4 AT Commands, S-Registers, and Result Codes

AT Commands

AT commands are used to control the operation of your modem. They are so called because each command must be preceded by the charac- ters AT to get the ATtention 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 ex- change data. Online command mode is a temporary state in which you can issue commands to the modem while connected to another mo- dem. 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 the HyperTerminal applet in Windows 95 and NT 4.0, or the 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 con- figuring 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 re- configuring the modem, e.g., to turn autoanswer on or off.

The format for entering an AT command is ATXn, where X is the com- mand and n is the specific 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 Command Summary,” which begins on the next page.

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Multi-Tech Systems MT5634ZBAV manual AT Commands, S-Registers, and Result Codes