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Chapter 1 – AT Commands, S-Registers and Result Codes
Multi-Tech Systems, Inc. SocketModem MT5634SMIxx Reference Guide (S000263C) 5
Chapter 1 – AT Commands, S-
Registers and Result Codes
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., +++ATH 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 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 auto answer 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. The default values are
shown in the “AT Command Summary” (See below).
You must press ENTER (it could be some other key depending on the terminal program) 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. See a table of valid result
codes at the end of this chapter.
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.