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64 MultiModemISI Hybrid Series, ISIHI-2S
Chapter 4—MODEM AT Commands, S-Registers, Result Codes
Modem
Modem AT Commands
The ISIHI-2S’s modems are controlled by instructions called
AT commands
, so called
because the
attention characters,
AT
, precede each command or sequence of
commands (known as a
command string
). You can send commands to the modem from
your keyboard while in terminal mode, or you can use communications software to issue
these commands automatically.
The modem is in command mode when it is not dialing or online. When it is in command
mode, you have access to a complete communications system that allows you to use
several features, including the basic AT command set described in this chapter. Using the
basic AT command set, you can enter phone numbers for automatic dialing, configure
modem options, and monitor telephone activity. In addition, you can command your
modem to perform advanced features such as error correction, data compression, speed
conversion, and more.
This chapter describes the modem’s operational modes, and shows you how to use each
of the modem’s AT commands. These commands and responses are compatible with all
systems and with all data communications software using the AT command set.
Modes of Operation
The modem operates in two basic functional modes:
command mode
and
online mode
.
(There is also an in-between state,
wait-for-carrier
, in which the modem is out of
command mode but not yet online.) When you turn on the modem, it is in command
mode and is ready to accept and respond to commands from your keyboard or software.
The modem enters online mode after it dials, connects with another modem, and detects
a valid carrier signal. If it does not detect a carrier signal within the time frame controlled
in the modems by the S-register S7, the modem abandons the call and re-enters
command mode.
You can make the modem enter online mode without dialing by entering AT and then D
(dial) or A (force answer mode). The modem exits online mode if the carrier signal is lost
or intentionally dropped. When this happens, the modem hangs up and re-enters
command mode. By sending certain
escape
characters to the modem while online, you
can make it enter command mode without losing the carrier signal.
Command Structure
You can control a wide variety of modem operations and options when the modem is in
command mode. AT commands tell the modem to dial a number, to answer a call, to
operate at a certain speed, to use a certain compression technique, and many other
functions. AT commands consist of one or two letters, which may be preceded by an
ampersand (
&
), a percent character (
%
), or a slash character (
\
). The Q command, for
example, determines whether the modem returns result codes, while the &Q command
selects the asynchronous communications mode.
A parameter after a command (0, 1, 2, etc.) tells the modem which option to use. If you
do not specify a parameter, the modem assumes the 0 (zero) option. E, for example, is
the same as E0. You can issue several commands on a single line (a command string)
as long as the line does not exceed 40 characters.