The AT commands are the basic commands for dealing with a modem. The command set comes from the Hayes modem, the Smartmodem 300 which set the standard for the commands. This command set is still used by most modem manufacturers, with some additonal commands built in. The AT stands for attention and is follwed by another set of instructions. For example, ATDT, tells the modem "Attention" followed by the fact that it needs to dial by tone instead of by pulse, ATDP. These commands need to be followed by a carriage return.
Below are some basic commands with what they would do:
+++ -- This is the escape sequence
A/ -- Repeat the last sequence
DT -- Dial Tone, often used with a combination of AT, ATDT, a way to test if the modem is receiving a dial tone
En -- Echo, controls if the modem will respond to an echo from the DTE
In -- Report modem information, where n is a number 0-7, based of the number information will be reported
Mn -- Sets the speaker mode, where n is a number 0-3, 0 is disabled, and 2 is enable speaker
P -- Pulse dialing, can be duplicated by DP
Qn -- Result Codes to DTE, where n is a number 0-1, 0 is enable and 1 is disable
T -- Tone dialing, can be duplicated by DT
Wn -- Extended Result Codes, where n is a number 0-2
Xn -- Result Codes for Call Progress, where n is a number 0-4, slects tone detection and resuls in a code formating options
Yn -- Long Space disconnect, where n is a number 0-1, tells whether or not the modem should disconnect if a break of 1.6 seconds occurs
Zn -- Restores modem profiles that are saved in non-volatile RAM, where n is a number 0-1
Information is taken from www.modem.com and www.modemhelp.net