Universal synchronous and asynchronous receiver-transmitter: Difference between revisions

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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]] (BSC), [[Synchronous Data Link Control]] (SDLC) and the ISO-standard [[High-Level Data Link Control]] (HDLC) synchronous link-layer protocols, which were used with synchronous voice-frequency [[modem | modems]]. 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 bps, while synchronous modems could run at speeds up to 9600 bps. 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 Those modems are obsolete, having been replaced by modems with which convert asynchronous data to synchronous forms, but similar synchronous telecommunications protocols survive in numerous block-oriented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, though apparently no longer manufactured as standalone components, are often integrated with MCUs. (Does anyone know if any current protocol implementations rely on HDLC or SDLC framing?)
 
== Operation ==
The operation of a USART is intimately related to the various protocols; refer to those pages for details. This section covers only some general characteristics.
* 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 either relied on physical-layer signals, towhile definelater framedevices boundariestook over the physical-layer recognition of bit patterns.
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.
* 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.
 
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
|8251A
|Programmable Communications Interface
An|Intel example8251A ofData a USART was the Intel 8251ASheet<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>.
|-
|Zilog
|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|accessdate = 2015-12-16}}</ref>
|}
 
== References ==