| Asynchronous Serial Communication |
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| synchronization | |
| data transmission | |
| digital electronics | |
| physical layer protocols | |
| broadcast engineering | |
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In the diagram, a start bit is sent, followed by eight data bits, no parity bit and one stop bit, for a 10-bit character frame. The number of data and formatting bits, and the transmission speed, must be pre-agreed by the communicating parties. After the stop bit, the line ''may'' remain idle indefinitely, or another character may immediately be started. The minimum stop bit length required by the system can be larger than a "bit". In fact, old electromechanical teletypewriters could demand 2 stop bits to allow mechanical impression without buffering, and RTTY Ham Radio is known to historically require 1.5 stop bits. New hardware that doesn't support fractional stop bits can be configured to send 2 stop bits when transmitting and requiring 1 stop bit when receiving. ORIGINS WITH TELETYPEWRITERS The format is derived directly from the design of the . 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 ). Electronic start/stop operation Before the transmitter and receiver will work, they must agree on the values of five parameters. First, both sides must agree on the number of bits per character. Second, the speed or Baud of the line must be the same on both sides. Third, both sides must agree to use or not use parity. Fourth, if parity is used, both sides must agree on using odd or even parity. Fifth, the number of stop bits must be agreed upon for a typical RS-232 or the newer RS-423 data interface standard. 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 or 8B/10B Encoding . Asynchronous start-stop is the physical layer used with modems for many dial-up Internet access applications, using a data link framing protocol such as PPP to create packets made up out of asynchronous serial 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. SEE ALSO REFERENCES |
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