Asynchronous serial communication

In this diagram, two bytes are sent, each consisting of a start bit, followed by eight data bits (bits 0-7), and one stop bit, for a 10-bit character frame. The last data bit is sometimes used as a parity bit. The number of data and formatting bits, the order of data bits, the presence or absence of a parity bit, the form of parity (even or odd) and the transmission speed (frequency) must be pre-agreed by the communicating parties. The "stop bit" is actually a "stop period"; the stop period of the transmitter may be arbitrarily long. It cannot be shorter than a specified amount, usually 1 to 2 bit times. The receiver requires a shorter stop period than the transmitter. At the end of each character, the receiver stops briefly to wait for the next start bit. It is this difference which keeps the transmitter and receiver synchronized.

Asynchronous serial communication is a form of serial communication in which the communicating endpoints' interfaces are not continuously synchronized by a common clock signal. Synchronization (clock recovery) is done by data-embedded signal: the data stream contains synchronization information in a form of start and stop signals set before and after each payload transmission. The start signal prepares the receiver for arrival of data and the stop signal resets its state to enable triggering of a new sequence.

A common kind of start-stop transmission is ASCII over RS-232, for example for use in teletypewriter operation.