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74 MultiModemISI Hybrid Series, ISIHP-2S/2U/4S/4U/4SD
Modem AT Commands & S-Registers
Modem AT Commands
ISIHP 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 modems operational modes and shows you how
to use each modem 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 by the S-register S7, the modem abandons the call
and reenters 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 reenters command mode. By sending certain escape
characters to the modem while online, you can make it enter command