AT Commands are text commands for communicating with a modem. Historically also called Hayes Command Set. For questions about the `at` command line tool use the tag [at-utility].
The most important, general specification covering AT commands is the ITU specification V.250 (formerly known as V.25ter). AT commands related to mobile phones are standardized by 3GPP in 27.007 (with some commands specified in 27.005). Historically, early AT commands were developed by modem manufacturer Hayes.
An AT command can be one of two fundamental types, action command or parameter command. This is important to distinguish because valid syntax for such commands is different (although specifications unfortunately sometimes mess up here).
Responses from the modem can be of three fundamental types:
Final result code
- Indicates that the execution of the currently running AT command is
finished. Examples:
OKandERROR.
Intermediate result code or Information text
- Responses related to a command that comes before the final result
code. Examples:
CONNECTand+CPBR.
Unsolicited result code
- Responses that the modem issues independently of any running AT
command as response to certain events (in the modem). Examples:
RINGand+CREG.
References
