Asynchronous start-stop describes an asynchronous transmission protocol in which a start signal is sent prior to each code symbol and a stop signal is sent after each code symbol. The start signal serves to prepare the receiving mechanism for the reception and registration of a symbol and the stop signal serves to bring the receiving mechanism to rest in preparation for the reception of the next symbol. A common kind of start-stop transmission is ASCII over RS-232, for example for use in teletypewriter operation. Asynchrony is the state of not being synchronized. ... In telecommunication, the term stop signal has the following meanings: 1. ... For other uses, see ASCII (disambiguation). ... RS-232 (also referred to as EIA RS-232C or V.24) is a standard for serial binary data interchange between a DTE (Data terminal equipment) and a DCE (Data communication equipment). ... A teleprinter (teletypewriter, teletype or TTY) is a now largely obsolete electro-mechanical typewriter which can be used to communicate typed messages from point to point through a simple electrical communications channel, often just a pair of wires. ...

In the diagram, a start bit is sent, followed by eight data bits, a parity bit and one stop bit, for an 11-bit character frame. The number of data and formatting bits, and the transmission speed, must be pre-agreed by the communicating parties.

Origins with teletypewriters

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. A teleprinter (teletypewriter, teletype or TTY) is a now largely obsolete electro-mechanical typewriter which can be used to communicate typed messages from point to point through a simple electrical communications channel, often just a pair of wires. ... Synchronization is coordination with respect to time. ... The Baudot code, named after its inventor Émile Baudot, is a character set predating EBCDIC and ASCII and used originally and primarily on teleprinters. ...

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, (an electrical engineer and son of Charles Krum) who patented the start-stop method of synchronization (Patent No. 1,286,351, December 3, 1918). Shortly afterward a practical teleprinter was patented(Patent No. 1,232,045; July 3, 1917). Charles L. Krum was a key figure in the development of the Teletype, a machine which played a key role in the history of telegraphy and computing. ... Teletype machines in World War II A teleprinter (teletypewriter, teletype or TTY) is a now largely obsolete electro-mechanical typewriter which can be used to communicate typed messages from point to point through a simple electrical communications channel, often just a pair of wires. ...

Electronic start/stop operation

Asynchronous start-stop signalling was widely used for dial-up modem access to time-sharing computers and BBS systems. These systems used either seven or eight data bits. A modem (a portmanteau constructed from modulator and demodulator) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. ... Alternate uses: see Timesharing Time-sharing is an approach to interactive computing in which a single computer is used to provide apparently simultaneous interactive general-purpose computing to multiple users by sharing processor time. ... A bulletin board system or BBS is a computer system running software that allows users to dial into the system over a phone line and, using a terminal program, perform functions such as downloading software and data, uploading data, playing games, reading news, and exchanging messages with other users. ...

The most common configuration used was (and still is) "8N1": eight bit characters, with one stop bit and no parity bit. Thus 10 bits are used to send a single character, which has the nice side-effect that dividing the signalling bit-rate by ten results in the overall transmission speed in characters per second. 8-N-1 is a common shorthand notation for a Serial port parameter setting or configuration in asynchronous mode, in which there are eight (8) data bits, no (N) parity bit, and one (1) stop bit. ...

The alternative to asynchronous start-stop was the use of synchronous protocols such as HDLC. It has been suggested that the section HDLC from the article Measuring_data_throughput be merged into this article or section. ...

Asynchronous start-stop is still used to communicate with modems for many dial-up Internet access applications, using a framing protocol such as PPP to create packets made up out of characters. The performance loss relative to synchronous access is negligible, as most modern modems will use a private synchronous protocol to send the data between themselves, and the asynchronous links at each end are operated faster than this data link, with flow control being used to throttle the data rate to prevent overrun. In computing, the Point-to-Point Protocol, or PPP, is commonly used to establish a direct connection between two nodes. ... The flow control mechanism is used for controlling the flow of data in a network under well-defined conditions, while congestion control is used for controlling the flow of data when congestion has actually occurred . ...

References

Wikibooks has more about this subject:

Programming:Serial Data Communications

• HISTORY OF TELETYPEWRITER DEVELOPMENT by R. A. Nelson and K. M. Lovitt, October 1963, Teletype Corporation, retrieved April 14, 2005