Universal synchronous and asynchronous receiver-transmitter: Difference between revisions

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[[File:USART.png | thumb | right | alt=An example of a USART | An example of a USART]]A '''Universaluniversal Synchronous/Asynchronoussynchronous Receiver/Transmitterand asynchronous receiver-transmitter''' ('''USART''', '''programmable communications interface''' or '''PCI''')<ref>{{Cite web |date=2017-12-04 |title=8251A-Programmable Communication Interface Notes - Computer Science Engineering (CSE) |url=https://edurev.in/studytube/8251A-Programmable-Communication-Interface-Micropr/75b44fe5-ea73-4bc7-88e4-4a74b1f65084_t |access-date=2022-07-02 |website=EDUREV.IN |language=en}}</ref> is a type of a serial interface device that can be programmed to communicate asynchronously or synchronously. See [[Universaluniversal asynchronous receiver/-transmitter]] (UART) for a discussion of the asynchronous capabilities of these devices.

== Purpose and Historyhistory ==

The USART's synchronous capabilities were primarily intended to support synchronous protocols like IBM's [[Synchronoussynchronous transmit-receive]] (STR), [[Binary Synchronous Communications|binary synchronous communications]] (BSC), [[Synchronoussynchronous Datadata Linklink Controlcontrol]] (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 | modems]]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) bpsmodulation, while synchronous modems could run at speeds up to 9600{{nbsp}}bit/s bps.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 Those modems are obsolete, having been replaced by modems with which convert asynchronous data to synchronous forms, but similar synchronous telecommunications protocols survive in numerous block-oriented technologies, includingsuch as the widely- used [[IEEE 802.2]] (Ethernet) link-level protocol. USART's,USARTs thoughare apparentlystill nosometimes longerintegrated manufacturedwith asMCUs. standalone componentsUSARTs are oftenstill integratedused within MCUs.routers that (Doesconnect anyoneto knowexternal ifCSU/DSU anydevices, currentand protocolthey implementationsoften relyuse oneither Cisco's proprietary HDLC implementation or SDLCthe framing?[[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 covers only someprovides a few general characteristicsnotes.

* 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-oriented mode(STR and BSC) modes, the device relied on particular characters to define frame boundaries; in bit-oriented mode(HDLC theand SDLC) modes earlier devices relied on physical-layer signals, towhile definelater framedevices boundariestook over the physical-layer recognition of bit patterns.

* A synchronous line is never silent; when the modem is transmitting, data is flowing. When Inthe character-orientedphysical layer indicates that the modem is modeactive, a USART will send a steady seriesstream of synchronizationpadding, either characters; inor bit-orientedbits modeas thisappropriate paddingto isthe provideddevice byand the physical layerprotocol.

USARTs operating as synchronous devices used either character-oriented or bit-oriented mode. In character-oriented mode, the device relied on particular characters to define frame boundaries; in bit-oriented mode the devices relied on physical-layer signals to define frame boundaries.

A synchronous line is never silent; when the modem is transmitting, data is flowing. In character-oriented mode, a USART will send a steady series of synchronization characters; in bit-oriented mode this padding is provided by the physical layer.

== Devices ==

{| class="wikitable"

(Not a complete list!)

!Manufacturer

An example of a USART was the Intel 8251A<ref>{{Cite web|title = Intel 8251A Programmable Communications Interface |url = http://www.datasheetarchive.com/dlmain/Datasheets-14/DSA-278171.pdf|website = www.datasheetarchive.com|accessdate = 2015-12-16}}</ref>.

!Device

!Description

!Device data

|-

|Intel

|[[Intel 8251|8251]]A<ref>{{Cite book |last1=Khalid |first1=Saifullah |url=https://books.google.com/books?id=DVHPhOTrAWcC |title=Microprocessor System |last2=Agrawal |first2=Neetu |publisher=Laxmi Publications Pvt Limited |year=2009 |isbn=9788131807521}}</ref>{{Rp|page=396}}

|Programmable communications interface

|Intel 8251A data sheet<ref>{{Cite web |title = Intel 8251A Programmable Communication Interface |url = http://www.datasheetarchive.com/dlmain/Datasheets-14/DSA-278171.pdf |website = www.datasheetarchive.com |access-date = 2015-12-16 |archive-url=https://web.archive.org/web/20151222120842/http://www.datasheetarchive.com/dlmain/Datasheets-14/DSA-278171.pdf |archive-date=22 December 2015 |url-status=dead}}</ref>

|-

|Signetics / Philips

|2651

|Programmable communications interface

An example of a USART|Philips wasSemiconductors theSCN2651 Inteldata 8251Asheet<ref>{{Cite web | title = IntelPhilips 8251ASemiconductors SCN2651 Programmable Communications Interface | url = httphttps://wwwdatasheet.datasheetarchive.com/dlmainoriginals/distributors/Datasheets-1425/DSA-278171495164.pdf | website = www.datasheetarchive.com |accessdate access-date = 20152020-1204-1605}}</ref>.

|-

|Zilog

|"SIO" Z8440&ndash;4/Z84C40&ndash;4

|Serial input/output controller

|Zilog #ps0183, Z8440/1/2/3/4 and Z84C40/1/2/3/4 data sheet<ref name="zilog_com-ps0183">{{cite web|title=Zilog Product specification Z8440/1/2/4, Z84C40/1/2/3/4. Serial input/output controller|url=http://www.zilog.com/docs/z80/ps0183.pdf}} 090529 zilog.com</ref>

|-

|Zilog

|[[Zilog SCC|"SCC"]] Z8530/Z85C30; Z85230/Z80230/Z8523L/Z85233

|Enhanced serial communications controller

|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>

|}

== References ==

<references />

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{{DEFAULTSORT:Universal Synchronous Asynchronous Receiver Transmitter}}

[[Category:Data transmission]]