AT Command Set

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What Does AT Command Set Mean?

The AT command set is a command language with a series of short text strings, which combine together to output complete commands for different operations such as hanging up, dialing and changing connection parameters for modems. A majority of personal computer modems follow the AT command set specifications.


This term is also known as Hayes command set.

Techopedia Explains AT Command Set

The AT command set is a standard developed by Hayes to control modems. AT stands for attention. A string holds multiple AT commands placed together, which optimally prepares the modem to dial out. Such strings are called initialization strings and are of the form AT&F&D2&C1S0=0X4. The V.250 specification requires all data communications equipment to accept bodies of at least 40 characters of concatenated commands.

The AT command sets are generally divided into four groups:

  • Basic Command Set
  • Extended Command Set
  • Proprietary Command Set
  • Register Commands

The basic command set is a capital character followed by a digit such as M1. Using zero in the command set is optional. For instance, L0 equates to plain L. The extended command set includes an ampersand and capital letter followed by a digit such as &M1. Proprietary command sets start with a backslash or with a percent sign and vary among modem manufacturers. An example of a register command is -Sr=n, where r represents the number of registers to be changed and n is the latest value assigned. Registers represent physical locations in memory. The register commands enter values into a particular memory location. Modems also have small amounts of memory on board. Registers hold a particular variable, which can be used by the modem and communication software.

Several AT commands can be used on the same line, eliminating the need to type AT before each command. AT is only required once at the beginning of the command line. Semicolons are used as command delimiters. If AT commands are to be entered on separate lines, a pause (comma) can be entered between previous and following commands until an OK is encountered. This avoids sending multiple AT commands at a time and waiting for every command response.


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Margaret Rouse
Technology Expert
Margaret Rouse
Technology Expert

Margaret is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical business audience. Over the past twenty years, her IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.