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ModemModule Developer’s Guide
MT3334SMI20
Introduction
The AT commands are used to control the operation of your modem. They are called AT commands
because each command must be preceded by the characters AT 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 98/95 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. The default values are
shown in the “AT Command Summary” (See below).
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. For a table of valid result
codes, see “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; they are ignored by the command interpreter. 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.
AT Command Summary
Command: AT Attention Code
Values: n/a
Description: The attention code precedes all command lines except A/, A:, and escape
sequences.
Command: ENTER Key
Values: n/a
Description: Press the E
NTER (RETURN) key to execute most commands.
Command: A Answer
Values: n/a
Description: Answer call before final ring.
Command: A/ Repeat Last Command
Values: n/a
Description: Repeat the last command string. Do not precede this command with AT. Do not
press E
NTER to execute.