Multitech MT5634MU, User Manual, Page: 32 / 80

32
MultiMobile USB User Guide
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 AT tention 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 95 and 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 the modem, e.g., to turn autoanswer 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 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 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.