1. Introduction
The AT commands are used to control the operation of your modem. They are called AT commands because the characters AT must precede each command 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 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 the HyperTerminal applet in Windows 98 and NT 4.0, or some other available terminal program. 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 auto answer on or off.
The format for entering an AT command is ATXn, where X is the command 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 Section 3.
You must press [Enter] (it could be some other key depending on the terminal program) 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. See a table of valid result codes at the end of this chapter.
