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 Universal asynchronous receiver/transmitter (UART) for a discussion of the asynchronous capabilities of these devices.
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 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 Those modems are obsolete, having been replaced by modems with which convert asynchronous data to synchronous forms, but similar synchronous telecommunications protocols survive in numerous block-oriented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, including 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 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, 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
(Not a complete list!) An example of a USART was the Intel 8251A[1].
References
- ^ "Intel 8251A Programmable Communications Interface" (PDF). www.datasheetarchive.com. Retrieved 2015-12-16.