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== Purpose and history ==
The USART's synchronous capabilities were primarily intended to support synchronous protocols like IBM's [[synchronous transmit-receive]] (STR), [[Binary Synchronous Communications|binary synchronous communications]] (BSC), [[synchronous data link control]] (SDLC), and the ISO-standard [[High-Level Data Link Control|high-level data link control]] (HDLC) synchronous link-layer protocols, which were used with synchronous voice-frequency [[modem]]s. These protocols were designed to make the best use of bandwidth when modems were analog devices. In those times, the fastest asynchronous voice-band modem could achieve at most speeds of 300{{nbsp}}bit/s using [[frequency-shift keying]] (FSK) modulation, while synchronous modems could run at speeds up to 9600{{nbsp}}bit/s using [[phase-shift keying]] (PSK). Synchronous transmission used only slightly over 80% of the bandwidth of the now more-familiar asynchronous transmission, since start and stop bits were unnecessary. Those modems are obsolete, having been replaced by modems which convert asynchronous data to synchronous forms, but similar synchronous telecommunications protocols survive in numerous block-oriented technologies such as the widely used [[IEEE 802.2]] (Ethernet) link-level protocol. USARTs are still sometimes integrated with MCUs. USARTs are still used in routers that connect to external CSU/DSU devices, and they often use either Cisco's proprietary HDLC implementation or the [[Internet Engineering Task Force|IETF]] standard [[point-to-point protocol]] (PPP) in HDLC-like framing as defined in RFC{{nbsp}}1662.
== Operation ==
The operation of a USART is intimately related to the various protocols; refer to those pages for details. This section only provides a few general notes.
* USARTs in synchronous mode transmits data in [[Frame (networking)|frames]]. In synchronous operation, characters must be provided on time until a frame is complete; if the controlling processor does not do so, this is an ''"underrun error''," and transmission of the frame is aborted.
* USARTs operating as synchronous devices used either character-oriented or bit-oriented mode. In character (STR and BSC) modes, the device relied on particular characters to define frame boundaries; in bit (HDLC and SDLC) modes earlier devices relied on physical-layer signals, while later devices took over the physical-layer recognition of bit patterns.
* A synchronous line is never silent; when the modem is transmitting, data is flowing. When the physical layer indicates that the modem is active, a USART will send a steady stream of padding, either characters or bits as appropriate to the device and protocol.
== Devices ==
|Intel
|8251A
|Programmable Communicationscommunications Interfaceinterface
|Intel 8251A Datadata Sheetsheet<ref>{{Cite web | title = Intel 8251A Programmable Communications Interface, | url = http://www.datasheetarchive.com/dlmain/Datasheets-14/DSA-278171.pdf | website = www.datasheetarchive.com | access-date = 2015-12-16}}</ref>
|-
|Signetics / Philips
|2651
|Programmable Communicationscommunications Interfaceinterface
|Philips Semiconductors SCN2651 Datadata Sheetsheet<ref>{{Cite web | title = Philips Semiconductors SCN2651 Programmable Communications Interface, | url = https://datasheet.datasheetarchive.com/originals/distributors/Datasheets-25/DSA-495164.pdf | website = www.datasheetarchive.com | access-date = 2020-04-05}}</ref>
|-
|Zilog
|Z85230/Z80230/Z8523L/Z85233
|Enhanced Serialserial Communicationscommunications Controllercontroller
|IXYS web page<ref>{{Cite web | title = Enhanced Serial Communications Controllers, | url = http://www.zilog.com/index.php?option=com_cutsheet&task=view&cid=8&id=8|website = www.zilog.com | access-date = 2015-12-16}}</ref>
|}
| 