Universal synchronous and asynchronous receiver-transmitter: Difference between revisions

Revision as of 17:58, 16 December 2015

Revision as of 06:36, 17 December 2015

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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]]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 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 Those modems are obsolete, having been replaced by modems with which convert asynchronous data to synchronous forms, but similar synchronous telecommunications protocols survive in numerous block-oriented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 such as the widely-used [[IEEE 802.2]] (Ethernet) link-level protocol. USARTs are still sometimes integrated with MCUs.{{Expand section|current protocols using synchronous framing, if any exist, or a statement that none do.|date=December 2015}}

== 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.

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|8251A

|Programmable Communications Interface

|Intel 8251A Data Sheet.<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

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|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 ==

<references />

{{DEFAULTSORT:Universal Synchronous Asynchronous Receiver Transmitter}}

[[Category:Data transmission]]