Definition
Transmitting and receiving serial data
Bits have to be moved from one place to another using wires or some other medium. Over many miles, the expense of the wires becomes large. To reduce the expense of long communication links carrying several bits in parallel, data bits are sent sequentially, one after another, using a UART to convert the transmitted bits between sequential and parallel form at each end of the link. Each UART contains a shift register which is the fundamental method of conversion between serial and parallel forms.
By convention, teletype-style UARTs send a "start" bit, five to eight data bits, least-significant-bit first, an optional "parity" bit, and then one, one and a half, or two "stop" bits. The start bit is the opposite polarity of the data-line's idle state. The stop bit is the data-line's idle state, and provides a delay before the next character can start. (This is called asynchronous start-stop transmission). In mechanical teletypes, the "stop" bit was often stretched to two bit times to give the mechanism more time to finish printing a character. A stretched "stop" bit also helps resynchronization. The parity bit can either make the number of "one" bits between any start/stop pair odd, or even, or it can be omitted. Odd parity is more reliable because it assures that there will always be at least one data transition, and this permits many UARTs to resynchronize.
The UART usually does not directly generate or receive the external signalling levels (such as voltages on wires) that are used between different equipment. Typically, an interface is used to convert the logic level signals of the UART to the external signalling levels. "Signalling levels" is a very broad term encompassing all the various possible schemes to convey a level from one place to another. Voltage is by far the most common kind of signalling used. Examples of standards for voltage signalling are RS-232, RS-422 and RS-485 from the EIA. Historically, the presence or absence of current (in current loops) was the dominant kind of signalling used. Depending on the limits of the communication channel to which the UART is ultimately connected, communication may be "full duplex" (both send and receive at the same time) or "half duplex" (devices take turns transmitting and receiving). Some signalling schemes do not use electrical wires. Examples of such are optical fiber, infrared, and (wireless) Bluetooth in its Serial Port Profile (SPP). Some signalling schemes use modulation (with or without wires). Examples are modulation of audio signals with phone line modems, RF modulation with data radios, and the DC-LIN for power line communication.
As of 2006, UARTs are commonly used with RS-232 for embedded systems communications. It is useful to communicate between microcontrollers and also with PCs. Many chips provide UART functionality in silicon, and low-cost chips exist to convert logic level signals (such as TTL voltages) to RS-232 level signals (for example, Maxim MAX232).