Asynchronous communications system - Definition and Overview

In telecommunication, an asynchronous communications system (start-stop system) is a data communications system that uses asynchronous operation.

• In an asynchronous communications system, extra signal elements are usually appended to the data for the purpose of synchronizing individual data characters or blocks.
• The time spacing between successive data characters or blocks may be of arbitrary duration.

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