AT Command Set Definition

The AT command set, officially known as the Hayes Standard AT Command Set, is the de facto standard set of instructions for configuring and controlling modems.

Pronounced ay-tee, this set was developed by Hayes Microcomputer Products, a pioneer in the modem field, and it is used in its entirety or in part by almost every manufacturer of modems for use with personal computers.

The commands are short sequences of ASCII characters. All command strings (i.e., sequences of characters) must be prefaced by the letters AT, an abbreviation for attention, thereby accounting for the name of the set.

The commands can be divided into four categories: basic, extended, proprietary and register. The basic commands consist of a capital letter followed by an integer, such as M1 (which stands for speaker on until remote carrier connected). The extended commands are an extension of the basic command set, and they are preceded by an ampersand to indicate that the characters that follow have a different meaning from what they would have in the basic command set (e.g., &M1 has a different meaning from M1). The proprietary commands typically begin with either a backslash or a percentage sign. The register commands begin with a capital S followed by the number of the register to be changed, an equals sign, and finally the new value that is to be assigned to it.

A register is a physical location in the small amount of memory that is used to store currently or frequently used data. It is typically built into microprocessors, but it could be part of a separate memory chip in a modem. For example, the command S5=80 instructs the computer to set the modem's fifth register to the value 80.

There is frequently some variation in the command set according to the modem manufacturer. This is particularly true for the proprietary commands. Thus, it is best to consult the command set for a specific model of modem rather than relying on any generic listing of commands. Places where information on command sets for specific modems might be found include the manual or floppy disk that came with the modem, the Internet and the modem itself (accessed via a computer).

One of the most familiar AT commands is the ATZ that typically appears at the start of the modem initialization. This string is a combination of AT, which initiates the commands, and Z, which resets the modem to its default state.

The command ATDT represents the initialization string followed by D, which represents dial, and T, which represents touch tone dialing (as opposed to P for pulse dialing).

Modems are configured by sending AT commands to them from the computer on the same serial lines that are used by the computer to send data to the modem.

Created November 27, 2005.
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