Editing Asynchronous serial communication (section)

== 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).