4 AT Commands, S-Registers, & Result Codes
20
MultiModemZBA-V-V92 User Guide
Chapter 4- AT Commands, S-Registers, andResult 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.