A universal synchronous and asynchronous receiver-transmitter (USART, programmable communications interface or PCI) is a type of a serial interface device that can be programmed to communicate asynchronously or synchronously. See

universal asynchronous receiver-transmitter A universal asynchronous receiver-transmitter (UART ) is a computer hardware device for asynchronous serial communication in which the data format and transmission speeds are configurable. It sends data bits one by one, from the least significan ...

(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 Synchronous transmit-receive (STR) was an early IBM character-oriented communications protocol which preceded Bisync. STR was point-to-point only, and employed a ''four-of-eight'' transmission code, communicating at up to 5100 characters per se ...

(STR),

binary synchronous communications Binary Synchronous Communication (BSC or Bisync) is an IBM character-oriented, half-duplex link protocol, announced in 1967 after the introduction of System/360. It replaced the synchronous transmit-receive (STR) protocol used with second gene ...

(BSC),

synchronous data link control Synchronous Data Link Control (SDLC) is a computer communications protocol. It is the layer 2 protocol for IBM's Systems Network Architecture (SNA). SDLC supports multipoint links as well as error correction. It also runs under the assumption th ...

(SDLC), and the ISO-standard

high-level data link control High-Level Data Link Control (HDLC) is a bit-oriented code-transparent synchronous data link layer protocol developed by the International Organization for Standardization (ISO). The standard for HDLC is ISO/IEC 13239:2002. HDLC provides both c ...

(HDLC) synchronous link-layer protocols, which were used with synchronous voice-frequency

modem A modulator-demodulator or modem is a computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio. A modem transmits data by Modulation#Digital modulati ...

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 300bit/s using

frequency-shift keying Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier signal. The technology is used for communication systems such as telemetry, weather ball ...

(FSK) modulation, while synchronous modems could run at speeds up to 9600bit/s using

phase-shift keying Phase-shift keying (PSK) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency reference signal (the carrier wave). The modulation is accomplished by varying the sine and cosine inputs at a ...

(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 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 IEEE 802.2 is the original name of the ISO/IEC 8802-2 standard which defines logical link control (LLC) as the upper portion of the data link layer of the OSI Model. The original standard developed by the Institute of Electrical and Electronics Engi ...

(Ethernet) link-level protocol. USARTs are still sometimes integrated with MCUs. USARTs are still used in routers that connect to external CSU/DSU devices, and they often use either Cisco's proprietary HDLC implementation or the

IETF The Internet Engineering Task Force (IETF) is a standards organization for the Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster or requirements and a ...

standard point-to-point protocol (PPP) in HDLC-like framing as defined in RFC1662.

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 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.

Devices

References

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