Editing Hayes AT command set (section)

=== Background ===
Before the introduction of the [[bulletin board system]] (BBS), modems typically operated on direct-dial [[telephone line]]s that began and ended with a known modem at each end. The modems operated in either "originate" or "answer" modes, manually switching between two sets of frequencies for data transfer. Generally, the user placing the call would switch their modem to "originate" and then dial the number by hand. When the remote modem answered, already set to "answer" mode, the telephone handset was switched off and communications continued until the caller manually disconnected.

When automation was required, it was commonly only needed on the answer side; for instance, a bank might need to take calls from a number of branch offices for end-of-day processing. To fill this role, some modems included the ability to pick up the phone automatically when it was in answer mode, and to clear the line when the other user manually disconnected. The need for automated outbound dialling was considerably less common, and was handled through a separate peripheral device: a "dialler". This was normally plugged into a separate [[computer port (hardware)|input/output port]] on the computer (typically an [[RS-232]] port) and programmed separately from the modem itself.

This method of operation worked satisfactorily in the 1960s and early 1970s, when modems were generally used to connect dumb devices like [[computer terminal]]s (dialling out) with smart [[mainframe computer]]s (answering). However, the [[microcomputer revolution]] of the 1970s led to the introduction of low-cost modems and the idea of a semi-dedicated point-to-point link was no longer appropriate. There were potentially thousands of users who might want to dial any of the other thousands of users, and the only solution at the time was to make the user dial manually.

The [[computer industry]] needed a way to tell the modem what number to dial through software. The earlier separate dialers had this capability, but only at the cost of [[out-of-band signaling|a separate port]], which a [[microcomputer]] might not have available. Another solution would have been to use a separate set of "command pins" dedicated to sending and receiving commands; another could have used a signal pin indicating that the modem should interpret incoming data as a command. Both of these had hardware support in the [[RS-232]] standard. However, many implementations of the RS-232 port on microcomputers were extremely basic, and some eliminated many of these pins to reduce cost.