Universal synchronous and asynchronous receiver-transmitter: Difference between revisions

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A Universal Synchronous/Asynchronous Receiver/Transmitter (USART) is a type of a serial interface device that can be programmed to communicate asynchronously or synchronously. See [[UART]] for a discussion of the asynchronous capability of this device.

The USART's synchronous capabilities were primarily intended to support IBM's [[SDLC]] and the ISO-standard [[HDLC]] synchronous link-layer protocols, which were used with synchronous voice-frequency modems. These protocols were designed to make the best use of bandwidth while transmitting blocks of data 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. In addition, 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 entirely obsolete, but similar synchronous telecommunications protocols survive in numerous block-oriented technologies, including the widely-used [[IEEE 802.2]] (Ethernet) link-level protocol. An example of a USART was the [[Intel 8251]]8251A<ref>{{Cite web|title = 8251A ... - Datasheet Search Engine Download|url = http://www.datasheetarchive.com/dlmain/Datasheets-14/DSA-278171.pdf|website = www.datasheetarchive.com|accessdate = 2015-12-16}}</ref>. 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?)

A synchronous line is never silent; data was always flowing. When the USART has nothing to transmit in synchronous mode, it sends a continuous stream of ''flag characters''.

USARTs in synchronous mode transmit in [[Frame (networking) |frames]], which are delimited by flag characters. The flag character is only transmitted as a frame delimiter; any flag characters in data are carefully transformed on transmission by the addition of bits (bit stuffing), a process which is reversed on reception. Control and data characters are then sent continuously without start and stop bits. At the end of frames, a [[frame check sequence]] is inserted, so that errors in transmission can be reliably detected. 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.

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