone lines when originating a call). 
With speed conversion on, this transmission speed can be 
different from the serial port speed.

When the ZDX answers a call from another modem, it automati-
cally switches its phone line transmission (baud) rate to 
match the calling modem. However, if the ZDX originates a 
call to another modem that is unable to connect at the baud 
rate of the ZDX, the ZDX will automatically drop to a lower 
baud rate in an attempt to match the speed of that modem.

For example, if the ZDX is set for 14,400 bps and calls 
another modem that has a top speed of 2400 bps, it will drop 
to 2400 bps.

The $MBn command sets the modem baud rate, where n can be 75 
(in ITU V.23 mode) 300, 1200, 2400, 4800, 7200, 9600, 12000, 
14400, 16800, 19200, or 28800 as listed below:

$MB75 = ITU V.23 mode
$MB300 = 300 bps
$MB1200 = 1200 bps
$MB2400 = 2400 bps
$MB4800 = 4800 bps
$MB7200 = 7200 bps
$MB9600 = 9600 bps
$MB12000 = 12,000 bps
$MB14400 = 14,400 bps
$MB16800 = 16,800 bps (MT1932ZDX only)
$MB19200 = 19,200 bps (MT1932ZDX only) 	
$MB28800 = 28,800 bps (MT2834ZDX only)

The factory default setting is 14,400 bps for the MT1432ZDX, 
19,200 bps for the MT1932ZDX and 28,800 for the MT2834ZDX.

If baud adjust is on ($BA1), speed conversion is off and a 
modem baud rate command will be ignored, since it is part of 
speed conversion.


Serial Port Baud Rate	$SBn
n = speed
Default: $SB57600

The #SB command presets the speed of the ZDX's serial (RS-
232C) port in both originate and answer modes. Speed conver-
sion allows you to set this serial port baud rate at a fixed 
speed of up to 115,200 bps, regardless of the modem's trans-
mission speed setting.

For this command to work, the modem's baud adjust feature 
must first be turned off with the $BA command. When baud ad-
just is on, the ZDX automatically adjusts its serial port 
baud rate to match the speed of the computer or terminal to 
which it is connected, as soon as it receives its first AT 
command. However, in applications such as automatic answer, 
the modem may not receive AT commands. In this case it is 
helpful to be able to preset the serial port baud rate with 
the $SB command.

In addition to setting the ZDX's serial port speed, this 
command sets the speed at which the modem issues command 
mode responses (result codes).

The ZDX accepts AT commands at any speed, regardless of the 
speed preset by the $SB command. If the modem receives a $SB 
command at a speed different than the preset speed, the mo-
dem switches its serial port baud rate to match the new AT 
command speed, (even though the baud rate value stored by 
the $SB command remains the same). This provides you with a 
convenient way to switch the serial port speed, while still 
making it easy to go back to the original speed automati-
cally the next time the modem is powered up or reset with an 
ATZ command.

The $SBn command sets the serial port baud rate, where n can 
be 300, 1200, 2400, 4800, 9600, 12,000, 19,200, 38,400, 
57,600, or 115,200 bps as listed below:

$SB300 = 300 bps
$SB1200 = 1200 bps
$SB2400 = 2400 bps
$SB4800 = 4800 bps
$SB9600 = 9600 bps
$SB12000 = 12,000 bps
$SB19200 = 19,200 bps
$SB38400 = 38,400 bps
$SB57600 = 57,600 bps (factory default)
$SB115200 = 115,200 bps


IMMEDIATE ACTION COMMANDS

Use these commands to get information about AT commands and 
the current settings of the modem. 

Help Screens		$Hn
n = 1, 2, or 3
Default: none

The help ($H) command gives you short explanations on how to 
use each ZDX command. The help command can be quite useful 
if your manual is not handy and you are in the middle of a 
communications session. Although the help explanations are 
shorter than those in this manual, they should prove to be 
helpful reminders when needed.

The ZDX has three screens of help information (Screen #1, #2 
and #3).

AT$H1 displays Help Screen #1. 

AT$H2 displays Help Screen #2. 

AT$H3 displays Help Screen #3.


Inquire Product Code	In
n = 0, 1, or 2
Default: none

Some systems or software packages automatically check the 
identification of the modem by using the I command. You can 
also use the I command to check the version number of the 
modem's built-in software, known as firmware.

I0 or I reveals the modem's ID number.

I1 reveals the modem's firmware version number. You can use 
this command to identify your modem's firmware level when 
calling Tech Support. 

I2 reveals the product name, such as MT1932ZDX.


List Current Operating Parameters	L5, L7
Default: none

The L5 and L7 commands cause your modem to list its current 
operating parameters. These lists are useful when you are 
changing communications software, or when you are changing 
modem default settings. All of the modem configuration com-
mand parameters are listed.

ATL5 lists the ZDX's current basic operating parameters. 

ATL7 lists additional ZDX parameters.


List Current S-Register Values	L6 
Default: none

ATL6 lists the current values stored in the modem's S-
Registers. This information can be very useful if you wish 
to change S-Register values.


List On-Line Diagnostics 	L8

The L8 command displays the current on-line CONNECT status 
of the ZDX. This display can be printed and used as a modem 
status report or as diagnostic information (such as when 
calling Tech Support). This command can be entered only when 
the modem is on-line. To activate this command, first type 
+++AT<CR> (the on-line escape command while maintaining Com-
mand mode), then type ATL8<CR>. Your modem's on-line status 
is displayed, listing current operating parameters (e.g., 
Link Type, Line Speed, Serial Speed, Line Quality, and 
Reason for Last Disconnect). 


Repeat Last Command	A/
Default: None

Enter A/ to repeat the previous command. Do not precede this 
command with AT or hit RETURN to execute it.


FLOW CONTROL

Flow control refers to the techniques used by data terminal 
equipment and the modem to pause and resume the flow of in-
formation between them. It prevents a device from accepting 
more data than it can handle. The ZDX implements flow con-
trol in both directions. When the modem halts the flow of 
data, it is called flow control, and when the computer halts 
the flow, it is called pacing.


Maximum Reliable Block Size	&BSn
n = 0 or 1
Default: &BS1

When the modem is in Reliable mode, use the &BS command to 
set the maximum size of a data block. MNP Class 3 uses a 64-
character block, while Classes 4 and 5 normally send blocks 
of 256 characters or less. Reducing the block size for MNP 
Classes 4 and 5 may improve the overall throughputs when the 
phone lines are noisy; however, it could slow down the data 
transfer if the line quality is good.

&BS0 sets the maximum transmit block size to 64 characters.

&BS1 sets the maximum transmit block size to 256 characters 
(factory default).


Asynchronous Word Length 	$EBn
n = 0 or 1
Default: $EB0

The ZDX has an 11-bit capability when operating asynchro-
nously. The $EB command selects between 11-bit and 10-bit 
operation, and it is functional in both command and on-line 
modes. 

$EB0 enables the modem to operate in 10-bit format (one 
start bit, seven data bits, one parity bit, and one stop 
bit). It automatically detects parity when an AT command is 
issued. This is the factory default. 

$EB1 enables the modem to use an 11-bit format (one start 
bit, eight data bits, one parity bit, and one stop bit).


Flow Control Disabled	&E3

The &E3 command completely disables data flow control initi-
ated by the modem.


Hardware Flow Control	&E4

The &E4 command enables the modem's use of the Clear to Send 
(CTS) signal on pin 5 of the RS-232C/V.24 interface to regu-
late the flow of data. When CTS is brought low, the data 
flow is suspended until the signal goes high again. This 
method of flow control works in conjunction with pacing 
(i.e., computer-initiated flow control), which uses the 
Request to Send (RTS) signal on pin 4 of the RS-232C/V.24 
interface. Hardware flow control cannot be enabled unless an 
active error correction protocol is selected.


Xon/Xoff Flow Control	&E5

Xon/Xoff flow control is an in-band method of data flow 
regulation used by the modem or computer (i.e., by flow con-
trol or pacing). If you issue the &E5 command to the modem, 
it will respond to Xon/Xoff pacing, and use Xon/Xoff charac-
ters as its own method of flow control to the computer.

In-band data regulation means that the Xon and Xoff charac-
ters (^Q and ^S, respectively) are inserted into the stream 
of data rather than using separate control lines. When an 
Xon character is detected, the data stream is suspended un-
til an Xoff character is detected. The drawback to using 
this method of pacing is that some files may contain these 
characters. If such a file is transferred using a modem, the 
file transfer can be suspended indefinitely.


Xon/Xoff Pass-Through	&En
n = 6 or 7
Default: &E6

When Xon/Xoff pacing is active, the local modem has two op-
tions regarding the Xon and Xoff characters. It can respond 
to and discard the characters from the computer, or it can 
respond to the characters and pass them through the datacomm 
link to the remote modem, thereby pacing the remote modem as 
well. 

&E6 causes the modem to respond to and discard the Xon and 
Xoff characters (factory default).

&E7 causes the modem to respond to the characters and pass 
them through the data link to the receiving modem.


Hewlett Packard ENQ/ACK Pacing	&En
n = 8 or 9
Default: &E8

If the ZDX is used with Hewlett Packard (or compatible) 
equipment, the modem can be configured to respond to ENQ/ACK 
pacing as a complement to any other form of active flow con-
trol or pacing. In that case, pacing from the Hewlett 
Packard equipment is processed according to the HP protocol.

&E8 causes the modem to ignore ENQ/ACK pacing (factory 
default).

&E9 causes the modem to respond to the ENQ (^E) and ACK (^F) 
characters.


Non-Error Correction Mode Flow Control 	&En
n = 10 or 11
Default: &E10

When two modems are connected in Non-Error Correction mode, 
Xon/Xoff flow control can be used as a complement to 
Xon/Xoff pacing.

&E10 disables Non-Error Correction mode flow control 
(factory default).

&E11 enables Non-Error Correction mode flow control.


Pacing (Computer-Initiated Flow Control) 	&En
n = 12 or 13
Default: &E12

Pacing is the means by which the data terminal equipment 
(DTE -- your computer or terminal) regulates the stream of 
incoming data. It is only enabled when the modem has some 
form of flow control active, such as RTS/CTS, or Xon/Xoff. 
When enabled, pacing complements the flow control used by 
the modem. When it is disabled, the DTE is not allowed to 
pace the modem.

&E12 disables pacing (factory default).

&E13 enables pacing.


Number of Xoff Characters Sent	#Xn
n = 0 or 1
Default: #X0

The #X command allows the ZDX to send either a single or 
multiple Xoff character(s) when the modem's buffer becomes 
full. When the buffer is full, the next character received 
causes an Xoff character to be sent. In default mode, only 
one character is sent until the buffer reaches the Xon 
level; however, it is also possible to send one Xoff charac-
ter for every character received after the buffer is full.

#X0 causes only a single Xoff character to be sent after the 
buffer is full (factory default).

#X1 causes one Xoff character to be sent for every character 
received after the buffer is full.


ESCAPE SEQUENCES: ENTERING COMMAND MODE WHILE ON-LINE


+++AT<CR>

You can cause the ZDX to enter command mode, after the modem 
has gone on-line with a remote modem, without disconnecting 
the call. This is accomplished by typing an escape code. The 
default escape code used by the ZDX is three plus signs 
(+++) followed by the letters AT, up to four command charac-
ters (most typically H, to hang up), and RETURN. The modem 
will then escape to command mode, execute the command (if 
any), and then remain in command mode. For example, to hang 
up the modem at the end of a call, type +++ATH <CR>.


<BREAK>AT<CR>

The ZDX provides an alternate escape method, using a break 
signal as the escape code. The break signal allows the 
start-stop DTE to signal the modem without the risk of the 
modem interpreting data characters as modem control charac-
ters. With this method, a break signal is used instead of 
the three plus signs. The BREAK is followed by the letters 
AT, up to 60 command characters, and RETURN. When this is 
done, the modem executes the command and immediately returns 
to on-line mode unless the command was to hang up and/or 
reset the modem (an H or Z command). In the latter case, the 
modem would "escape" to command mode after executing the H 
or Z command.


Escape Sequence Options	%En
n = 0 - 5
Default: %E1

The default setting is for the modem to respond to the +++ 
escape method. Optional settings are for the modem to re-
spond to the break method, for the modem to respond to 
either the +++ or the break methods, and for the modem to 
ignore both methods and not escape. The %E command is used 
to select these options, as follows:

%E0 = Modem won't escape
%E1 = +++ AT<CR> escape method (factory default).
%E2 = BREAK escape method
%E3 = Either +++ or BREAK escape methods
%E4 = Disables OK response to +++
%E5 = Enables OK response to +++
%E1, %E5 = Enables +++ and OK responses to +++
%E3, %E5 = Enables +++ or BREAK and OK responses to +++ 


Force Answer Mode	A 

You can use the A command to force the ZDX into answer mode. 
Type ATA when in command mode to immediately bring your mo-
dem off-hook, out of command mode, and into on-line answer 
mode, and cause it to transmit its carrier signal over the 
phone line. If no responding carrier tone is received by 
your modem within forty-five seconds (or by the time you 
have specified in S-Register S7), your modem stops transmit-
ting its tone, hangs up, and goes back into command mode.


Go Back On-Line		O 

You can use the O command to bring the ZDX out of command 
mode and back into on-line mode. The O command reverses the 
result of entering the escape code. The O command brings the 
modem into whichever on-line mode (originate or answer) that 
it was in prior to entering command mode.


DIAGNOSTIC COMMANDS

The ZDX has two diagnostic commands and four diagnostic 
modes. The power-on self-test is activated every time power 
is applied to the modem. The U command selects a loopback 
test, while the &T command enables your modem to respond to 
a loopback test from another modem. See "Testing Your Modem" 
for a fuller discussion. Use the L8 command to list the di-
agnostic on-line status. 


Respond to Remote Digital Loopback Signal	&Tn
n = 4 or 5
Default: &T5

The ZDX has several self-test features, discussed in Chapter 
5. The &T command enables or disables the modem's ability to 
recognize the remote digital loopback (RDL) test signal.

AT&T4 lets the ZDX respond to an RDL signal, and places it 
in digital loopback. 

AT&T5 causes the ZDX to ignore the RDL signal (factory 
default).


Loopback Test Modes	Un
n = 0-4
Default: None

Loopback tests send data through the modem in such a way 
that it loops back to the terminal where it was entered, 
testing all the circuits along the way. The U command se-
lects from several loopback tests. See "Testing Your modem" 
for more information.

ATU0 places the modem in originate mode for the local analog 
loopback test.

ATU1 places the modem in answer mode for the local analog 
loopback test.

ATU2 performs the remote digital loopback test.

ATU3 performs the local digital loopback test.


DIFFERENCES BETWEEN AT COMMAND SETS

If you bought your ZDX after owning a modem made by another 
manufacturer, you may be tempted to use your old initializa-
tion string or other AT commands. Don't do it without check-
ing your old command set against the commands described in 
this chapter. Though the most common AT commands act much 
the same way in most modems, some commands differ or are 
missing altogether.


S-Registers
-----------

This section covers S-Registers, which are special memory 
areas where variables such as escape code characters and 
delay times are stored. Each S-Register is assigned a number 
(S0, S1, S2, etc.). Use the S command to read and/or change 
the value stored in an S-Register (ATSr? to read and ATSr=n 
to change S-Register values).


S0: Number of Rings Until Modem Answers
Unit: 1 ring
Range: 0-255
Default: 1

S0 defines the number of rings the modem waits before 
answering an incoming call. The default value is one ring, 
which means that the modem answers the call immediately 
after the first ring. The maximum number of rings that can 
be configured is 255. Setting the value to zero means the 
modem will not automatically answer a call.


S1: Ring Count
Unit: 1 ring
Range: 0-255
Default: 0

S1 counts the number of rings that have occurred. It is a 
"read" type of register and is seldom, if ever, used in 
typical operation. Each time an incoming ring signal is de-
tected, S1 increases its value by one, up to a maximum of 
255. If you set S1 to a value other than its default value 
of zero, or if the value is increasing with rings, this new 
value remains stored in S1 for eight seconds after the last 
ring is counted, after which the value reverts back to zero.


S2: Escape Code Character
Unit: Decimal
Range: 0-127
Default: 43 (+ sign)

S2 defines the escape code character by its decimal ASCII 
code. The default character is the plus (+) sign (decimal 
43). It may be set for any ASCII character. Setting an S2 
value greater than 127 results in no escape character, and 
therefore no means of entering command mode from on-line 
mode without breaking the on-line connection, unless you use 
the BREAK method. See Appendix F, "Escape Methods Used by 
Multi-Tech Systems." 

NOTE: If you change the S2 value, you must make correspond-
ing changes in your datacomm software.


S3: Return Character
Unit: Decimal
Range: 0-127
Default: 13 (^M)

S3 defines by its decimal ASCII code the character recog-
nized as carriage return. The default setting is Control-M 
(decimal 13), which is the ASCII code for the RETURN or 
ENTER key on most keyboards. S3 may be set for any ASCII 
character. 

NOTE: If you change the S3 value, you must make correspond-
ing changes in your datacomm software.


S4: Line Feed Character
Unit: Decimal
Range: 0-127
Default: 10 (^J)

S4 defines by its decimal ASCII code the character recog-
nized as line feed. The default setting is Control-J 
(decimal 10), which is the ASCII code for the line feed key 
on most keyboards that have such a key. S4 may be set for 
any ASCII character.


S5: Backspace Character
Unit: Decimal
Range: 0-127
Default: 8 (^H)

S5 defines by its decimal ASCII code the character recog-
nized as BACKSPACE. The default setting is CTRL+H (decimal 
8), which is the BACKSPACE key on most keyboards. S5 may be 
set for any ASCII character. 

NOTE: If you change the S5 value, you must make correspond-
ing changes in your datacomm software.


S6: Wait Time for Dial Tone
Unit: 1 second
Range: 2-255 (ZDX), 4-255 (ZDXI), or 4-7 (ZDXK)
Default: 2 (ZDX) or 4 (ZDXI and ZDXK)

S6 defines the length of time the modem waits after the 
RETURN key is pressed before executing a dial command. The 
default setting is two seconds for North America; four sec-
onds elsewhere.


S7: Time for Carrier (Abort Timer)
Unit: 1 second
Range: 1-255 (ZDX), 1-45 (ZDX DOC and ZDXI), or 1-55 (ZDXK)
Default: 45 (ZDXI) or 55 (ZDXK)

S7 determines the abort timer delay time, which is the 
amount of time your modem will wait for a carrier signal be-
fore it disconnects. The default value is 45 seconds, except 
ZDXK models, for which it is 55 seconds. This means that, 
after dialing, the modem waits for a carrier signal for up 
to 45 or 55 seconds and, if none is detected, aborts the 
call. The maximum S7 value is 255 seconds for ZDX models, 45 
seconds for ZDX DOC and ZDXI models, and 55 seconds for ZDXK 
models.


S8: Pause Time for Comma
Unit: 1 second
Range: 0-255 (ZDX), 4-255 (ZDXI), or 4-7 (ZDXK)
Default: 2 (ZDX) or 4 (ZDXI and ZDXK)

S8 determines the length of the pause caused by a comma in a 
dialing command. The default setting is two seconds for do-
mestic modems; four seconds for international modems. S8 may 
be set for up to 255 seconds. S8 also defines the length of 
time the modem waits before retrying a call after it detects 
a busy signal. Some computer systems need more than two sec-
onds to reset, in which case you should increase the value 
of S8.


S9: Carrier Detect Response Time
Unit: 100 ms
Range: 1-255
Default: 6

S9 determines the time delay between when the modem first 
detects a valid incoming carrier signal and when the modem 
turns on its Carrier Detect circuit. The default setting is 
600 milliseconds (six units of 100 ms each). S9 may be set 
for up to 25.5 seconds.


S10: Carrier Loss Disconnect Delay Time
Unit: 100 ms
Range: 0, 1-255
Default: 7

S10 defines the length of time without a carrier signal be-
fore the modem disconnects. The default setting is 700 ms 
(seven units of 100 ms each). Maximum delay is 25.4 seconds 
(decimal 254). Setting the S10 value at 255 causes the modem 
to not disconnect with loss of carrier.


S11: Tone Dialing Spacing and Duration
Unit: 1 ms
Range: 1-255 (ZDX) or 80-255 (ZDX DOC, ZDXI, and ZDXK)
Default: 70 (ZDX) or 80 (ZDX DOC, ZDXI, and ZDXK)

S11 sets the speed of tone dialing (spacing and tone dura-
tion times). The default value is 70 units for domestic mo-
dems and 80 ms for Canadian and international modems, where 
each unit is one ms, meaning that each tone is sustained for 
70 ms followed by a 70 ms pause. The minimum S11 value al-
lowed by most telephone systems is 50 ms (50 units). Very 
few telephone systems can handle anything faster than that. 
The maximum S11 value is 255 ms (255 units).


S13: Remote Configuration Escape Character 
Unit: ASCII character
Range: 0-127
Default: 37 (%)

S13 defines the remote configuration escape character. The 
S13 default is the percent sign (%). When you enter the S13-
defined character three consecutive times from a remote 
site, your modem responds with its remote configuration pro-
cedure. S13 is available for the MT1932ZDX, MT1932ZDXI, and 
MT1932ZDXK only.


S17: Changing Break Time
Unit: 10 ms
Range: 0-2.5 seconds
Default: 25 (250 ms)

S17 defines the break time (space) the modem sends the DTE 
when it receives a break signal from a remote modem. Break 
duration is adjustable in 10 millisecond increments. The de-
fault is set to 25, for a 250 millisecond break.


S24: PBX/CBX Disconnect Drop Time for DSR/CTS/CD
Unit: 50 ms
Range: 0-255
Default: 20 (1 second)

Some PBX and CBX phone systems require the modem's DSR, CTS, 
and/or CD signals to drop briefly when calls are discon-
nected. The ZDX's &R, &S, and &C commands cause the modem to 
drop these signals for a specified time upon disconnect, and 
then bring the signals up again. S24 defines the length of 
time that the signals drop. The default setting of 20 re-
sults in a one-second drop time, which is what most PBX/CBX 
systems with this requirement need.


S25: DTR Dropout Time
Unit: 100 ms
Range: 0, 1-255
Default: 0 (50 ms)

S25 sets the amount of time that DTR must be dropped before 
the modem disconnects. Typically, a disconnect occurs when 
DTR is dropped for 50 milliseconds or more. This dropout 
time can be increased up to a maximum of 25.5 seconds. The 
S25 unit value for zero is 50 ms. For values from 1 through 
255, the unit value is 100 ms.


S30: Inactivity Timer
Unit: 1 minute
Range: 0, 1-255 minutes
Default: 0

S30 can be used to cause the modem to disconnect if no data 
is transmitted or received for a specified time. S30 runs 
during both Reliable and Non-Error Correction connections. 
The timer restarts any time a data character passes through 
the serial port (either sent or received). If noise on the 
phone line causes an error to be received during Non-Error 
Correction mode, this also will restart the S30 timer. The 
factory default of zero disables the timer.


S32: Time Elapse for Escape Sequence
Unit: 100 ms
Range: 0-255
Default: 20 (2 seconds)

S32 sets the time period to validate the escape sequence. If 
the S32 time interval expires before you hit RETURN, then 
the escape sequence is aborted. The default is two seconds.


S34: Command Buffer Length After On-Line Escape Sequence
Unit: ASCII character
Range: 0-60
Default: 10

If the number of characters after "AT" exceeds the S34 
buffer length value, then the buffer is flushed and the 
escape sequence is aborted.


S36: DTR Busy-Out Time Length
Unit: 1 second
Range: 0-255
Default: 0

S36 sets the time between when DTR goes inactive and when 
the modem goes off-hook. The DTR busy-out feature is inac-
tive when S36=0. Also, refer to S37 for the DTR busy-in 
control. 


S37: DTR Busy-In Time Length 
Unit: 1 second
Range: 0-255
Default: 5

S37 sets the time between when DTR goes active and when the 
modem goes on-hook. The DTR busy-out time is controlled with 
S36.


S48: Program V.34 Connect Speeds
Unit: N/A
Range: 28, 26, 21, 19, 16, 14, 12, 96, and 48
Default: 0

S48 sets the maximum speed at which the modem connects in 
V.34 mode (e.g., S48=21 sets the maximum connect speed to 
21.6K). This register allows you to set lower speeds for 
line conditions that will not support higher V.34 speeds 
such as 28K, 26.4K, or 24K. The factory default of zero sets 
a connection attempt of 28.8K.


READING AND ASSIGNING S-REGISTER VALUES
---------------------------------------

The S command is used to assign a value to, and to read the 
current value of, an S-Register. To read an S-Register 
value, enter the letter S followed by the S-Register number 
and a question mark (?), then press RETURN. For example, 
entering ATS7? and pressing RETURN displays the value of S-
Register S7 in a 3-digit decimal form. The number 8 would 
appear as 008, the number 30 would appear as 030, and the 
number 255 would appear as 255.

To assign a value to an S-Register, enter the letter S fol-
lowed by the S-Register number and an equals sign (=), and 
then a decimal number. Convert all ASCII characters to their 
decimal equivalents before entering them. S-Register decimal 
values are 0-127 for ASCII characters, and 0-255 for numeric 
values. 


EXAMPLES OF ASSIGNING VALUES

1. Let's say you want to have longer pauses caused by the 
   comma in a dial command, five seconds instead of two. 
   Entering ATS8=5 will assign 5 as the value for S-Register 
   S8, after which the modem will pause five seconds for 
   every comma in a dial command.

2. In a second example, let's say that you wish to configure 
   the ZDX modem to answer incoming calls after the thirti-
   eth ring instead of after the first ring. To configure S-
   Register S0 with a value of 30, enter ATS0=30 and press 
   RETURN.

3. In a third example, let's say you are calling long dis-
   tance to another country code, and it is taking a long 
   time to connect. The S-Register S7 (Abort Timer) factory 
   default setting of 45 seconds is insufficient; the abort 
   timer times out and cancels the call before a connection 
   is made. To change the S7 value to 55 seconds, enter com-
   mand mode, then enter ATS7=55 and press RETURN. Now, 
   after dialing, the modem allows 10 more seconds for a 
   carrier signal before aborting the call. The additional 
   10 seconds should provide enough time for international 
   calls.


EXAMPLES OF READING VALUES

1. To verify that you entered the value correctly in the 
   above examples, enter ATS8? and hit RETURN in the first 
   example, ATS0? in the second, and ATS2? in the third 
   example. You should receive the response 005 in the first 
   example, 030 in the second example, and 055 in the third 
   example.

2. When configuring the S-Registers, it is a good practice 
   to include the verification read-entry in the same com-
   mand line as the configuration assignment entry. In the 
   three preceding examples, enter ATS8=5S8?, ATS0=30S3?, 
   and ATS7=55S7?.


COMMAND AND S-REGISTER SUMMARY
------------------------------

Your ZDX's default configuration is for originating a call 
to another high speed modem that supports error correction 
and data compression. If the receiving modem is not compati-
ble, the ZDX can match any ITU or Bell Standard modem, but 
not proprietary protocols.


------------------------------------------------------------

CHAPTER 7
SERVICE, WARRANTY, AND TECHNICAL SUPPORT

------------------------------------------------------------


INTRODUCTION
------------

This chapter begins with statements about your modem's war-
ranty. Read carefully the next section, Tech Support, if you 
have questions or problems with your modem. It includes the 
technical support telephone numbers, space for recording 
your modem information, and an explanation of how to send in 
your modem should you require service. The final two sec-
tions explain how to use our bulletin board service (BBS), 
and get support through CompuServe. 


LIMITED WARRANTY
----------------

Multi-Tech (R) Systems, Inc. ("MTS") warrants that this 
product will be free from defects in material or workmanship 
for the period specified on the enclosed Warranty Registra-
tion Card. Please complete the card and return to Multi-Tech 
postage pre-paid. MTS MAKES NO OTHER WARRANTY, EXPRESSED OR 
IMPLIED, AND ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND 
FITNESS FOR A PARTICULAR PURPOSE ARE HEREBY DISCLAIMED. This 
warranty does not apply to any products that have been dam-
aged by lightning storms, water, or power surges, or that 
have been neglected, altered, abused, used for a purpose 
other than the one for which they were manufactured, re-
paired by the customer or any party without MTS's written 
authorization, or used in any manner inconsistent with MTS's 
instructions. 

MTS's entire obligation under this warranty shall be limited 
(at MTS's option) to repair or replacement of any products 
that prove to be defective within the warranty period, or, 
at MTS's option, issuance of a refund of the purchase price. 
Defective products must be returned by Customer to MTS's 
factory transportation prepaid.

MTS WILL NOT BE LIABLE FOR CONSEQUENTIAL DAMAGES AND UNDER 
NO CIRCUMSTANCES WILL ITS LIABILITY EXCEED THE PURCHASE 
PRICE FOR DEFECTIVE PRODUCTS.


TECH SUPPORT
------------

Multi-Tech has an excellent staff of tech-support personnel 
available to help you get the most out of your Multi-Tech 
product. If you have any questions about the operation of 
this unit, please call 1-800-972-2439. Please fill out the 
modem information (below), and have it available when you 
call. If your modem requires service, the tech support spe-
cialist will guide you on how to send in your modem.


RECORDING MODEM INFORMATION

Please fill in the following information on your Multi-Tech 
modem. This will help Tech Support in answering your ques-
tions. (The same information is requested on the Warranty 
Registration card.)

Modem model # ______________________________________________

Modem serial # _____________________________________________

Modem firmware version # ___________________________________

MEWFAX software version # __________________________________

MEW software version # _____________________________________

The modem model number and serial number are printed on a 
label on the bottom of your modem. The software version num-
bers are printed on the diskette labels. You can also find 
the software version numbers by selecting Help >> About... 
from the software menu bars. Enter the ATI1 command to dis-
play the modem firmware version number.

Before calling tech support, please check your cables to en-
sure they are connected properly; then note the status of 
your modem including status indicators, screen messages, di-
agnostic test results (e.g., L8 command), problems with a 
specific application, etc. Use the space below to note modem 
status:

____________________________________________________________

____________________________________________________________

____________________________________________________________

____________________________________________________________

____________________________________________________________

____________________________________________________________

____________________________________________________________

____________________________________________________________


SERVICE

If your tech support specialist decides that service is re-
quired, modems may be sent (freight prepaid) to our factory. 
Return shipping charges will be paid by Multi-Tech Systems.

Include the following with your modem:

* A description of the problem.

* Return billing and return shipping addresses.

* Contact name and phone number.

* Check or purchase order number for payment if the modem is 
  out of warranty. (The standard repair charge for this mo-
  dem is $95. This price is valid at the time of this publi-
  cation, but could change in the future. Check with your 
  technical support specialist.)

* If possible, note the name of the technical support spe-
  cialist with whom you spoke.

If you need to inquire about the status of the returned 
product, be prepared to provide the serial number of the 
product sent (see "Recording Modem Information."

Send modems to the following address:

	Multi-Tech Systems, Inc.
	2205 Woodale Drive
	Mounds View, MN 55112
	Attn.: Service or Repairs

You should also check with the supplier of your modem on the 
availability of local service and/or loaner units in your 
part of the country. You may call us at 1-800-972-2439 (in 
Canada and USA) or at 612-785-3500 (in Minnesota).


ABOUT THE MULTI-TECH BBS
------------------------

Multi-Tech Systems maintains a bulletin board system (BBS) 
for its customers. The information available via the BBS in-
cludes new product information, product upgrade data, prob-
lem-solving tips, and a message service for you to leave 
questions for which you would like additional information. 
The phone number for the Multi-Tech BBS is (612) 785-3702 or 
(800) 392-2432 (USA and Canada). 

The BBS can be accessed by any asynchronous modem operating 
at speeds of 28,800 to 1200 bps at a setting of 8 bits word 
length, 1 stop bit, and no parity.


HOW TO USE THE MULTI-TECH BBS

To use Multi-Tech's BBS, perform the following steps.

1. Set your communications program to 8-N-1, and to emulate 
   ANSI (e.g., with MultiExpress software, press ALT+S and 
   choose "ANSI").

2. Dial our BBS at 1-800-392-2432 (USA and Canada) or 612-
   785-3702 (international).

3. password; then hit RETURN. If you are a first-time cal-
   ler, after you hit RETURN, the BBS will ask if your name 
   is spelled correctly. If you answer yes, our question-
   naire will be displayed. You can use our BBS on your 
   first call.

4. There are four BBS areas: the Main Menu, the Files Menu, 
   Bulletins (from the Main Menu), and the Message Menu. All 
   bulletins are menu-driven. To read the bulletins, enter 
   the number of the bulletin you wish to read.

5. Files Menu: From the Main Menu, type F to display the 
   Files Menu. If you want a list of directories, type L 
   (list directory) and then type L again for a list of all 
   directories. If you do not type the second L, you will 
   list all of the files on the BBS. At the list of direc-
   tories, select the number of the directory required. A 
   list of files and a description for each of the files 
   will be displayed. Select a file that you would like to 
   download; if you already know the file name, type D at 
   the Files Menu to download the selected file(s). Type V 
   to view a text file.

6. At the Message Menu, you can leave a message to the sysop 
   (you cannot read messages at this point). The BBS will 
   tell you if you have a personal message (mail). At the 
   prompt Would you like to read it now?, type R for read 
   now. You must read your message(s) when you first access 
   the BBS.


ABOUT COMPUSERVE
----------------

In addition to the BBS, Multi-Tech provides support through 
CompuServe's Modem Vendor Forum (GO MODEMVEN) under GO 
MULTITECH. Refer to your CompuServe documentation for 
special operating procedures.


------------------------------------------------------------

APPENDICES

------------------------------------------------------------


Appendix A  Specifications
--------------------------

Your MultiModemZDX was designed to meet the following 
specifications.

Model Number:
MT1432ZDX, MT1932ZDX, MT2834ZDX (USA); MT1432ZDXK, 
MT1932ZDXK, MT2834ZDXK (UK); MT1432ZDXI, MT1932ZDXI, 
MT2834ZDXI (International)

Data Rates (Modem):
0-300, 1200, 2400, 4800, 9600, 12,000, 14,400 bps 
(MT1432ZDX), and 16,800 and 19,200 bps (MT1932ZDX), and 
21,600, 24,000, 26,400, and 28,800 (MT2834ZDX)

Data Rates (Fax):
4800, 9600, 14,400 bps

Data Format (Modem):
Serial, binary, asynchronous

Compatibility (Modem):
Bell 212A and 103/113, ITU V.22; V.22bis, V.29, V.32, 
V.32bis, V.42, V.42bis (MT1432ZDX), and AT&T V.32 terbo 
(MT1932ZDX), and V.34 (MT2834ZDX); ITU V.21 & V.23 in ZDXI 
and ZDXK versions

Compatibility (Fax):
ITU Group 3, T.4, T.30, V.21, V.27ter, V.29, V.17, and EIA 
TR 29.2

Error Correction:
ITU V.42 (LAP-M or MNP 2- 4)

Data Compression:
ITU V.32 (4:1 throughput) or MNP 5 (2:1 throughput)

Speed Conversion:
Serial port data rates adjustable to 300, 1200, 2400, 4800, 
9600, 12,000, 19,200, 38,400, 57,600, and 115,200 bps

Mode of Operation:
Half or full duplex over dial-up lines; automatic or manual 
dialing, automatic or manual answer

Flow Control:
Xon/Xoff, hardware (RTS/CTS), ENQ/ACK (HP)

Intelligent Features:
Fully AT command compatible, autodial, redial, repeat dial, 
pulse or tone dial, dial pauses, call status display, auto-
parity and data rate selections, keyboard-controlled modem 
options, on-screen displays for modem option parameters and 
command lines of up to 60 digits each, help menus

Command Buffer:
60 characters

Modulation (Modem):
FSK at 300 bps, PSK at 1200 bps, QAM at 2400, 4800, and 9600 
bps (non-trellis), QAM with Trellis Coded Modulation (TCM) 
at 9600, 12,000, 14,400 bps (MT1432ZDX), and 16,800 and 
19,200 bps (MT1932ZDX)

Fax Modulations:
V.21 CH2 FSK at 300 bps
V.27ter DPSK at 2400 and 4800 bps
V.29 QAM at 7200 and 9600 bps
V.17 TCM at 7200, 9600, 12,000, and 14,400 bps

Carrier Frequencies, 28.8K/16.8K bps:
1700/1800 Hz V.34

Carrier Frequencies 19.2K/16.8K/14.4K/ 12K/9.6K/4.8K bps:
1800 Hz V.32/V.32bis/V.32 terbo

Carrier Frequencies, 2400 & 1200 bps (V.22bis/V.22 or Bell 
212A Standard):
Transmit originate: 1200 Hz
Transmit answer: 2400 Hz
Receive originate: 2400 Hz
Receive answer: 1200 Hz

Carrier Frequencies, 300 bps (Bell Standard):
1270 Hz mark, 1070 Hz space for transmit originate
2225 Hz mark, 2025 Hz space for receive originate
2225 Hz mark, 2025 Hz space for transmit answer
1270 Hz mark, 1070 Hz space for receive answer

Fax Carrier Frequencies:
V.21 Ch2 (Half Duplex) 
1650 Hz mark, 1850 Hz space for transmit originate
1650 Hz mark, 1850 Hz space for transmit answer
V.27ter 1800 Hz originate/answer
V.29 QAM 1700 Hz originate/answer
V.17 TCM 1800 Hz originate/answer

Transmit Level:
-10 dBm

Frequency Stability:
0.01%

Receiver Sensitivity:
-43 dBm under worst case conditions

AGC Dynamic Range:
43 dB

Interface:
EIA RS-232C/ITU V.24/V.28

Connectors:
DB-25S RS-232C connector; two RJ-11 phone jacks for line and 
phone set (one RJ-11 jack on UK and international models), 
power jack.

Cables:
One 14-foot RJ-11 phone cable (USA); country-specific cord 
for UK and international models; external power transformer 
and cord.

NOTE: 	Any cables connected to the computer should be 
shielded to reduce interference.

Diagnostics:
Power-on self test, local analog loop, local digital loop, 
remote digital loop.

Indicators:
LEDs for Transmit Data, Receive Data, Carrier Detect, 28,800 
(MT2834ZDX), 19,200 bps (MT1932ZDX), 14,400 bps, 9600 bps, 
2400 bps (MT1432ZDX), Off-Hook, Terminal Ready, Error 
Correction, and Fax.

Speaker:
Command-controlled speaker for call progress monitoring.

Manual Control:
Power On/Off

Environmental:
Temperature range 0  to 50 C (32  to 120 F); humidity range 
20-90% (non-condensing)

Power Requirements:
100-130VAC, 50/60 Hz, 0.1A/5W; two-prong outlet-mounted 
transformer (included); 230V/50 Hz optional (international)

Dimensions:
10.8 cm wide ( 14.8 cm long ( 2.9 cm high (4.25( ( 5.8(( 
1.15 ()

Limited Warranty:
Ten years, US, UK, Canada & Mexico, 5 years elsewhere



APPENDIX B  MULTIMODEMZDX DEFAULTS
----------------------------------

AT COMMAND DEFAULTS:

$A0	Auto-Reliable Buffering: Discard data received 
	during establishment of Reliable connection.

#A0	Auto Speed Detection in Answer Mode: Start at maxi-
	mum speed and fall back incrementally to 14400 to 
	12000 to 9600 to 4800 to 2400 to 1200 to 300 bps.

B0	Answer Tone: ITU answer tone.

&BS1	Maximum Reliable Block Size: 256 characters.

$BA0	Baud Adjust: Baud adjust off, serial conversion on 

&C1	Carrier Detect Control: CD goes high when carrier 
	is detected, low when carrier is lost.

&D2	Data Terminal Ready Control: Modem hangs up when 
	DTR drops and returns to command mode when DTR goes 
	high again.

$D0	DTR Dialing: Disabled.

E1	Echo Command Mode Characters: Off.

&E1	V.42 Error Correction: Auto-Reliable mode.

&E4	Modem-Initiated Flow Control: Hardware flow control

&E6	Xon/Xoff Pass-Through: Off.

&E8	Hewlett-Packard ENQ/ACK Pacing: Off.

&E10	Non-Error Correction Mode Flow Control: Off.

&E12	Pacing (Computer-Initiated Flow Control): Off.

&E15	Data Compression: On.

$EB0	Asynchronous Word Length: 10-bit mode.

$E0	V.42 Error Correction at 300 bps: Off.

%E1	Escape Sequence Options: +++ method.

&F8	Load Default Configurations: Read factory defaults 
	in ROM.

$F1	Auto-Reliable Fallback: Fall back to Non-Error 
	Correction mode connect if <CR> received.

#F2	Fallback Modes When On-Line: Fall back incremen-
	tally to 4800 bps; fall forward when line improves.

&G0 or 	Guard Tones. ZDXI models: &G0 (turn off ITU guard
&G2	tones). ZDXK models: &G2 (ITU 1800 Hz guard tone) 
	only.

#L0	V.42 Error Correction Mode: Modems negotiate V.42 
	mode.

M1	Modem Speaker Control: Speaker on until carrier 
	signal detected.

$MB14400  Modem Baud Rate: 14,400 bps (MT1432ZDX).

$MB19200  Modem Baud Rate: 19,200 bps (MT1932ZDX).

$MB28800  Modem Baud Rate: 28,800 bps (MT2834ZDX).

P	Pulse Dial.

&P0 or 	Set Pulse Dial Ratios: 60-40 ratio. ZDXK models:
&P1	&P1 (67-33 ratio) only.

Q0	Result Codes Enable/Disable: Result codes sent.

&Q0	Result Codes (Multi-Tech or Standard AT): Multi-
	Tech responses with modifiers.

R0	Reverse Originate/Answer Modes: Off.

&R1	Clear to Send Control: Force CTS high (on).

$R0	Retransmit Count: Disconnect after 12 retransmits.

&RF1	CTS/RTS Interaction Control: CTS acts 
	independently.

&S1	Data Set Ready Control: DSR follows CD. 

$SB57600  Serial Port Baud Rate: 57,600 bps.

&SF0	DSR/CD Interaction Control: DSR follow CD.

&T5	Respond to Remote Digital Loopback Signal: Off.

#T1	Trellis Coded Modulation: On.

V1	Result Codes (Verbose/Terse): Verbose response.

&W1	Store Configuration: Off.

X0	Result Codes and Call Progress Selection: Basic 
	result codes (CONNECT only); modem does not look 
	for dial or busy tones.

#X0	Number of Xoff Characters Sent After Buffer Is 
	Full: One.

Y0	Long Space Disconnect: Disable sending or respond-
	ing to long space break signal on disconnect.


S-REGISTER DEFAULTS

S0 = 1	  Number of rings until modem answers.

S1 = 0	  Ring count.

S2 = 43	  Escape code character (+).

S3 = 13	  Return character (^M).

S4 = 10	  Line feed character (^J).

S5 = 8	  Backspace character (^H).

S6 = 2 or 4
	  Dial tone wait: 2 seconds ZDX, 4 seconds ZDXK and 
	  ZDXI models.

S7 = 45 or 55
	  Wait for carrier: 45 seconds ZDX and ZDXI, 55 
	  seconds ZDXK models.

S8 = 2 or 4
	  Comma pause time: 2 seconds ZDX, 4 seconds ZDXK
	  and ZDXI models.

S9 = 6	  Carrier detect response time: 600 ms.

S10 = 7	  Disconnect delay time: 700 ms.

S11 = 70  Tone duration and spacing: 70 ms ZDX, 80 ms all
   or 80  other models, including ZDX DOC.

S13 = 37  Remote configuration escape character (%).

S17 = 25  Length of break time to PC: 250 ms.

S24 = 20  DSR/CTS/CD dropout time: 1 second.

S26 = 0	  Number of failed attempts allowed.

S30 = 0	  Inactivity timer: disabled.

S32 = 20  Time modem will wait for <CR> during escape 
	  sequence execution: 2 seconds.

S34 = 10  Buffer length of command mode after on-line escape 
	  sequence: 10 characters.

S36 = 0	  Time between DTR inactive and modem off-hook.

S37 = 5	  Time between DTR active and modem on-hook. 

S48 = 0	  V.34 maximum connect speed.



APPENDIX C  AT COMMAND SUMMARY
------------------------------


Command Values	Default	Description
------------------------------------------------------------
AT			Attention Code
			The attention code precedes all 
			command lines except A/, A: and 
			escape codes.

RETURN			RETURN Key
			Press the RETURN (ENTER) key to 
			execute most commands.

A			Force Answer Mode
			Answer call immediately without 
			waiting for ring.

A/			Repeat Last Command. 
			Do not precede this command with 
			AT. Do not hit RETURN to execute.

A:			Continuous Redial
			Continuous redial (10 redials in 
			DOC modems) of last number until 
			answered. (Not used on ZDXK or ZDXI 
			modems.)

&An	n = 0 or 1	Answerback Caller ID
		   ***	&A0  Answerback off.
			&A1  Answerback on.

$An	n = 0 or 1	Auto-Reliable Buffering 
		   ***	$A0  Discard data received during
			     establishment of Reliable
			     connection
			$A1  Buffer data received during 
			     establishment of Reliable 
			     connection

#An	n = 0-3		Auto Speed Detection in Answer Mode
		   ***	#A0  Start at maximum speed and 
			     fall back to 14400 to 12000 to 
			     9600 to 4800 to 2400 to 1200 
			     to 300 bps.
			#A1  Maximum speed only.
			#A2  Start at maximum speed and 
			     fall back incrementally to 
			     4800 bps only.
			#A3  Start at 2400 bps and fall 
			     back to 1200 to 300 bps only.

Bn	n = 0 or 1	Answer Tone (ZDXI modems only)
		   ***	B0   Select ITU V.21 answer tone.
			B1   Select Bell 103 answer tone.

&Bsn	n = 0 or 1	Maximum Reliable Block Size
			&BS0 Maximum transmit block size of 
			     64 characters.
		   ***	&BS1 Maximum transmit block size of 
			     256 characters.

$Ban	n = 0 or 1	Baud Adjust
		   ***	$BA0 Set baud adjust off, speed 
			     conversion on. (Serial port 
			     speed is independent of modem 
			     data rate.)
			$BA1 Set baud adjust on, speed 
			     conversion off. (Serial port 
			     speed is same as modem data 
			     rate.)

&Cn	n = 0, 1, 2,	Carrier Detect Control
	    or 4 	&C0  Force Carrier Detect on.
		   ***	&C1  Let Carrier Detect follow 
			     carrier signal.
			&C2  Let Carrier Detect drop for
			     time set by S24 on discon-
			     nect, then go high again.
			&C4  Reset modem when Carrier
			     Detect drops.

Ds	s = phone #	Dial
			Dial telephone number s, where s 
			may include up to 60 digits or T, 
			P, R, comma, colon, and semicolon 
			characters.

DsNd	s = phone #	Store Phone Number
	d = 0 or 1	To store, enter D followed by 
			telephone number s, then N followed 
			by directory number d.

&Dn	n = 0, 1, 2,	Data Terminal Ready Control
	    or 3	&D0  Modem ignores DTR signal.
			&D1  Modem hangs up when DTR drops, 
			     disables auto-answer, and 
			     returns to command mode when 
			     DTR goes high again.
		   ***	&D2  Modem hangs up when DTR drops 
			     and returns to command mode 
			     when DTR goes high again.
			&D3  Modem hangs up and resets to 
			     default parameters when DTR 
			     drops.

$Dn	n = 0 or 1	DTR Dialing
		   ***	$D0  Disable DTR dialing.
			$D1  Enable DTR dialing.

En	n = 0 or 1	Echo Command Mode Characters
			E0   Do not echo command mode 
			     characters.
		   ***	E1   Do echo command mode 
			     characters.

&En	n = 0 thru	V.42 Error Correction Modes
	    15		&E0  Non-Error Correction mode
			     (V.42 disabled).
		   ***	&E1  Auto-Reliable mode.
			&E2  Reliable mode (V.42 enabled).

			Modem-Initiated Flow Control
			&E3  Flow control disabled.
		   ***	&E4  Hardware flow control.
			&E5  Xon/Xoff flow control.

			Xon/Xoff Pass-Through (&E5 
			selected)
		   ***	&E6  Xon/Xoff not passed through.
			&E7  Xon/Xoff passed through.

			Hewlett-Packard ENQ/ACK Pacing
		   ***	&E8  ENQ/ACK pacing off.
			&E9  ENQ/ACK pacing on.

			Non-Error Correction Mode Flow 
			Control
		   ***	&E10 Non-Error Correction mode flow 
			     control off.
			&E11 Non-Error Correction mode flow 
			     control on.

			Pacing (Computer-Initiated Flow 
			Control)
		   ***	&E12 Pacing off.
			&E13 Pacing on.

			Data Compression
			&E14 Data compression disabled.
		   ***	&E15 Data compression enabled.

$En	n = 0 or 1	V.42 Error Correction at 300 bps
		   ***	$E0  V.42 error correction at 300 
			     bps disabled.
			$E1  V.42 error correction at 300 
			     bps enabled.

$Ebn	n = 0 or 1	Asynchronous Word Length
		   ***	$EB0 10-bit mode enabled.
			$EB1 11-bit mode enabled.

%En	n = 0 thru	Escape Sequence Options
	    5		%E0  Modem won't escape.
		   ***	%E1  +++ method.
			%E2  Break method.
			%E3  Either +++ or Break methods.
			%E4  No "OK" response to +++.
			%E5  "OK" response to +++.

&Fn	n = 0, 8,	Load Default Configuration
	    or 9	&F0  Load factory default values 
			     from ROM.
		   ***	&F8  Read factory default values
			     and DIP switch settings when
			     &F is issued.
			&F9  Read parameters stored in non-
			     volatile memory when &F is 
			     issued.

$Fn	n = 0 or 1	Auto-Reliable Fallback Character 
			Enable/Disable
			$F0  Do not fall back to Non-Error 
			     Correction mode connect if CR 
			     received.
		   ***	$F1  Fall back to Non-Error 
			     Correction mode connect if CR 
			     received.

#Fn	n = 0, 1,	Fallback Modes When On-Line
	    or 2	#F0  No fallback when on-line.
			#F1  Fall back incrementally from 
			     maximum speed to 4800 bps.
		   ***	#F2  Fall back incrementally to
			     4800 bps, fall forward when
			     line improves.

&Gn	n = 0, 1, 	Guard Tones (International only)
	    or 2   ***	&G0  Turn off ITU guard tones.
			&G1  Turn on ITU 550 Hz guard tone.
			&G2  Turn on ITU 1800 Hz guard tone

			NOTE:  The ZDXK is locked to &G2, 
			which turns on the ITU 1800 Hz 
			guard tone.

Hn	n = 0 or 1	On-Hook/Off-Hook
			H0   Go on-hook (hang up).
			H1   Go off-hook.

$Hn	n = 1, 2, 	Help Screens
	    or 3	$H1  Bring up Help Screen #1.
			$H2  Bring up Help Screen #2.
			$H3  Bring up Help Screen #3.

In	n = 0, 1, 	Inquire Product Code
	    or 2	I0   Request modem ID #.
			I1   Request firmware revision #.
			I2   Request modem description.

Ln	n = 0, 5, 6,	List Commands
	    7, or 8	L    List stored telephone numbers.
			L5   List current operating 
			     parameters.
			L6   List current S-Register values
			L7   List additional parameters
			L8   List on-line diagnostic 
			     parameters

#Ln	n = 0, 1, 2,	V.42 Error Correction Modes
	    3, or 8 *** #L0  Modems negotiate V.42 mode.
			#L1  MNP on and LAP-M off
			     (originate mode only).
			#L2  LAP-M on and MNP off
			     (originate mode only).
			#L3  Disable detection phase and go 
			     directly to LAP-M.

Mn	n = 0, 1, 2,	Modem Speaker Control
	    or 3	M0   Modem speaker always off.
		   ***	M1   Modem speaker on until carrier 
			     signal detected.
			M2   Modem speaker always on.
			M3   Monitor speaker on during dia-
			     ling, off during handshaking.

$Mbn	n = speed	Modem Baud Rate
			$MB75	 Select CCITT V.23 mode.
			$MB300	 Select 300 bps on-line.
			$MB1200	 Select 1200 bps on-line.
			$MB2400	 Select 2400 bps on-line.
			$MB4800	 Select 4800 bps on-line.
			$MB7200	 Select 7200 bps on-line.
			$MB9600	 Select 9600 bps on-line.
			$MB12000 Select 12,000 bps on-line.
		   ***	$MB14400 Select 14,400 bps on-line.
			$MB16800 Select 16,800 bps on-line.
		   ***	$MB19200 Select 19,200 bps on-line.
		   ***	$MB28800 Select 28,800 bps on-line.

Nd	d = 0 or 1	Dial a Stored Number
			Dial stored telephone number d.

NdNe...	d = 0 or 1	Number Linking
	e = 1 or 0	If first number dialed is busy, 
			other stored numbers may be auto-
			matically dialed. In example, 
			stored number d is dialed; if it is 
			busy, stored number e is dialed.

O			Go Back On-Line
			Exit command mode and go into on-
			line mode after using the escape 
			code to do the reverse.

P	In dialing ***	Pulse-Dial
	command 	Modem will pulse-dial numbers that 
			follow the P.

&Pn	n = 0 or 1	Set Pulse Dial Ratios
		   ***	&P0  60-40 pulse ratio.
			&P1  67-33 pulse ratio.

			NOTE:	The ZDXK is locked to &P1, 
			a 67-33 pulse ratio.

Qn	n = 0, 1, 	Result Codes Enable/Disable
	    or 2   ***	Q0   Result codes sent.
			Q1   Result codes suppressed 
			     (quiet).
			Q2   No Response answer mode.

&Qn	n = 0 or 1	Result Codes (Multi-Tech or 
			Standard AT)
		   ***	&Q0  Multi-Tech responses with 
			     modifiers.
			&Q1  AT responses with no modifiers

Rn	n = 0 or 1	Reverse Originate/Answer Mode
		   ***	R0   Modem will not reverse modes.
			R1   Modem will reverse modes.

&Rn	n = 0, 1,	Clear to Send Control
	    or 2	&R0  Let CTS state follow RTS state 
			     when on-line.
		   ***	&R1  Force CTS high (on).
			&R2  Let CTS drop on disconnect for 
			     time set by S24, then go high 
			     again.

$Rn	n = 0 or 1	Retransmit Count
		   ***	$R0  Disconnect after 12 
			     retransmits.
			$R1  Do not disconnect after 12 
			     retransmits.

&Rfn	n = 0 or 1	CTS/RTS Interaction Control
			&RF0 Let CTS follow RTS.
		   ***	&RF1 Let CTS act independently.

Sr=n	r = 0-11, 13,	Set Register Value
	24, 25, 30, 	Set value of S-Register r to value
	32, 34, 36, 	of n, where n is entered in decimal
	37 or 48; 	format.
	n varies

 Sr?	r = 0-11, 13,	Read Register Value
	24, 25, 30, 	Read value of S-Register r and
	32, 34, 36, 	display value in 3-digit decimal
	37 or 48; 	format.
	n varies

&Sn	n = 0, 1, 	Data Set Ready Control
	    or 2	&S0  Force DSR high (on).
		   ***	&S1  Let DSR follow CD.
			&S2  DSR drops on disconnect for 
			     time set by S24, then goes
			     high again.

$Sbn	n = speed	Serial Port Baud Rate
	Baud Adjust	$SB300	  Select 300 bps at serial 
	($BA) must		  port
	be off. 	$SB1200	  Select 1200 bps at serial 
				  port.
			$SB2400	  Select 2400 bps at serial 
				  port.
			$SB4800	  Select 4800 bps at serial 
				  port.
			$SB9600	  Select 9600 bps at serial 
				  port.
			$SB12000  Select 12,000 bps at 
				  serial port.
			$SB19200  Select 19,200 bps at 
				  serial port.
			$SB38400  Select 38,400 bps at 
				  serial port.
		   ***	$SB57600  Select 57,600 bps at 	
				  serial port.
			$SB115200 Select 115,200 bps at 
				  serial port.

&Sfn	n = 0 or 1	DSR/CD Interaction Control
		   ***	&SF0 Select DSR to follow CD.
			&SF1 Select DSR to be independent.

T			Tone-Dial
			Modem tone-dials numbers following 
			the T.

&Tn	n = 4 or 5	Respond to Remote Digital Loopback 
			Signal
			&T4  Enable response to remote
			     digital loopback signal.
		   ***	&T5  Disable response to remote 
			     digital loopback signal.

#Tn	n = 0 or 1	Trellis Coded Modulation
			#T0  Disable Trellis Coded 
			     Modulation.
		   ***	#T1  Enable Trellis Coded 
			     Modulation.

Un	n = 0, 1, 2,	Loopback Test Modes
	    or 3	U0   Enable analog loop originate 
			     mode.
			U1   Enable analog loop answer mode
			U2   Enable remote digital loopback 
			     mode.
			U3   Enable local digital loopback 
			     mode.

Vn	n = 0 or 1	Result Codes (Verbose/Terse)
			V0   Result codes sent as digits 
			     (terse response).
		   ***	V1   Result codes sent as words 
			     (verbose response).

W			Wait for New Dial Tone
			Inserted in dialing command, causes 
			modem to wait for new dial tone. 
			(X2 or X4 must be selected.)

&Wn	n = 0 or 1	Store Configuration
			&W0  Store current settings in 
			     NVRAM; modem will load these
			     at power-on or with the ATZ
			     command instead of reading
			     factory ROM defaults.
		   ***	&W1  Do not store settings.

Xn	n = 0 thru	Result Codes and Call Progress 
	    4		Selection
		   ***	X0   Basic result codes (CONNECT  
			     only); does not look for dial 
			     tone or busy.
			X1   Extended result codes (w/ 
			     CONNECT 1200, CONNECT 2400, 
			     etc.); does not look for dial 
			     tone or busy signal
			X2   Extended result codes with NO 
			     DIAL TONE; does not look for 
			     busy signal
			X3   Extended result codes with 
			     BUSY; does not look for dial 
			     tone.
			X4   Extended result codes with NO 
			     DIAL TONE and BUSY.

#Xn	n = 0 or 1	Number of Xoff Characters Sent
		   ***	#X0  Single Xoff character sent 
			     after buffer is full.
			#X1  Multiple Xoff characters sent 
			     (one for every character
			     received after buffer is full)

Yn	n = 0 or 1	Long Space Disconnect
		   ***	Y0   Disable sending or responding 
			     to long space break signal on 
			     disconnect.
			Y1   Enable sending or responding
			     to long space break signal on
			     disconnect. (Both modems must
			     have Y1 set.)

Z			Modem Reset
			Reset modem to default values. 
			Defaults come from NVRAM if &W0 is 
			set, from factory ROM if &W1 is 
			set.

,	In Dial 	Dialing Pause
	command		Comma; causes dialing pause for 
			time set by S8.

:	At end of 	Continuous Redial
	Dial command	Colon; causes continuous redial (10 
			in DOC modems) of number until 
			answered. Not used in ZDXK or ZDXI 
			modems.

;	At end of 	Return to Command Mode After 
	Dial command	Dialing
			Semi-colon; causes immediate return 
			to command mode after dialing.

!	In Dial 	Flash On-Hook 
 	command		Exclamation point; causes modem to 
			flash on-hook.

@	In Dial 	Quiet Answer
	command		Causes modem to wait for a ring-
			back, then 5 seconds of silence 
			before processing next part of 
			command.

+++AT<CR>		Escape Code
			Puts modem in command mode while 
			still remaining on-line. Enter +++ 
			followed by the letters A and T, up 
			to ten command characters (or as 
			defined by S34), and a RETURN.

<BREAK>AT<CR>		Break Escape Code
			Alternate escape method. Puts modem 
			in command mode while still remain-
			ing on-line. Enter BREAK followed 
			by the letters A and T, up to sixty 
			command characters, and a RETURN.



Appendix D  S-Register Summary
------------------------------

Regis-
ter 	Unit	Range	Default	Description 
------------------------------------------------------------
S0	1 ring	0-255	 1	Sets number of rings until 
				modem answers.

S1	1 ring	0-255	 0	Counts rings that have 
				occurred.

S2	decimal	0-127	43 (+)	Sets escape code character.

S3	decimal	0-127	13 (^M)	Sets character recognized 
				as carriage return.

S4	decimal	0-127	10 (^J)	Sets character recognized 
				as line feed.

S5	decimal	0-127	 8 (^H)	Sets character recognized 
				as backspace.

S6	1 sec.	2-255	 2	Determines wait-time for
		4-255**	 4**	dial tone.
		4-7***	 4***	

S7	1 sec.	1-255	45 	Determines how long modem
		1-45*	45* 	will wait for carrier 
		1-55**	55**	before aborting call.

S8	1 sec.	0-255	 2	Sets pause time caused by a 
		4-255**	 4**	comma character in a dial 
		4-7***	 4***	command.

S9	100 ms	1-255	 6	Sets carrier detect 
				response time.

S10	100 ms	0-255	 7	Sets delay time between 
				when carrier is lost and 
				when modem disconnects.

S11	1 ms   1-255	70	Sets time duration of and
	       80-255*	80*	spacing between tones in 
	       80-255** 80**	tone-dialing.

S13	decimal	0-127	37 (%)	Determines remote config-
				uration escape character.

S17	10 ms	1-255	25	Determines length of break 
				time (space) sent to local 
				PC.

S24	50 ms	0-255	20	Sets DSR/CTS/CD dropout 
				time. 20 default equals one 
				second.

S25	100 ms	0, 1-255 0	Sets DTR dropout time. 0 
				default equals 50 ms.

S30	1 min.	0-255	 0	Inactivity timer used to 
				disconnect modem.

S32	100 ms	0-255	20	Sets duration in which mo-
				dem will wait for a RETURN 
				to be entered during escape 
				sequence execution.

S34	No. of 	0-60	10	Buffer length of command
	charac-			mode after on-line escape
	ters.			sequence.

S36	1 sec.	0-255	 0	Time between DTR inactive 
				and modem off-hook.

S37	1 sec.	0-255	 5	Time between DTR active and 
				modem on-hook.

S48	decimal	28, 26,	 0	Maximum V.34 connect speed.
		21, 19,		
		16, 14,		
		12, 96,		
		or 48
------------------------------------------------------------
*   Values for ZDXI modems and ZDX-DOC modems only.
**  Values for ZDXI modems only. 
*** Values for ZDXK modems only.



APPENDIX E  RESULT CODE SUMMARY
-------------------------------

Terse  Verbose		Meaning
------------------------------------------------------------
0      OK		Command was executed without error, 
			ready for next command.

1      CONNECT		Modem has detected carrier and gone 
			on-line.

2      RING		Modem has detected ring caused by 
			incoming call.

3      NO CARRIER	No carrier signal has been detected 
			within allowed time.

4      ERROR		Error in command line (too many, or 
			invalid characters).

5      CONNECT 1200	Modem has detected carrier at 1200 
			bps and gone on-line.

6      NO DIAL TONE	No dial tone has been detected.

7      BUSY		A busy signal has been detected.

8      NO ANSWER 	Remote system did not answer.

9      CONNECT 2400	Modem has detected carrier at 2400 
			bps and gone on-line.

10     (Not used)		

11     CONNECT 4800	Modem has detected carrier at 4800 
			bps and gone on-line.

12     CONNECT 9600	Modem has detected carrier at 9600 
			bps and gone on-line.

13     CONNECT 14400	Modem has detected carrier at 14400 
			bps and gone on-line.

19     CONNECT 19200	Modem has detected carrier at 19200 
			bps and gone on-line.

21     CONNECT 21600	Modem has detected carrier at 21600 
			bps and gone on-line.

24     CONNECT 24000	Modem has detected carrier at 24000 
			bps and gone on-line.

26     CONNECT 26400	Modem has detected carrier at 26400 
			bps and gone on-line.

28     CONNECT 28800	Modem has detected carrier at 28800 
			bps and gone on-line.
------------------------------------------------------------


Reliable Mode: If the ZDX is used in Reliable mode, the 
following responses change:
------------------------------------------------------------
1R	CONNECT 	(As above, except Reliable.)
	RELIABLE

5R	CONNECT 1200 	(As above, except Reliable.)
	RELIABLE

9R	CONNECT 2400 	(As above, except Reliable.)
	RELIABLE

11R	CONNECT 4800 	(As above, except Reliable.)
	RELIABLE

12R	CONNECT 9600 	(As above, except Reliable.)
	RELIABLE

13R	CONNECT 14400 	(As above, except Reliable.)
	RELIABLE

19R	CONNECT 19200 	(As above, except Reliable.)
	RELIABLE

21R	CONNECT 19200 	(As above, except Reliable.)
	RELIABLE

24R	CONNECT 24000 	(As above, except Reliable.)
	RELIABLE

26R	CONNECT 26400 	(As above, except Reliable.)
	RELIABLE

28R	CONNECT 28800 	(As above, except Reliable.)
	RELIABLE
------------------------------------------------------------


LAP-M Reliable mode: If the ZDX is used in LAP-M Reliable 
mode, the following responses change:
------------------------------------------------------------
1L	CONNECT LAPM	     (As above, except Reliable.)

5L	CONNECT 1200 LAPM    (As above, except Reliable.)

9L	CONNECT 2400 LAPM    (As above, except Reliable.)

11L	CONNECT 4800 LAPM    (As above, except Reliable.)

12L	CONNECT 9600 LAPM    (As above, except Reliable.)

13L	CONNECT 14400 LAPM   (As above, except Reliable.)

19L	CONNECT 19200 LAPM   (As above, except Reliable.)

21L	CONNECT 21600 LAPM   (As above, except Reliable.)

24L	CONNECT 24000 LAPM   (As above, except Reliable.)

26L	CONNECT 26400 LAPM   (As above, except Reliable.)

28L	CONNECT 28800 LAPM   (As above, except Reliable.)
------------------------------------------------------------


Data Compression: If the ZDX is used with data compression, 
the word COMPRESSED or letter C will be added to result 
codes 1, 5, 9, 11, 12, 13, 19, 21, 24, 26, and 28.


Standard AT Responses: If standard AT command set 2400 re-
sponses are selected with the &Q1 command, the following 
responses change:
------------------------------------------------------------
9			(Not used)

10	CONNECT 2400	Modem has detected carrier at 2400 
			bps and gone on-line.
------------------------------------------------------------



APPENDIX F  MULTI-TECH SYSTEMS' ESCAPE METHODS
----------------------------------------------


INTRODUCTION

You may sometimes find it necessary to issue AT commands to 
your modem, while you are on-line with a remote modem, with-
out disconnecting the call. If so, you will want to take ad-
vantage of escape methods that allow you to change the mo-
dem's mode of operation from on-line mode to command mode. 
After you issue your AT commands, you may return to on-line 
mode, but typically most users escape so that they may hang 
up a modem upon completion of a call.

Our modems offer two escape methods: in-band and out-of-
band. Both incorporate Time Independent Escape Sequence 
(TIES) methodology. An escape sequence is a pattern or se-
quence that the modem recognizes as its signal to shift from 
on-line mode to command mode. "Time independent" means that 
the modem recognizes the escape sequence without a prefixed 
and/or suffixed delay.

In an in-band escape, the modem recognizes the escape se-
quence as a pattern sent to it as part of the data stream or 
band (hence its name).

In an out-of-band escape, the escape sequence is a pattern 
that cannot and does not occur in the data stream.

Our in-band escape method is +++AT<CR>

Our out-of-band escape method is <BREAK>AT<CR>

A break signal cannot be sent as part of a data file; in-
stead it is sent by a direct program command to the UART 
used by the computer.

The break signal is defined as either the transmission of 
binary 0 for a minimum of 10 bits; or as a minimum interval 
of 135 milliseconds as established in the ITU X.28 standard. 
There are routines in high level languages and keys on most 
computers that have been established to send BREAK for fixed 
intervals, but you may control the break's duration by re-
ferring to your UART's specifications.


HOW TO SELECT AN ESCAPE METHOD

If you want your modem to escape and then wait for you to 
issue a command before it will return to on-line mode, then 
use +++AT<CR>. You might use this method if you find you 
need to review a help screen in the middle of a communica-
tions session. 

If you wish to combine the escape with a command (or com-
mands) and with an immediate return to on-line mode, then 
use the <BREAK>AT<CR> method. You may also use this method 
to have the modem wait before it will return on-line.

The following AT commands are used to select the modem's 
escape method: 

%E0 = Escape disabled		%E3 = Both methods enabled
%E1 = +++AT method (default)	%E4 = Disable "OK" to +++ 
				      escape
%E2 = BREAK AT method 		%E5 = Enable "OK" to +++ 
				      escape


METHOD 1: +++AT<CR>

In the following example, a user who is in on-line mode 
decides to set S0 to 1 to configure the modem to answer on 
the first ring.

1.  The user enters the sequence +++AT<CR>. The sequence is 
    sent to the modem 

2.  The modem transmits the +++.

3.  The modem buffers AT and starts the Wait for <CR> Timer. 

4.  Upon receiving the <CR> the modem escapes to command 
    mode. 

5.  The modem responds OK.

6.  The user enters the command ATS0=1<CR>. This sequence is 
    sent to the modem. 

7.  The modem buffers ATS0=1<CR> and identifies it as a 
    valid command.

8.  The modem executes the command, setting S0=1. 

9.  The user sends ATO<CR> to the modem.

10. The modem returns to on-line mode. 


ESCAPE METHOD 2: <BREAK>AT<CR>

In the following example, a user who is in on-line mode 
decides to set S0 to 1, to configure the modem to answer on 
the first ring.

1.  The BREAK signal is sent to the modem.

2.  The modem buffers BREAK .

3.  The modem starts the S32 Wait for <CR> Timer.

4.  ATS0=1<CR> is sent to the modem.

5.  The modem buffers ATS0=1<CR> and identifies it as a 
    valid command.

6.  The modem escapes to command mode.

7.  The modem executes the command, setting S0=1.

8.  The modem returns to on-line mode.


S-REGISTERS AND ESCAPE SEQUENCE

There are two S-Registers that you may set to modify the 
functioning of your escape sequences. The first is S-
Register S32, which establishes a value for how much time 
may elapse between the receipt of the beginning of the 
escape sequence, whether BREAK AT or +++AT, and the receipt 
of a <CR>. This interval is known as wait-for-<CR>-time, or 
BREAK passthrough.

You may assign a value to S32 in increments of 100 milli-
seconds. The default value is 20, or 2 seconds.

In the +++AT<CR> method, the wait-for-<CR>-time interval 
begins once the A in +++AT is received by the modem.

In the BREAK AT method, the wait-for-<CR>-time interval 
begins once the modem has received the break signal.

You may use S-Register S34 in conjunction with our in-band 
escape sequence, +++AT<CR>, to establish the maximum number 
of characters that your modem can buffer following an AT, 
before a <CR> must be received. The default value is 10 
characters. Do not confuse this buffer size with our regular 
command mode buffer length of 60 characters.

S-Register S34 does not affect our out-of-band escape se-
quence's buffer length, which is fixed at 60 characters.


ABORTING AN ESCAPE SEQUENCE

The +++AT<CR> escape will abort if you do not issue a <CR> 
before the wait-for-<CR>-time interval expires.

The <BREAK>AT<CR> method will also abort if you do not issue 
a <CR> before the wait-for-<CR>-time interval expires, and 
also if any of the following occurs:

1.  An illegal sequence is detected, including:

    a)	A character other than A follows the BREAK ;

    b)	A character other than T follows BREAK A;

    c)	Two BREAK s are received in succession;

2.  The command buffer overflows before a <CR> occurs. 

If you have any questions regarding the information in this 
appendix, please call Tech Support (Chapter 7). 



APPENDIX G  ASCII CONVERSION CHART
----------------------------------

Ctrl Code Hex Dec |Code Hex Dec |Code Hex Dec |Code Hex Dec
------------------------------------------------------------
 @   NUL  00  0   | SP  20  32  | @   40  64  | '   60  96
 A   SOH  01  1   | !   21  33  | A   41  65  | a   61  97
 B   STX  02  2   | "   22  34  | B   42  66  | b   62  98
 C   ETX  03  3   | #   23  35  | C   43  67  | c   63  99
 D   EOT  04  4   | $   24  36  | D   44  68  | d   64  100
 E   ENQ  05  5   | %   25  37  | E   45  69  | e   65  101
 F   ACK  06  6   | &   26  38  | F   46  70  | f   66  102
 G   BEL  07  7   | '   27  39  | G   47  71  | g   67  103
 H   BS   08  8   | (   28  40  | H   48  72  | h   68  104
 I   HT   09  9   | )   29  41  | I   49  73  | i   69  105
 J   LF   0A  10  | *   2A  42  | J   4A  74  | j   6A  106
 K   VT   0B  11  | =   2B  43  | K   4B  75  | k   6B  107
 L   FF   0C  12  | ,   2C  44  | L   4C  76  | l   6C  108
 M   CR   0D  13  | -   2D  45  | M   4D  77  | m   6D  109
 N   SO   0E  14  | .   2E  46  | N   4E  78  | n   6E  110
 O   SI   0F  15  | /   2F  47  | O   4F  79  | o   6F  111
 P   DLE  10  16  | 0   30  48  | P   50  80  | p   70  112
 Q   DC1  11  17  | 1   31  49  | Q   51  81  | q   71  113
 R   DC2  12  18  | 2   32  50  | R   52  82  | r   72  114
 S   DC3  13  19  | 3   33  51  | S   53  83  | s   73  115
 T   DC4  14  20  | 4   34  52  | T   54  84  | t   74  116
 U   NAK  15  21  | 5   35  53  | U   55  85  | u   75  117
 V   SYN  16  22  | 6   36  54  | V   56  86  | v   76  118
 W   ETB  17  23  | 7   37  55  | W   57  87  | w   77  119
 X   CAN  18  24  | 8   38  56  | X   58  88  | x   78  120
 Y   EM   19  25  | 9   39  57  | Y   59  89  | y   79  121
 Z   SUB  1A  26  | :   3A  58  | Z   5A  90  | z   7A  122
 [   ESC  1B  27  | ;   3B  59  | [   5B  91  | {   7B  123
 \   FS   1C  28  | <   3C  60  | \   5C  92  | |   7C  124
 ]   GS   1D  29  | =   3D  61  | ]   5D  93  | }   7D  125
 ^   RS   1E  30  | >   3E  62  | ^   5E  94  | -   7E  126
 --  US   1F  31  | ?   3F  63  | _   5F  95  |DEL  7F  127
------------------------------------------------------------



APPENDIX H  RS-232C INTERFACE
-----------------------------


RS-232C SPECIFICATIONS

The ZDX's RS-232C interface circuits have been designed to 
meet the electrical specifications given in EIA (Electronic 
Industries Association) RS-232C standards. All signals gen-
erated by the modem are approximately 10 volts when measured 
across a load of 3000 ohms or greater. The receiving cir-
cuits of the modem accept signals in the 3- to 25-volt 
range. The voltage thresholds are:

Negative:  Voltage more negative than 3 volts with respect 
	   to signal ground
Positive:  Voltage more positive than +3 volts with respect 
	   to signal ground


		Table H-1.  Signal polarities

Signal Information:		Negative	Positive 
------------------------------------------------------------
Binary State			One		Zero
Signal Condition 		Mark		Space
Control and Timing Function	Off		On
------------------------------------------------------------


All modem circuits that accept signals from the data proc-
essing terminal or CPU equipment have DC input impedances of 
4.7K. For more specific details, consult the EIA RS-232C 
standard.

The following chart lists the EIA RS-232C signals present on 
the ZDX's RS-232C interface connector. All other pins are 
unused.


	   Table H-2.  RS-232C Pin Assignments

     Multi-Tech	    EIA	   Signal	
Pin  Designation  Circuit  Source*  Circuit Function
------------------------------------------------------------
 1	 PG	    -- 	     --     Protective Ground
 2	 TD	    BA	    DTE     Transmitted Data
 3	 RD	    BB	    DCE     Received Data
 4	 RTS	    CA	    DTE     Request to Send
 5	 CTS	    CB	    DCE     Clear to Send
 6	 DSR	    CC	    DCE     Data Set Ready
 7	 SG	    AB	     --     Signal Ground
 8	 CD	    CF	    DCE     Carrier Detect
 9	 +V	    +V	    DCE     Test Voltage
20    DTR or TR	    CD	    DTE     Data Terminal Ready
22	 RI	    CE	    DCE     Ring Indicator
------------------------------------------------------------
* DTE = Data Terminal Equipment (terminal or computer)
  DCE = Data Communications Equipment (the modem)


The computer or terminal should be supplied with a cable 
terminated with a Cinch DB25P (or equivalent) connector 
mounted in a Cinch DB51226-1 (or equivalent) hood assembly 
as specified by the EIA RS232C standard.


FUNCTIONAL DESCRIPTION OF ZDX RS-232C SIGNALS:

Transmitted Data -- Pin 2, TD (BA)
Direction: To modem

Signals on this circuit are generated by a local computer or 
terminal and passed to the ZDX 's transmitter. A positive 
signal is a space (binary 0) and a negative signal is a mark 
(binary 1). The transmitting terminal should hold this line 
in the marking state when no data is being transmitted, in-
cluding intervals between characters or words. The TD LED 
indicates the status of this circuit.


Received Data -- Pin 3, RD (BB)
Direction: From modem

This circuit is the data output of the ZDX. Data signals 
received from the remote modem are passed to the local com-
puter on this line. When no carrier signal is being received 
(pin 8 negative), this line will be forced into a marking 
condition. The RD LED indicates the status of this signal.


Request To Send -- Pin 4, RTS (CA)
Direction: To modem

This signal from the terminal tells the ZDX that the termi-
nal has data that it wants to transmit.


Clear To Send -- Pin 5, CTS (CB)
Direction: From modem

This line tells the terminal that the ZDX is ready to trans-
mit any data present on the Transmit Data line (pin 2). 


Data Set Ready -- Pin 6, DSR (CC) 
Direction: From modem

This signal indicates that the ZDX is in the data mode and 
is connected to the communications channel. This signal will 
be on during analog-loop-test mode.


Signal Ground -- Pin 7, SG (AB)

The SG pin is connected to the ZDX's signal ground. It 
establishes the common ground reference for all other 
interface signals.


Carrier Detect -- Pin 8, CD (CF) 
Direction: From modem

CD on (positive voltage) indicates that a carrier signal has 
been received from the other modem. This circuit does not 
typically turn on in the presence of message circuit noise 
or out-of-band signals. There is a one second delay between 
when the carrier tone has been detected and when the CD cir-
cuit is turned on. 

This signal goes off if the received data carrier falls 
below the receiver threshold for more than 37 ms. When CD is 
off, the Received Data circuit (pin 3) is held to the 
marking state.


Test Voltage -- Pin 9, +V
Direction: from modem

This test pin has 330 ohms of resistance to +12 volts DC. It 
may be used to strap other signals high. For example, if the 
terminal does not supply a DTR (Data Terminal Ready) signal, 
pin 9 may be jumpered to pin 20 (DTR) in the RS232 connector 
or cable to force DTR on.


Data Terminal Ready -- Pin 20, DTR (CD)
Direction: To modem

The DTR (TR) signal provides a means for the terminal or 
computer to control the ZDX's connection to the communica-
tions channel. A high DTR signal is required by the modem to 
be able to communicate. Turning DTR off for more than 50 
milliseconds will force the modem to disconnect.

The most common use of DTR is in automatic answer applica-
tions. A high DTR signal is required by the modem to answer 
a call. A frequently used method is to have the computer 
turn on DTR in response to RI (Ring Indicator), which allows 
the modem to answer. Later, DTR is turned off at the conclu-
sion of the log-off procedure, which forces the modem to 
disconnect, enabling it to receive another call. In non-auto 
answer applications, it is advisable to leave DTR on. This 
is the standard factory setting. An alternative is to pro-
vide a constant high DTR from the terminal or computer. The 
TR LED indicates the status of this signal.


Ring Indicator -- Pin 22, RI (CE)
Direction: From modem

This signal remains on for the duration of the ringing sig-
nal. When a ring signal is received by the modem, the modem 
automatically answers after the first ring. The modem can 
answer after a specified number of rings, which can be pro-
grammed in AT command mode, but if none is specified, the 
modem answers after the first ring.



RS-232C CABLES

Serial cables can be purchased for your ZDX wherever comput-
ers are sold. The following diagrams illustrate the three 
cables most commonly used with the ZDX.


IBM PCS AND COMPATIBLES

IBM personal computers typically have DB-9P connectors on 
their serial ports. PC-compatible computers, however, may 
have either a DB-9P connector, a DB-25S connector, or one of 
each. In both cases, the connections are straight-forward. 
See Fig. H-1 and H-2.


    DTE DB-25				DCE DB-25
    connector				connector
   |---------				---------|
   |  TD  2 |---------------------------| 2  TD  |
   |  RD  3 |---------------------------| 3  RD  |
   | RTS  4 |---------------------------| 4  RTS |
   | CTS  5 |---------------------------| 5  CTS |
   | DSR  6 |---------------------------| 6  DSR |
   | GND  7 |---------------------------| 7  GND |
   |  CD  8 |---------------------------| 8  CD  |
   | DTR 20 |---------------------------| 20 DTR |
   |  RI 22 |---------------------------| 22 RI  |
   |---------				---------|

      Fig. H-1.  DB-25P-to-DB-25P serial cable


    DTE DB-9				DCE DB-25
    connector				connector
   |---------				---------|
   |  TD  3 |---------------------------| 2  TD  |
   |  RD  2 |---------------------------| 3  RD  |
   | RTS  7 |---------------------------| 4  RTS |
   | CTS  8 |---------------------------| 5  CTS |
   | DSR  6 |---------------------------| 6  DSR |
   | GND  5 |---------------------------| 7  GND |
   |  CD  1 |---------------------------| 8  CD  |
   | DTR  4 |---------------------------| 20 DTR |
   |  RI  9 |---------------------------| 22 RI  |
   |---------				---------|

      Fig. H-2.  DB-9S-to-DB-25P serial cable



APPLE MACINTOSH COMPUTERS

Macintosh serial cables are complicated by the fact that 
Macintoshes follow the RS-422 protocol for serial communica-
tions instead of the RS-232C protocol. Additionally, the 
128K and 512K Macintoshes use a DB-9 connector and do not 
support RTS/CTS hardware flow control, whereas all later 
Macintoshes use a circular mini-DIN 8-pin connector and sup-
port hardware flow control. The cable in Fig. H-3 supports 
RTS/CTS flow control in the Macintosh Plus and later 
Macintoshes. The connection of DIN-8 pin 1 to DB-25 pin 20 
gives you the option of using a DTR disconnect. You cannot 
have DTR and RTS enabled at the same time, however. Nor-
mally, DTR is turned off by including &D0 in the initializa-
tion string.


  DTE Mini-DIN-8 			DCE DB-25
    connector				connector
    -------- 				---------|
   | TxD- 3 |---------------------------| 2  TD  |
   | RxD- 5 |---------------------------| 3  RD  |
   | GND  4 |---+		    +---| 4  RTS |
   | HSKi 2 |---|-------------------|---| 5  CTS |
   | (CTS)  |   |		    |   | 6  DSR |
   | RxD+ 8 |---+-------------------|---| 7  GND |
   |        |			    |   | 8  CD  |
   | HSKo 1 |-----------------------+---| 20 DTR |
   | (RTS)  |				| 22 RI  |
    -------- 				---------|

         Fig. H-3.  Macintosh serial cable


                      6   7   8 

                     3   4     5 

                        1   2

      Fig. H-4.  Mini-DIN-8 pin identification



Appendix I  FCC, DOC, and BABT Information
------------------------------------------


FCC REGULATIONS FOR TELEPHONE LINE INTERCONNECTION

This equipment complies with Part 68 of the Federal Communi-
cations Commission (FCC) rules. On the outside surface of 
this equipment is a label that contains, among other infor-
mation, the FCC registration number and ringer equivalence 
number (REN). If requested, this information must be pro-
vided to the telephone company.

The suitable USOC jack (Universal Service Order Code con-
necting arrangement) for this equipment is shown below.

The ringer equivalence number (REN) is used to determine the 
quantity of devices that may be connected to the telephone 
line. Excessive RENs on the telephone line may result in the 
devices not ringing in response to an incoming call. In 
most, but not all areas, the sum of the RENs should not 
exceed five (5.0). To learn the number of devices that may 
be connected to the line, contact the telephone company to 
determine the maximum REN for the calling area.

If this equipment causes harm to the telephone network, the 
telephone company will notify you in advance. But if advance 
notice isn't practical, the telephone company will notify 
you as soon as possible. Also, you will be advised of your 
right to file a complaint with the FCC if you believe it is 
necessary.

The telephone company may make changes in its facilities, 
equipment, operations, or procedures that could affect the 
operation of the equipment. If this happens, the telephone 
company will provide advance notice in order for you to make 
necessary modifications in order to maintain uninterrupted 
service.

If trouble is experienced with this equipment (the model of 
which is indicated below) please contact Multi-Tech Systems, 
Inc. at the address shown below for details of how to have 
repairs made. If the trouble is causing harm to the tele-
phone network, the telephone company may request you remove 
the equipment from the network until the problem is 
resolved.

No repairs are to be made by you. Repairs are to be made 
only by Multi-Tech Systems or its licensees. Unauthorized 
repairs void registration and warranty.

This equipment cannot be used on the public coin service 
provided by the telephone company. Connection to Party Line 
Service is subject to state tariffs. (Contact the state 
public utility commission, public service commission or 
corporation commission for information.)


Manufacturer:		Multi-Tech Systems, Inc.
Model Number:		MT1432ZDX, MT1932ZDX, or MT2834ZDX
FCC Registration No:	AU7USA-75711-MM-E (MT1432ZDX and 
			MT1932ZDX) AU7USA-20673-MM-E 
			(MT2834ZDX)
Ringer Equivalence:	0.5B
Modular Jack (USOC):	RJ11C or RJ11W (single line) 

Service Center in USA:	Multi-Tech Systems Inc. 
			2205 Woodale Drive
			Mounds View, MN 55112
			(800) 328-9717 
			(612) 785-3500
			(612) 785-9874 FAX



DOC TERMINAL EQUIPMENT WARNINGS

NOTICE: The Canadian Department of Communications label 
identifies certificated equipment. This certification means 
that the equipment meets certain telecommunications network 
protective, operational and safety requirements. The Depart-
ment does not guarantee the equipment will operate to the 
user's satisfaction.

Before installing this equipment, users should insure that 
it is permissible to be connected to the facilities of the 
local telecommunications company. The equipment must also be 
installed using an acceptable method of connection. In some 
cases, the company's inside wiring associated with a single 
line individual service may be extended by means of a certi-
fied connector assembly (telephone extension cord). The cus-
tomer should be aware that compliance with the above condi-
tions may not prevent degradation of service in some 
situations.

Repairs to certified equipment should be made by an author-
ized Canadian maintenance facility designated by the sup-
plier. Any repairs or alterations made by the user to this 
equipment; or equipment malfunctions, may give the telecom-
munications company cause to request the user to disconnect 
the equipment.

Users should insure for their own protection that the elec-
trical ground connections of the power utility, telephone 
lines and internal metallic water pipe system, if present, 
are connected together. This precaution may be particularly 
important in rural areas.

The Load Number (LN) assigned to each terminal device de-
notes the percentage of the total load to be connected to a 
telephone loop which is used by the device, to prevent over-
loading. The termination on a loop may consist of any combi-
nation of devices subject only to the requirement that the 
total of the Load Numbers of all the devices does not exceed 
100. The Load Number for this product is 4.

CAUTION: Users should not attempt to make such connections 
themselves, but should contact the appropriate electric in-
spection authority, or electrician, as appropriate.

This digital apparatus does not exceed the Class B limits 
for radio noise for digital apparatus set out by the 
Department of Communications.



COMPLIANCE WITH BABT REQUIREMENTS

Approved for connection to telecommunications system speci-
fied in the instructions for use subject to the conditions 
set out in them.

EUROPEAN LOW VOLTAGE DIRECTIVE

When correctly installed and maintained, the modem will pre-
sent no hazard to the user. When correctly installed, the 
modem will be connected to the PSTN and to a Data Terminal 
Equipment (DTE), whose modem connections comply with ITU 
recommendations V.28. The DTE connections are therefore 
taken to be safe voltages (less than (30 volts).

COMPLIANCE WITH BS6305 CLAUSE 6.2, BS6320 CLAUSE 7.2, AND 
BABT/SITS/82/005S/D

a. The modem is suitable for connection to the Public 
   Switched Telephone Network (PSTN) provided by British 
   Telecommunications plc or Kingston Communications (Hull) 
   plc. Circuit supply by British Communications, Mercury 
   Communication, or Hull City Council. Only direct exchange 
   lines may be used, not shared service.

b. The modem is suitable for household, office, and similar 
   general indoor use. It is not suitable for use as an 
   extension to a pay phone.

c. BT lines supplied must support either loop disconnect or 
   multifrequency tone signaling.

d. REN (Ringer Equivalence Number).

   The REN value of a unit is calculated from 3/n where n is 
   the total number of units which can be connected in 
   parallel which will still cause the standard bell (as 
   defined in BS6305 Appendix D) to ring.

   REN values of less than 0.3 cannot be assigned.

   For apparatus which is not capable of forming part of 
   multiple installation, a REN value of 3 is assigned.

			REN = 1

   If a telephone or other device is connected in parallel 
   with the modem, the combined REN must not exceed 4. A BT 
   supplied telephone may be assumed to have REN of 1.0 
   unless otherwise noted.

The approval of this modem for connection to the British 
Telecom public switched telephone network is INVALIDATED if 
the apparatus is subject to any modification in any material 
way not authorized by BABT or if it is used with or con-
nected to:

i.  internal software that has not been formally accepted by 
    BABT.

ii. external control software or external control apparatus 
    which cause the operation of the modem associated call 
    set-up equipment to contravene the requirements of the 
    standard set out in BABT/SITS/82/005S/D.

All apparatus connected to this modem and thereby connected 
directly or indirectly to the British Telecom public 
switched telephone network must be approved apparatus as 
defined in Section 22 of the British Telecommunications Act 
1984.


COMPLIANCE WITH BS6789: SECTION 3.1 AND PART 2

a. The modem is not capable of allowing Auto Call using 
   '999' or other PABX emergency numbers.

b. Modes other than modes 1, 2, or 3 should not be used on 
   the BT PSTN. This modem is a mode 1 device.

c. Users are advised to check the numbers entered during the 
   Auto Call set up phase prior to dialing.

d. The user should not issue any sequence of commands to the 
   modem which would cause the modem to exceed the maximum 
   allowable pause of 8 seconds from the time the modem goes 
   off-hook until dialing begins.

e. For correct operation of the call progress monitor, the 
   power has to be properly connected and switched on.


COMPLIANCE WITH DTI 83/009

a. The apparatus is only approved for compatible PBXs. 
   Consult the supplier for an up-to-date list of compatible 
   PBXs.

b. There is no guarantee of correct working in all 
   circumstances. Any difficulties should be referred to 
   Multi-Tech Systems.

c. If sockets are required for connexion to the PBX, use the 
   BT post card only if BT owns the wiring to the PBX.

This apparatus has been approved for the use of the 
following facilities:

* Auto-calling
* Loop disconnect and MF dialing
* Phone number storage and retrieval by a predetermined code
* Operation in the absence of proceed indication
* Automatic storage of last number dialed
* Tone detection-busy
* Auto clear from the originating end
* DTR dialing
* Modem
* PBX timed break register recall

Any other usage will invalidate the approval of the appara-
tus if, as a result, it then ceases to comply with the stan-
dards against which approval was granted.



APPENDIX J  SOFTWARE USER LICENSE AGREEMENT
-------------------------------------------

The software contained in the MultiModemZDX package is li-
censed by Multi-Tech Systems, Inc. to the original end-user 
purchaser of the product, hereafter referred to as 
"Licensee." The License includes the distribution diskette, 
other accompanying programs, and the documentation.

The software programs and installation utilities, hereafter 
referred to as "Software," consists of the computer program 
files included on the original distribution diskettes.

Licensee agrees that by purchase and/or use of the Software, 
he hereby accepts and agrees to the terms of this License 
Agreement.

In consideration of mutual covenants contained herein, and 
other good and valuable considerations, the receipt and suf-
ficiency of which is acknowledged, Multi-Tech Systems, Inc. 
does hereby grant to the Licensee a non-transferable and 
non-exclusive license to use the Software and accompanying 
documentation on the following conditions and terms:

The software is furnished to the Licensee for execution and 
use on a single computer system only and may be copied (with 
the inclusion of the Multi-Tech Systems, Inc. copyright no-
tice) only for use on that computer system. The Licensee 
hereby agrees not to provide or otherwise make available any 
portion of this software in any form to any third party 
without prior express written approval of Multi-Tech 
Systems, Inc.

Licensee is hereby informed that this Software contains con-
fidential, proprietary, and valuable trade secrets developed 
by or licensed to Multi-Tech Systems, Inc. and agrees that 
sole ownership shall remain with Multi-Tech Systems, Inc.

The Software is copyrighted. Except as provided herein, the 
Software and documentation supplied under this agreement may 
not be copied, reproduced, published, licensed, sub-
licensed, distributed, transferred, or made available in any 
form, in whole or in part, to others, without express writ-
ten permission of Multi-Tech Systems, Inc. Copies of the 
Software may be made to replace worn or deteriorated copies 
for archival or back-up procedures.



APPENDIX K  DIAL PULSE AND TONE-DIAL FREQUENCIES
------------------------------------------------


DIAL PULSES

                   B
Closed ----+     +---+     +--------------------+     +----
           |  A  |   |     |                    |     |
Open       +-----+   +-----+                    +-----+
                Digit 2                         Digit 1

                   Fig. K-1.  Dial pulses


In the example above, the digit 2 is pulse dialed, followed 
by the digit 1. Each pulse consists of an A ms Open and a B 
ms Closed, Where A is either 60 or 67 ms, and B is either 40 
or 33 ms, for a total of 100 ms per pulse, or a rate of 10 
pulses per second. The interdigital pause time is 800 ms. 
The pulse ratios are controlled by the &P command. 

TONE DIAL FREQUENCIES

The tone dialing method combines two frequencies for each of 
the twelve digits found on a Touch Tone dial pad. This 
method is referred to as Dual-Tone Multi-Frequency (DTMF) 
dialing. 

The four horizontal rows on a touch-tone keypad use the four 
low-frequency (697, 770, 852, 941 Hz), while the three ver-
tical columns use the three high frequencies (1209, 1336, 
1477 Hz). The generally-accepted tone frequency tolerance is 
+/- 0.02%. 

For example, the digit 4 would be dialed by combining two 
tone frequencies. It would use the frequency 770 Hz from the 
second row, and the frequency 1209 Hz from the first column. 
In another example, the digit 9 would be dialed with the 
tone frequencies of 852 Hz and 1477 Hz.


		Hz         Digits
		       
		     +---+  +---+  +---+
		697  | 1 |  | 2 |  | 3 |
		     +---+  +---+  +---+
		     +---+  +---+  +---+
		770  | 4 |  | 5 |  | 6 |
		     +---+  +---+  +---+
		     +---+  +---+  +---+
		852  | 7 |  | 8 |  | 9 |
		     +---+  +---+  +---+
		     +---+  +---+  +---+
		941  | * |  | 0 |  | # |
		     +---+  +---+  +---+

		Hz   1209   1336   1477

         Fig. K-2.  Standard DTMF frequencies


The extended DTMF characters (A, B, C, D) are the high-end 
frequencies (1633 Hz) defined on some telephone sets with a 
fourth vertical column of buttons. This fourth column pro-
vides for extended PBX control functions; the actual func-
tions provided depend on the PBX manufacturer's implementa-
tion and feature set. 


	      Hz            Digits

	           +---+  +---+  +---+  +---+
	      697  | 1 |  | 2 |  | 3 |  | A |
	           +---+  +---+  +---+  +---+
	           +---+  +---+  +---+  +---+
	      770  | 4 |  | 5 |  | 6 |  | B |
	           +---+  +---+  +---+  +---+
	           +---+  +---+  +---+  +---+
	      852  | 7 |  | 8 |  | 9 |  | C |
	           +---+  +---+  +---+  +---+
	           +---+  +---+  +---+  +---+
	      941  | * |  | 0 |  | # |  | D |
	           +---+  +---+  +---+  +---+
  
	      Hz   1209   1336   1477   1633

           Fig. K-3.  Extended DTMF frequencies



------------------------------------------------------------

GLOSSARY

------------------------------------------------------------


Algorithm

A sequence of mathematical processes that must be performed 
in order to obtain a desired numerical value. An algorithm 
may also be known as a complex formula or function which is 
usually executed by a computer.
 

Amplitude

The difference between the maximum and minimum voltages of a 
waveform expressed as a "peak-to-peak" voltage.


Analog Loopback

A test performed on the internal circuitry of either the lo-
cal modem (via the local analog loopback), or of the remote 
modem (via the remote analog loopback). Analog loopback is a 
feature included in some modems for diagnostic purposes.


Analog Signal

A waveform that has amplitude, frequency, and phase, and 
that takes on a range of values between its maximum and 
minimum values. Analog is the continuous movement from point 
A to point B, as opposed to discrete jumps.


Analog Transmission

One of two types of telecommunications which uses an analog 
signal as a carrier of data, voice, video, et al. An analog 
signal becomes a carrier when it is modulated by altering 
its phase, amplitude, or frequency to correspond with the 
source signal. Compare with digital transmission.


ANSI (American National Standards Institute)

An American standards committee best known for its ANSI code 
character set. This committee operates on a national level 
developing and publishing standards for the United States, 
and it has contributed to the International Standards 
Organization (ISO) for the development of world-wide 
standards.


Application Software

A program that interfaces between the user and a computer's 
machine (assembly) language to perform data input/output, 
processing, or manipulation. The applications software 
accepts input from the user and converts the requests into a 
format that can be processed by the computer. Common appli-
cation software packages include word processors, databases, 
spreadsheets, datacomm, and fax programs. 


ASCII (American Standard Code for Information Interchange)

A hexadecimal (base 16) numbering code used to represent 
numbers, letters, punctuation, and control characters. The 
basic ASCII code is a 7-bit character set that defines 127 
characters. The extended 8-bit ASCII set defines 255 charac-
ters, but is not as standardized as the basic set, which is 
recognized by most computers, editors, and word processors.


Asynchronous Transmission

The oldest, cheapest method of data transfer, used in both 
analog and digital transmissions. Async communications are 
timed by start and stop bits that frame each character 
instead of by using a clocking signal; thus, it is also 
referred to as a character-based protocol. Although asyn-
chronous transmissions have been considered less efficient 
than synchronous transmissions (due to the overhead of the 
timing bits), advances in data compression (which are not 
applicable to synchronous data streams) have narrowed the 
gap.


Auto Answer

A feature offered with some modems that enables the modem to 
go "off-hook" when it detects an incoming call. This option 
can usually be toggled on or off in addition to being con-
figured for the number of rings the modem will ignore before 
going off-hook.


Autobaud

A feature offered on some modems that allows the modem to 
dynamically negotiate the speed used over the datacomm link. 
This applies to the initial call and during the call, if the 
quality of the line degenerates.


Band

The range of frequencies between two defined end points.


Bandwidth

The range of usable frequencies that a transmission medium 
will pass without unacceptable attenuation or distortion. 
Bandwidth is a factor in determining the amount of informa-
tion and the speed at which a medium can transmit.


Baseband Signal

An unmodulated signal that is transmitted at its original 
frequency. Baseband signals may be digital or analog.


Baud Adjust

A feature available in some modems that enables the modem to 
detect the maximum speed at which another modem is capable 
of transmitting, and to adjust its own "baud rate" accord-
ingly. The baud adjust feature changes both the link speed 
and the serial port baud rate.


Baud Rate

A synonym for the frequency of the carrier signal used in 
data transmission. It refers to the number of cycles per 
second of a carrier signal. When a modulator uses one cycle 
of the carrier to transmit one bit, the baud rate and the 
transmission speed (in bits per second -- bps) are the same.


Bell 103

A de facto standard used by American modem makers for 300 
bps transmission.


Bell 212A

A de facto standard used by American modem makers for 1200 
bps transmission.


Binary

A numbering system based on two digits, usually represented 
as 0 and 1, that has positional weighting based on powers of 
two. Binary also refers to a file format that uses 8-bit 
characters, to allow for control characters (e.g., .BAT, 
.EXE, or .COM files), as opposed to ASCII files which allow 
only for text and punctuation.


Bit (Binary Digit)

A bit, or binary digit, is the basis of the binary numbering 
system. It can take a value of one or zero. The high-low 
nature of digital pulses lends itself well to encoding data 
as bits. Bits are generally recognized as electrical charges 
generated or stored by a computer that represent usable 
information.


Block Check Character (BCC)

An error control method used in character-oriented or byte-
synchronous protocols. Two eight-bit BCCs are used to create 
the cyclic redundancy check (CRC) field of a synchronous 
data packet.


Bps (Bits Per Second)

A unit of the speed at which data bits can be transmitted 
and/or received. Bps differs from baud rate when more than 
one bit is represented by a single cycle of the carrier.


Buffer

A temporary storage register, or section of memory, used in 
data communications to prevent data from being lost due to 
differences in transmission speed. Keyboards, serial ports, 
modems, muxes, and printers are a few examples of devices 
that include buffers. A buffer allows a device to dump data 
at high speed for a low-speed device to accept at its own 
rate. In this way, the high-speed device can continue its 
work without having to keep pace with the slower device. 


Byte

A term coined to represent the number of bits a computer 
could "bite off" at one time. Since many of the original 
microcomputers had 8-bit addresses and data buses, this was 
their "bite size." The original definition is now usually 
applied to the term word, while byte has come to mean a 
group of eight bits.


Carrier Signal

An analog signal with known frequency, amplitude, and/or 
phase characteristics used to transport information. By 
knowing the original characteristics of the carrier signal, 
a receiver can interpret any changes as modulation, and 
thereby recover the encoded information.


CCITT (International Consultative Committee on Telephone and 
Telegraph)

An advisory group of the International Telecommunication 
Union (ITU), created and controlled by the United Nations, 
whose purpose was to develop and publish recommendations for 
world-wide standardization of telecommunications devices. In 
1993 the CCITT became the ITU-T group of the ITU, and the 
CCITT Secretariat became the Telecommunication Standardiza-
tion Bureau (TSB).


Checksum

A control field found in synchronous data packets that 
contains the results of the error control algorithm used.


Clock

A timing signal generated by an oscillating circuit that is 
used to synchronize data transmissions.


COM Port

The standard serial port available on a PC. PCs usually have 
two serial ports, which are referred to as COM1 and COM2.


Command Mode

One of two states of an intelligent (i.e., programmable) 
DCE, the other being the on-line mode. The mode in which 
commands can be issued to alter operating parameters.


Cyclic Redundancy Check (CRC)

A field used in packetized data that contains two 8-bit 
block check characters (BCCs) as the binary result of an 
algorithm performed on the data bits in the package. A CRC 
is used for error-detection by many synchronous protocols.


DB-25

A very common 25-pin connector often referred to as an RS-
232C connector because it is the connector over which an RS-
232C interface commonly occurs.


Data Bits

One of the variables used in asynchronous data transmission 
used to define the contents of a character. Depending on the 
character code and/or data compression used, each character 
may be represented by 5, 6, 7, or 8 bits.


Data Communications Equipment (DCE)

Any device that serves as the portal of entry from the user 
equipment to a telecommunications facility. A modem is a DCE 
for the public telephone network that is commonly on site at 
the user's premises. Packet switched networks have another 
level of DCE which is most often located at a central 
office.


Data Compression

The process of reducing the number of data bits necessary to 
represent useful information. By compressing files, less 
storage is used and higher throughputs are realized.


Data Pump

The digital-to-analog conversion (DAC), or modulation 
circuitry used within a modem.


Data Terminal Equipment (DCE)

Any device in a network that generates, stores, or displays 
user information. DTE is a telecommunications term that 
usually refers to PCs, terminals, printers, etc.


De Facto Standard

A de facto standard is one of two types of voluntary stan-
dards recognized by a given market. It is introduced by a 
single vendor and becomes a standard by its widespread use 
and acceptance by other vendors. AT&T's Bell 212A, IBM's 
Binary Synchronous Protocol, and DEC's VT100 terminal proto-
col are examples of de facto standards. Compare with de jure 
standards.


De Jure Standard

A de jure standard is one of two types of voluntary stan-
dards. It represents the collective consensus of the indus-
try and users for a particular aspect of manufacturing. The 
ITU's V and X standards (e.g., V.32 and X.25) are examples 
of de jure standards. Compare with de facto standards.


Decibel (dB)

A unit of measurement for signal strength based on logarith-
mic increments. One dB is a tenth of a Bel.


Dedicated

A term used to describe a telecommunications facility or a 
piece of equipment that is used exclusively for a particular 
application. A dedicated telephone line (e.g., a leased 
line) is a permanent connection reserved for communication 
between two points. A dedicated server in a LAN is a com-
puter that cannot be used as a workstation in addition to 
its server operations.


Digital Transmission

A method of electronic information transmission that uses 
the binary numbering system to represent data. Analog sig-
nals are waveforms made of continuously varying voltages. 
Digital signals may be only "high" or "low" at any given 
time, making it easy for digital signals to be "cleaned up" 
(noise and distortion removed) by regenerating the signal 
during transmission.


Direct Current (DC)

DC is an analog or digital signal that may vary in voltage 
level but never crosses the reference voltage (usually 
called ground). Direct current is usually understood to mean 
a constant voltage supply that fluctuates only a nominal 
amount.


DPSK (Differential Phase Shift Keying)

An improved method of phase modulation that performs phase 
shifts relative to the source signal. This reduces the 
errors occurring as a result of phase jitter during 
transmission.


Driver

A driver is a software module used by the operating system 
to access a specific hardware device such as a color moni-
tor, a printer, or a hard disk.


DTMF (Dual-Tone Multifrequency)

A dialing method in which each push-button generates a 
unique combination of two tones. Also known as touch tone 
dialing.


Echo

The reflection, or duplication, of a signal back toward its 
source. Echoing is useful when a terminal is transmitting 
data, in that the data can be echoed to the screen so the 
user can monitor what is being sent. Echoing is undesirable 
when it refers to the signal that results on a telephone 
line from impedance mismatches.


Echo Cancellation

A procedure done by modems that are attempting to transmit 
in full-duplex mode over a two-wire telephone line at speeds 
of 4800 bps and faster. To achieve these higher speeds, the 
modem requires all of the bandwidth available on a voice-
grade line for transmission, so each modem transmits and 
receives on an 1800 Hz carrier. Using precise timing and 
cancellation circuits for the near and far echoes, an echo-
canceling modem removes its own echo from the received 
signal.


Emulation

The process of one type of device imitating another via a 
hardware or software package. Terminal emulation, for exam-
ple, allows a personal computer to access a mainframe com-
puter by generating and accepting data like a "dumb" 
terminal.


Environment

The connectivity scheme of terminals, printers, controllers, 
and the host in an on-line, active system. The elements of 
an environment establish connections and transfer data in 
the same way.


Error Correction

The process of detecting distorted data and requesting a 
retransmission or interpolating to correct the error. Errors 
may be introduced by line conditions or by external 
interference.


Facsimile (Fax)

A term used when referring to the bit-mapped rendition of a 
graphics-oriented document (fax) or to the electronic trans-
mission of the image over telephone lines (faxing). Fax 
transmission differs from data transmission in that the for-
mer is a bit-mapped approximation of a graphical document 
and, therefore, cannot be accurately interpreted according 
to any character code.


Fax Modem

A multifunction device capable of transmitting either data 
or faxes. Fax modems adhere to ITU and/or Bell standards for 
the modem circuitry and to ITU class and group specifica-
tions for the fax circuitry.


Flow Control

The process of regulating the speed at which data enters a 
serial port. Software flow control is implemented by commu-
nications software or by the user sending predefined charac-
ters or packets that are recognized as "pause" and "resume" 
indicators. Hardware flow control is achieved by using the 
Request to Send (RTS) and Clear to Send (CTS) control lines 
of the RS-232C interface.


Frequency

Frequency is the number of cycles or repetitions occurring 
in a given time period. Frequency is inversely proportional 
to the wavelength of the signal; i.e., long wavelength sig-
nals have low frequencies, while short wavelength signals 
yield high frequencies. The international unit for frequency 
is the hertz, abbreviated Hz, which is equal to one cycle 
per second.


FSK (Frequency Shift Keying)

A technique that assigns two different frequencies to repre-
sent the binary values of 0 and 1. This was the earliest 
type of modulation used by 300 bps modems.


Full-Duplex (FDX)

A method of establishing a connection that allows simultane-
ous, two-way transmission.


Half-Duplex (HDX)

A method of establishing a connection that allows alternat-
ing, two-way transmission.


Handshaking 

Handshaking is a process two modems go through at the time 
of call setup to negotiate transmission speeds and other 
protocols over the datacomm link. The process is accom-
plished by the exchange of predefined, mutually recognizable 
control codes.


ISDN (Integrated Services Digital Network

A conceptual network in which all transmission lines (usable 
for both voice and data) handle digital or digitized data. 
It is, to date, still largely theoretical due to the expense 
of replacing customer-to-LEC (local exchange carrier) lines.


ITU (International Telecommunication Union)

The ITU was created and controlled by the United Nations. It 
is the parent body of the ITU-T group, formerly known as the 
CCITT, whose purpose is to develop and publish recommenda-
tions for world-wide standardization of telecommunications 
devices.


LAP (Link Access Procedure)

The basis of an error control protocol used in data communi-
cations. Examples are LAP-B (balanced), LAP-D (for digital 
connections), and LAP-M, the popular error correction proto-
col specified in the ITU's V.42 recommendation.


LED (Light Emitting Diode)

A solid-state device used mostly as an indicator light.


Line Conditioning

A feature available to leased-line customers in which the 
facility provider fine tunes the circuitry to reduce the 
noise on the line and provide better quality than a standard 
voice-grade leased line. The conditioning process provides 
faster and more reliable data transfer


Mainframe Computer

A large, powerful computer used in a centralized processing 
environment. Its use of a channel-attached front end proces-
sor (FEP) distinguishes it from a mid-range computer. A 
mainframe usually has file storage capacities in the giga-
byte range. 


MNP 3, 4 (Microcom Networking Protocol Class 3, Class 4)

MNP 3 and MNP 4 are protocols developed by Microcom. They 
are used in many other brands of modems as a de facto stan-
dard for error correction. They are also called out as the 
alternate to LAP-M error correction in the popular V.42 ITU 
standard for error correction.


MNP 5 (Microcom Networking Protocol Class 5)

MNP 5 is a popular data compression protocol that provides 
2-1 compression and relies on the MNP 3 and MNP 4 error cor-
rection protocols for a 100% error-free link.


Modem

A device that converts digital computer data into audio fre-
quency waves that can be transmitted over analog telephone 
circuits, and vice-versa. From the words MOdulator-
DEModulator.


Modulation

The process of changing one or more characteristics of a 
signal with known frequency, phase, and amplitude so that 
these changes represent the source signal. Modulation is 
commonly used in telecommunications when one type of signal 
must be encoded onto another (called the carrier) for trans-
mission over an otherwise incompatible medium.


Operating System (OS)

The core software in a computer that is responsible for man-
aging and coordinating the resources it provides the user. 
An OS may control access to hardware (printers, hard disks, 
etc.) and software (application programs) resources. Typi-
cally it interfaces with the user as a command language and 
with device drivers. Examples of popular operating systems 
are UNIX, MS-DOS, and System 7.


Parameter

A variable that is given a constant value for a specific 
purpose or process. The numerical suffix on an AT command is 
a parameter, as is any changeable value that affects the op-
eration of a device such as a modem.


Parity

A rudimentary form of hardware error detection used in some 
asynchronous transmissions (e.g., between a computer and its 
modem). When parity is used, the hardware counts the number 
of 1s in a character and appends either a 1 or a 0 to main-
tain an odd or even count. When using data communications 
software, the user must select a parity option of odd, even, 
or none.


PC (Personal Computer)

A small desktop or laptop computer, also known as a micro-
computer. PC can also refer to a brand of computer made by 
IBM, and to computers that can use hardware and software 
designed for the IBM PC computer.


Protocol

Generally, a set of rules. In data communications, a guide-
line used to determine the data format, the handshaking, pa-
rameter negotiation, and link maintenance procedures to be 
used over a specific type of datacomm link.


PSK (Phase Shift Keying)

A modulation technique that establishes two-bit elements, 
called "dibits," yielding four possible combinations: 00, 
01, 10, 11. These elements are then represented by changes 
in the carrier signal's phase at 0 , 90 , 180 , and 270  
phase angles. PSK produces the appropriate shift in phase 
relative to the original carrier. Differential PSK replaces 
this technology because it is less prone to error. Modems 
using PSK normally operate at 1200 bps.


PSTN (Public Switched Telephone Network)

The vast network of worldwide circuit-switching equipment 
used as a telecommunications medium for the transmission of 
voice, data, and other information. The phone system.


Pulse Width

Pertaining to a digital signal, pulse width refers to the 
duration of one state (high or low) between clocking 
signals.


QAM (Quadrature Amplitude Modulation)

A complex method of modulation that establishes 12 phase 
angles and four amplitudes, designating sixteen possible 
combinations. QAM encodes hexadecimal values (called "quad-
bits") into one cycle of the carrier. Using a 2400 bps car-
rier frequency, QAM provides 9600 bps effective transmission 
speed. However, because it uses amplitude modulation, QAM is 
susceptible to line noise.


Remote Configuration

A feature available in some modems that allows some of the 
modem's parameters to be configured through a datacomm link, 
in addition to local configuration via its serial port.


Remote Echo

Any signal that is reflected, or duplicated, back toward the 
source as a result of some equipment located near the desti-
nation. For example, when a terminal transmits keystrokes to 
the host, it is not capable of sending the data to two loca-
tions (e.g., to the host and to the user's display). For the 
user to view what is being typed, the host must reflect the 
keystrokes it receives over the datacomm link back to the 
user's display. This is a useful form of remote echoing. An 
undesirable example of a remote echo occurs when the four-
wire circuit at the remote exchange is converted back to a 
two-wire phone line at the destination. The mismatched im-
pedances result in some of the transmitted signal being re-
flected back into the modem that sent it, which becomes a 
problem if the modem is sending and receiving simultaneously 
(i.e., operating in full-duplex mode).


Retraining Sequence

The process of resynchronizing a data communications connec-
tion that has been interrupted or has lost equalization. Re-
training is similar to the handshaking procedure performed 
by communications devices at call setup.


RS-232 (Recommended Standard 232)

A physical-level serial interface standard drafted by the 
Electronic Industries Association (EIA) that details pin 
configuration, voltage levels, and other electrical charac-
teristics of analog data transfer. This recommended standard 
is often appended with a letter (e.g., RS-232C) to indicate 
the level of revision being used.


RS-422 (Recommended Standard 422)

A physical-level serial interface standard drafted by the 
Electronic Industries Association (EIA) that details pin 
configuration, voltage levels, and other electrical charac-
teristics of digital data transfer. RS-422 is a superset of 
RS-232C.


RJ-11 (Registered Jack 11)

The common six-pin modular connector used to attach user 
phone equipment to the telephone service. As a phone jack, 
four of the six pins are used for dial-up service 
connections.


Serial Transmission

One of two ways data can be transferred between points or 
that devices can be connected together. Serial transmission 
is a sequential data transfer that provides only one pathway 
for data. Thus, it is slower, but less costly, than parallel 
transmission.


Session

An interactive connection in which someone uses a terminal 
or computer to access, input, or exchange information with 
another computer or user. Also, in some datacomm software, a 
set of parameters for establishing such a connection.


Speed Conversion

A feature available in some modems that allows the modem to 
be configured for one speed through its serial port and an-
other speed over the phone line. This allows the user to set 
up the communications software (i.e., the serial port speed) 
regardless of the speed at which the modems connect.


Standard

A convention established for continuity in manufacturing, 
development, or operation. When developed by a private or-
ganization, the convention is known as a de facto standard, 
and is considered proprietary, or closed. When arrived at by 
a public organization commissioned to the project, or by 
industry consensus, the convention is known as a de jure 
standard.


Stop Bit

One of the variables used for timing in asynchronous data 
transmission. Depending on the devices, each character may 
be trailed by 1, 1.5, or 2 stop bits.


String

A sequence of characters or commands on one line.


Terminal

The screen and keyboard device used in a mainframe environ-
ment for interactive data entry. Terminals have no "box," 
which is to say they have no file storage or processing 
capabilities.


Trellis Code Modulation (TCM)

An enhanced version of QAM in which the modem generates a 
"convolutional code" (an error-checking algorithm) that re-
places every quadbit. The code creates dependencies between 
successive bits so that certain combinations are not possi-
ble. TCM modems actually transmit five bits per carrier cy-
cle; however, with the advanced error detection scheme, TCM 
greatly improves the performance of QAM modems. If a conven-
tional QAM modem incurred one error in every ten blocks, the 
same modem with TCM might have one error in 10,000.


TSB (Telecommunication Standardization Bureau)

The TSB is the former secretariat of the International 
Consultative Committee on Telephone and Telegraph (CCITT), 
now known as ITU-T. Both groups are part of the Interna-
tional Telecommunication Union (ITU), created and controlled 
by the United Nations for the purpose of developing and pub-
lishing recommendations for world-wide standardization of 
telecommunications devices


UART (Universal Asynchronous Receiver/Transmitter)

The chip commonly used to convert between a computer's 
internal bus, which uses parallel transmission, and its COM 
port, which uses serial transmission.


UNIX

UNIX is a multitasking, multi-user operating system usually 
installed on mainframe or mid-range computers, but also 
found increasingly on personal computers.


V.21

The ITU recommendation for synchronous or asynchronous input 
to be transmitted at 300 bps over the PSTN. V.21 operates in 
half- or full-duplex mode.


V.22

The ITU recommendation for synchronous or asynchronous input 
to be transmitted at 1200 bps over the PSTN. V.22 operates 
in half- or full-duplex mode.


V.22bis

The ITU recommendation for synchronous or asynchronous input 
to be transmitted at 2400 bps over the PSTN. V.22bis oper-
ates in half- or full-duplex mode.


V.25bis

A dialing command set developed by the ITU for both synchro-
nous and asynchronous devices. Since V.25bis does not pro-
vide any modem configuration commands, it is primarily used 
for its synchronous dialing commands, while the AT commands 
are used for asynchronous dialing and for modem 
configuration.


V.32

The ITU recommendation for synchronous or asynchronous input 
to be transmitted at 9600 bps over the PSTN. V.32 operates 
in half- or full-duplex mode. A V.32 modem is capable of 
falling back to preset speeds (7200, 4800, 2400, or 1200 
bps) should the line quality degrade.


V.32bis

The ITU recommendation for synchronous or asynchronous input 
to be transmitted at 14,400 bps over the PSTN. V.32bis oper-
ates in half- or full-duplex mode. A V.32bis modem is capa-
ble of falling back to preset speeds (12,000, 9600, 7200, 
4800, 2400, or 1200 bps) should the line quality degrade. It 
is also able to step back up in the same increments as the 
line quality improves.


V.32 terbo

The AT&T de facto standard for synchronous or asynchronous 
input to be transmitted at 19,200 bps over the PSTN. V.32 
terbo operates in half- or full-duplex mode. A V.32 terbo 
modem is capable of falling back to preset speeds (16,800, 
14,400, 12,000, 9600, 7200, 4800, 2400, or 1200 bps) should 
the line quality degrade. It is also able to step back up in 
the same increments as the line quality improves.


V.34

The ITU recommendation for synchronous or asynchronous input 
to be transmitted at 28,800 bps over the PSTN. V.34 operates 
in half- or full-duplex mode. A V.34 modem is capable of 
falling back to preset speeds (26,400, 24,000, 21,600, 
19,200, 12,000, 9600, 7200, 4800 bps) should the line qual-
ity degrade. It is also able to step back up in the same 
increments as the line quality improves.


V.42

An ITU recommendation for error-control hardware on a modem 
that accepts asynchronous input. V.42 recommends for manu-
facturers to implement LAP-M, and makes a provision for MNP 
Classes 3 and 4 as an alternate because of their popularity. 
Most modem makers provide both.


V.42bis

An ITU recommendation for data-compressing hardware on a 
modem that accepts asynchronous input. V.42bis is based on a 
dynamically updated dictionary that looks up common strings 
and replaces the strings with code words. This reduces the 
quantity of characters actually transmitted. V.42bis has 
been found to be most effective for file transfers that con-
tain long strings of repetitive information and least effec-
tive for short strings of unique data and files that have 
already been compressed by software. It requires that V.42 
error correction has been enabled for it to be activated in 
the modem.



			    END
