Universal synchronous and asynchronous receiver-transmitter

A universal synchronous and asynchronous receiver-transmitter (USART, programmable communications interface or PCI)[1] is a type of a serial interface device that can be programmed to communicate asynchronously or synchronously.

In those times, the fastest asynchronous voice-band modem could achieve at most speeds of 300 bit/s using frequency-shift keying (FSK) modulation, while synchronous modems could run at speeds up to 9600 bit/s using phase-shift keying (PSK).

Those modems are obsolete, having been replaced by modems which convert asynchronous data to synchronous forms, but similar synchronous telecommunications protocols survive in numerous block-oriented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 used in routers that connect to external CSU/DSU devices, and they often use either Cisco's proprietary HDLC implementation or the IETF standard point-to-point protocol (PPP) in HDLC-like framing as defined in RFC 1662.

The operation of a USART is intimately related to the various protocols; refer to those pages for details.