Stop bit

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Start and stop bit in the Baudot code

A start bit and up to two stop bits are used in asynchronous serial data transmission to enable the receiver to synchronize with each transmitted character.

The start and stop bits always have the same but different logical levels. A start bit is sent before the data word to be transmitted, the stop bit or bits after.

1, 1.5 or 2 stop bits can be used as the end of the character. One stop bit is usually used in newer transmission protocols.

The 1.5 stop bit variant has historical reasons, it comes from the time of the mechanical teleprinter . These devices were set to their speed and thus to their transmission speed via mechanical centrifugal governors ; the half bit could compensate for speed fluctuations.

If there is no continuous data transmission, the logic level of the stop bit is transmitted in the pauses between the characters; this is the rest position of an asynchronous data transmission system.

The transition of the level from the stop bit to the start bit triggers a timer in the receiver, which now controls the reception of the individual data bits, which follow at fixed intervals determined by the walking speed . It is therefore necessary that the transmitter and receiver use the same transmission speed.

In the case of synchronous data transmission , start and stop bits are not required, since the transmitter and receiver are synchronized by other means.

literature

  • Dieter Conrads: Telecommunications. Basics, procedures, networks. 5th, corrected edition. Vieweg, Wiesbaden 2004, ISBN 3-528-44589-0 , p. 46.
  • Peter Welzel, Harald Schummy (ed.): Data transmission. LAN and Internet protocols for IT professions, 4th edition, Friedrich Vieweg & Sohn Verlag, Wiesbaden 2001, ISBN 978-3-528-34369-9 .
  • Günter Kemnitz: Technical computer science . Volume 2 Digital Circuit Design. Springer Verlag, Berlin / Heidelberg 2011, ISBN 978-3-642-17446-9 .

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