Editing Universal asynchronous receiver-transmitter (section)

{{short description|Computer hardware device}}

[[File:UART block diagram.svg|class=skin-invert-image|thumb|Block diagram for a UART]]

A '''universal asynchronous receiver-transmitter''' ('''UART''' {{IPAc-en|ˈ|juː|ɑr|t}}) is a peripheral device for [[asynchronous serial communication]] in which the data format and transmission speeds are configurable. It sends data [[bit]]s one by one, from the least significant to the most significant, framed by start and stop bits so that precise timing is handled by the communication channel. The electric signaling levels are handled by a driver circuit external to the UART. Common signal levels are [[RS-232]], [[RS-485]], and raw [[Transistor-transistor logic#Serial signaling|TTL]]<ref>{{cite web |title=RS-232 vs. TTL Serial Communication - SparkFun Electronics |url=https://www.sparkfun.com/tutorials/215 |website=www.sparkfun.com}}</ref> for short debugging links. Early teletypewriters used [[Digital current loop interface|current loop]]s.

It was one of the earliest computer communication devices, used to attach [[teletypewriter]]s for an [[command line interface|operator console]]. It was also an early hardware system for the [[Serial Line Internet Protocol|Internet]].

A UART is usually implemented in an [[integrated circuit]] (IC) and used for [[serial communications]] over a computer or peripheral device [[serial port]]. One or more UART peripherals are commonly integrated in [[microcontroller]] chips. Specialised UARTs are used for [[Local Interconnect Network|automobiles]], [[smart card]]s and [[SIM card|SIM]]s.

A related device, the [[universal synchronous and asynchronous receiver-transmitter]] (USART), also supports synchronous operation.

In [[OSI model]] terms, UART falls under layer 2, the [[data link layer]].{{Citation needed|date=April 2025}}