Editing Asynchronous serial communication

{{Use American English|date = March 2019}}
{{Short description|Form of serial communication lacking synchronization control signals}}
{{Use mdy dates|date = March 2019}}
{{one source|date=January 2013}}
[[File:Puerto serie Rs232.png|frame|In this diagram, two [[byte]]s are sent, each consisting of a start bit, followed by eight data bits (bits 0-7), and one stop bit, for a 10-bit character frame.   The last data bit is sometimes used as a [[parity bit]]. The number of data and formatting bits, the [[endianness|order of data bits]], the presence or absence of a parity bit, the form of parity (even or odd) and the transmission speed (frequency) must be pre-agreed by the communicating parties. The "stop bit" is actually a "stop period"; the stop period of the transmitter may be arbitrarily long. It cannot be shorter than a specified amount, usually 1 to 2 bit times. The receiver requires a shorter stop period than the transmitter. At the end of each character, the receiver stops briefly to wait for the next start bit. It is this difference which keeps the transmitter and receiver synchronized.]]

'''Asynchronous serial communication''' is a form of [[serial communication]] in which the communicating endpoints' interfaces are not continuously synchronized by a common clock signal. Synchronization ([[clock recovery]]) is done by data-embedded signal: the data stream contains synchronization information in a form of start and stop signals set before and after each payload transmission. 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.

== Origin ==
Mechanical [[teleprinter]]s using 5-bit codes (see [[Baudot code]]) typically used a stop period of 1.5 bit times.<ref>{{ cite book|url=http://www.bitsavers.org/pdf/teletype/144_Model15_Descr_Feb31.pdf |title=Description, Typebar Page Printer (Model 15) |publisher=Teletype Corporation |volume=Bulletin No. 144 |location=Chicago |page=11 |year=1931 |url-status=dead |archive-url=https://web.archive.org/web/20090320151231/http://www.bitsavers.org/pdf/teletype/144_Model15_Descr_Feb31.pdf |archive-date=2009-03-20 }}Dead link: 2015-Oct-03</ref> Very early electromechanical teletypewriters (pre-1930) could require 2 stop bits to allow mechanical impression without buffering.{{Citation needed|date=March 2009}} 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{{Citation needed|date=August 2009}} for [[Comparison of synchronous and asynchronous signalling|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 ({{Patent|US|1199011}}, granted September 19, 1916, then {{Patent|US|1286351}}, granted December 3, 1918). Shortly afterward a practical [[teleprinter]] was patented ({{Patent|US|1232045}}, granted July 3, 1917).

== Operation ==
Before signaling will work, the sender and receiver must agree on the signaling parameters:

* Full or half-[[duplex (telecommunications)|duplex]] operation
* The number of bits per character -- currently almost always [[8-bit character]]s, but historically some transmitters have used a [[five-bit character code]], [[six-bit character code]], or a [[7-bit ASCII]].
* [[Endianness]]: the order in which the bits are sent
* The speed or bits per second of the line (equal to the [[Baud]] rate when each symbol represents one bit). Some systems use automatic speed detection, also called [[automatic baud rate detection]].
* Whether to use or not use [[parity_bit|parity]]
** Odd or even parity, if used
* The number of stop bits sent must be chosen (the number sent must be at least what the receiver needs)
* Mark and space symbols (current directions in early telegraphy, later voltage polarities in [[Electronic Industries Alliance|EIA]] [[RS-232]] and so on, frequency-shift polarities in [[frequency-shift keying]] and so on)

Asynchronous start-stop signaling was widely used for dial-up [[modem]] access to [[time-sharing]] computers and [[Bulletin board system|BBS]] systems. These systems used either seven or eight data bits, transmitted [[least-significant bit]] first, in accordance with the [[ASCII]] standard.

Between computers, the most common configuration used was "8N1": eight-bit characters, with one start bit, one stop bit, and no parity bit. Thus 10 Baud times are used to send a single character, and so dividing the signaling bit-rate by ten results in the overall transmission speed in characters per second.

Asynchronous start-stop is the lower [[data-link layer]] used to connect computers to modems for many dial-up Internet access applications, using a second (encapsulating) data link [[frame (networking) | framing]] protocol such as [[Point-to-Point Protocol|PPP]] to create [[network packet | packets]] made up out of asynchronous serial characters. The most common physical layer interface used is RS-232D. 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 (data)|flow control]] being used to throttle the data rate to prevent overrun.

== See also ==
* [[Comparison of synchronous and asynchronous signalling]]
* [[Degree of start-stop distortion]]
* [[Synchronous serial communication]]
* [[Universal asynchronous receiver/transmitter]] (UART)

== References ==

{{reflist}}

== Further reading ==

* Nelson, R. A. and Lovitt, K. M. [http://www.rtty.com/history/nelson.htm ''History of Teletypewriter Development'' (October 1963)], Teletype Corporation, retrieved April 14, 2005
* Hobbs, Allan G. (1999) [https://web.archive.org/web/19991104220621/http://www.nadcomm.com/fiveunit/fiveunits.htm ''Five-unit codes''], accessed 20 December 2007
* Edward E. Kleinschmidt.  [http://www.gutenberg.org/files/53481/53481-h/53481-h.htm ''Printing Telegraphy ... A New Era Begins''], 1967, released Nov. 9, 2016 by [http://www.gutenberg.org Project Gutenberg].

== External links ==

* {{Wikibooks-inline|Programming:Serial Data Communications}}

[[Category:Synchronization]]
[[Category:Data transmission]]
[[Category:Digital electronics]]
[[Category:Physical layer protocols]]
[[Category:Broadcast engineering]]