Asynchronous communications system

In telecommunication, an asynchronous communications system is a data communications system that uses asynchronous operation.

Note 1: In an asynchronous communications system, extra signal elements are usually appended to the data for the purpose of synchronizing individual data characters or blocks.

Note 2: The time spacing between successive data characters or blocks may be of arbitrary duration. Synonym start-stop system.

Source: from Federal Standard 1037C and from MIL-STD-188