Introduction to Data Communications
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29. AT Command Set

Hayes modems were the first smart modems. They had built-in CPUs that could interpret a special series of commands. These commands are called the AT command set. The basic command for getting a modem's attention was the characters "AT" (older modems may only recognize lower case "at"). Once the modem's attention was available, character's are added immediately after that specify instructions.

Smart modems operate in two modes: command and communication mode. In command mode, the modem is waiting for AT command instructions. In communication mode, the modem is transferring data from sender to receiver.

To talk to a modem, you must use either a terminal or a terminal emulation software on a PC such as Procomm or Hyperterminal. A basic test to see if the modem is communicating properly with the terminal, is to type "AT". If the modem responds with "OK", then the software's configuration matches the modems configuration.

The following configuration issues must match before proper modem to terminal communication will work:

Configuration Point Typical value
Com port of modem: Com2 for external, Com4 for internal
IRQ of modem: IRQ3
Number of data bits: 8
Type of parity: n (none)
Number of stop bits: 1
Transfer speed: 56 kbps (depends on modem)
Terminal emulation: vt100

If the modem is on-line (communicaton mode), to enter command mode, type "+++" (3 pluses in a row) and wait. The modem should respond with "OK". This indicates that you have entered command mode. You then may enter AT command strings to the modem.

Introduction to Data Communications
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