Multi-Tech Systems ISIHP-4SD, ISIHP-4U manual Modem AT Commands, Modes of Operation

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 modem’s 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

74 MultiModemISI Hybrid Series, ISIHP-2S/2U/4S/4U/4SD