A UART sends a stream of bits over a serial line without sharing a separate clock signal between transmitter and receiver.

Both ends agree on a baud rate and frame format. A common frame contains a start bit, several data bits, an optional parity bit and one or more stop bits.

The idle line has a defined level. The start bit gives the receiver a timing reference, after which it samples later bit positions using its own local clock.

Because the clocks are independent, their frequencies must be close enough that sampling does not drift outside the intended bit intervals over one frame.

UART is typically full duplex with separate transmit and receive signals. Higher-level protocols may add message boundaries, checksums or commands; UART itself only transports framed serial characters or bytes.

Its small signal count and simple hardware make UART useful for device consoles, debugging links and communication between nearby components.