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1. 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 AT
Xn, 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 Section 3.
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.