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Chapter 4 - AT Commands, S-Registers, Result Codes
13
MultiModemDID User Guide
Chapter 4 - AT Commands, S -Registers,
and Result Codes
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 HyperTerminal
or the PhoneTools 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 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 Commands”
section, 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.