Asynchronous serial communication

Asynchronous serial communication is a form of serial communication in which the communicating endpoints' interfaces are not continuously synchronized by a common clock signal. Instead of a common synchronization signal, the data stream contains synchronization information in form of start and stop signals, before and after each unit of transmission, respectively. The start signal prepares the receiver for arrival of data and the stop signal resets its state to enable triggering of a new sequence.

A common kind of start-stop transmission is ASCII over RS-232, for example for use in teletypewriter operation.

Mechanical teleprinters using 5-bit codes (see Baudot code) typically used a stop period of 1.5 bit times. Very early electromechanical teletypewriters (pre-1930) could demand 2 stop bits to allow mechanical impression without buffering. Hardware which does not support fractional stop bits can communicate with a device that uses 1.5 bit times if it is configured to send 2 stop bits when transmitting and requiring 1 stop bit when receiving.

The format is derived directly from the design of the teletypewriter, which was designed this way because the electromechanical technology of its day was not precise enough for synchronous operation: thus the systems needed to be re-synchronized at the start of each character. Having been re-synchronized, the technology of the day was good enough to preserve bit-sync for the remainder of the character. The stop bits gave the system time to recover before the next start bit. Early teleprinter systems used five data bits, typically with some variant of the Baudot code.

Very early experimental printing telegraph devices used only a start bit and required manual adjustment of the receiver mechanism speed to reliably decode characters. Automatic synchronization was required to keep the transmitting and receiving units "in step". This was finally achieved by Howard Krum, who patented the start-stop method of synchronization (US 1199011 , granted September 19, 1916, then US 1286351 , granted December 3, 1918). Shortly afterward a practical teleprinter was patented (US 1232045 , granted July 3, 1917).

...
Wikipedia