Hayes modems were the first smart modems. They had built-in CPUs that could interpret a special series of commands, called the AT command set. The basic command for getting a modem's attention was the character set "AT" (older modems may only recognize lower case "at"). Once the modem's attention was available, characters are added immediately after that specific set of 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 terminal emulation software on a PC (such as Procomm or Hyperterminal). Typing in "AT" is a basic test to see if the modem is communicating properly with the terminal. 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 online (communication mode), type "+++" (3 pluses in a row) to enter the command mode, and then 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.
Basic AT commands
Modern modems require an initialization string for configuring themselves. The most common configuration string is "ATZ", the reset command. There are factory stored configurations that can usually be accessed by using the "ATF1" command (depending on the modem). If there is 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 at 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 chip sets: Rockwell and US Robotics. Both have excellent documentation on identifying and configuring the modem chip sets that they manufacture.
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