Filter
Top Products
38
PORTABLE USB MODEM
4. AT Commands, S-Registers, and
Result Codes
4.1 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 when it is not connected to
another modem. The modem is in data mode when 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 (for example, +++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 NT
®
4.0, or the communications
program included with your modem. 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 a modem (for example, 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 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 Section 4.3.