MultiModemDSVD User Guide 4 AT Commands, S-Registers, and Result Codes
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AT Commands
AT commands are used to control the operation of your modem. They are so called
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 mo-
dem 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 95 and NT 4.0, or the Trio communi-
cations program included with your modem. You can issue commands to the mo-
dem either directly, by typing them in the terminal window of the communications
program, or indirectly, by configuring the operating system or communications pro-
gram 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 mo-
dem, 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 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,” which begins on the next page.
You must press ENTER 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 sepa-
rate 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.