A family of modems developed by Hayes, which was the industry leader for many years (see
Hayes). Hayes developed the "intelligent modem" for the first personal computers in 1978, and its command language became the de facto standard for modem control (Hayes Standard AT Command Set).
Two States of Operation
An intelligent modem has a command state and an online state. In the command state, it accepts instructions. In the online state, it dials, answers, transmits and receives.
Handshaking
Once connected, the modem performs the handshaking with the remote modem, which are the whistles and tones you hear from the speaker. This is similar to the opening exchange of a human telephone call: called party says "hello;" calling party says "hello, this is..."; after that, the conversation begins. After the handshake is completed, you are online with the other computer, and data can be transmitted back and forth.
Escape Sequence
The escape sequence tells the modem to switch from online to command state. It consists of three plus signs in sequence (+++) with a Hayes-patented, one-second guard time interval before and after, which prevents the modem from mistaking a random occurrence of plus signs for an escape sequence. The escape sequence and guard time interval can be programmed in the modem's status registers.
To issue an escape sequence, hold down the Shift key and press + three times with a pause of at least one second before and after the sequence. The modem will return the OK result code, indicating it is ready to accept commands.