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 peripheral 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 significant to ...

(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 seco ...

(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 gener ...

(BSC),

synchronous data link control Synchronous Data Link Control (SDLC) is a computer serial communication, serial communications protocol first introduced by IBM as part of its Systems Network Architecture (SNA). SDLC is used as layer 2, the data link layer, in the SNA protocol s ...

(SDLC), and the ISO-standard

high-level data link control High-Level Data Link Control (HDLC) is a communication protocol used for transmitting data between devices in telecommunication and networking. Developed by the International Organization for Standardization (ISO), it is defined in the standard ...

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

modem The Democratic Movement (, ; MoDem ) is a centre to centre-right political party in France, whose main ideological trends are liberalism and Christian democracy, and that is characterised by a strong pro-Europeanist stance. MoDem was establis ...

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 encoded on a carrier signal by periodically shifting the frequency of the carrier between several discrete frequencies. The technology is used fo ...

(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 carrier wave. The modulation is accomplished by varying the sine and cosine inputs at a precise time. I ...

(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 (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 standard, Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster ...

standard

point-to-point protocol In computer networking, Point-to-Point Protocol (PPP) is a data link layer (layer 2) communication protocol between two routers directly without any host or any other networking in between. It can provide loop detection, authentication, transmissio ...

(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

__NOTOC__ {{DEFAULTSORT:Universal Synchronous Asynchronous Receiver Transmitter Data transmission