Hayes introduces AT command set enabling software control of modem dialing and connection

Before the AT command set, modems required manual switching between originate and answer modes and often needed a separate dialer peripheral for outbound calls. Hayes’ 1981 Smartmodem introduced a command language that used the existing data pins to switch between data mode and command mode via an escape sequence. This let software issue text strings to dial numbers, hang up, or adjust connection parameters without extra hardware. As a result, computers could automate both inbound and outbound modem interactions, paving the way for bulletin board systems and remote access.

Adopting the AT command set required only a firmware update in the modem, leaving the host computer’s serial port unchanged. Developers had to learn the AT syntax and implement handling of the escape sequence and pause to avoid false command triggers. Modem makers faced little hardware change but needed to ensure their firmware recognized the basic AT set and could add vendor-specific prefixes such as & or \. This avoided the expense and complexity of a separate dialer card or additional command pins that many microcomputers lacked.

The AT command set assumes an asynchronous serial link with reliable timing, so it can fail if the line introduces significant latency or noise. If the pause after the +++ escape sequence is too short, the modem may not recognize the command and remain in data mode, preventing software control. Because the original set only covered early 300 bit/s operations, later high-speed modems required vendor-specific prefixes and extensions, which sometimes diverged before being re-standardized.