Introduction to Data Communications
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29a. Basic AT commands

Modern modems require an initialization string for configuring themselves. The most common configuration string is "ATZ", which is the reset command. Usually and this will depend on the modem, there are factory stored configurations that can be accessed by using the "ATF1" command. If there are more than one available configuration, the others can be accessed by "ATF2" and so on.

Older modems, typically 14.4kbps and earlier, had elaborate initialization strings that differed for each modem and each manufacturer. It was quite a headache to support so many different types of modems and make them work with each other.

To dial out, the "ATD" command is used. "ATDT" uses tone dialing versus rotary dialing. Immediately after the "ATDT" command, the destination telephone number is entered, for example: "ATDT555-1234". Would command the modem to use tone dialing to dial the number 555-1234. To hang up a modem, the AT command string "ATH" can be used.

A partial listing of the AT command set is available in Appendix J. The AT command set is incredibly large and is constantly growing due to the improvements and innovations by the modem manufacturers. There are two main manufacturer's of modem chipsets: Rockwell and US Robotics. Both have excellent documentation on identifying and configuring the modem chipsets that they manufacture.

Introduction to Data Communications
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