AT Command Sets

　　Long ago, Hayes set a standard for modem commands with its Smartmodem 300. Most modem manufacturers adopted this command set in order to call themselves "Hayes compatible." The command set used by the Smartmodem 300, as well as most modems today (with a few advanced commands), is known as the AT command set. AT stands for attention, and precedes all (with the exception of A/) commands directed to the modem. For example, when dialing, it is necessary for either the software or the user to issue an ATDT or ATDP command followed by the number and enter. AT tells the modem that it is about to receive a command. DT tells it to dial by tone, while DP tells it to pulse dial. Finally, the modem dials the number given to it after the command. Different modems do have slightly different command sets, but generally most modems follow the standard set by Hayes.