Asynchronous start-stop
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The asynchronous start-stop is the more common of two basic modes of teletypewriter operation, allowing a common means of encoding characters over a serial link such as a telegraph circuit or an RS-232 interface. Asynchronous start-stop uses a "start bit" followed by some number of data bits, possibly a "parity" bit, and one or two "stop bits".
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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
This 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 experiemental 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).
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.
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.
The alternative to asynchronous start-stop was the use of synchronous protocols such as HDLC.
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.
References
[HISTORY OF TELETYPEWRITER DEVELOPMENT by R. A. Nelson and K. M. Lovitt, October 1963, Teletype Corporation, retrieved April 14, 2005 (http://www.rtty.com/history/nelson.htm)]