Chapter 1 - At Commands; Introduction - Multitech 5634 Series Reference Manual

Chapter 1 - AT Commands

Introduction

The AT commands are used to control the operation of your modem. They are called AT commands because the
characters AT must precede each command to get the ATtention of the modem.
AT commands can be issued only when the modem is in command mode or online command mode. The modem
is in command mode whenever it is not connected to another modem. The modem is in data mode whenever it is
connected to another modem and ready to exchange data. Online command mode is a temporary state in which
you can issue commands to the modem while connected to another modem. To put the modem into online
command mode from data mode, you must issue an escape sequence (+++) followed immediately by the AT
characters and the command, e.g., +++ to hang up the modem. To return to data mode from online command
mode, you must issue the command ATO.
To send AT commands to the modem you must use a communications program, such as the HyperTerminal
applet in Windows 98/95 and NT 4.0, or some other available terminal program. You can issue commands to the
modem either directly, by typing them in the terminal window of the communications program, or indirectly, by
configuring the operating system or communications program to send the commands automatically. Fortunately,
communications programs make daily operation of modems effortless by hiding the commands from the user.
Most users, therefore, need to use AT commands only when reconfiguring the modem, e.g., to turn autoanswer
on or off.
The format for entering an AT command is ATXn, where X is the command and n is the specific value for the
command, sometimes called the command parameter. The value is always a number. If the value is zero, you
can omit it from the command; thus, AT&W is equivalent to AT&W0. Most commands have a default value,
which is the value that is set at the factory.
You must press ENTER (depending on the terminal program it could be some other key) to send the command
to the modem. Any time the modem receives a command, it sends a response known as a result code. The
most common result codes are OK, ERROR, and the CONNECT messages that the modem sends to the
computer when it is connecting to another modem.
You can issue several commands in one line, in what is called a command string. The command string begins
with AT and ends when you press ENTER. Spaces to separate the commands are optional; the command
interpreter ignores them. The most familiar command string is the initialization string, which is used to
configure the modem when it is turned on or reset, or when your communications software calls another modem.
Multi-Tech Systems, Inc. AT Commands Reference Guide (PN S000272E)
Chapter 1 – AT Commands
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