STOP BIT

Asynchronous serial communication

From Wikipedia, the free encyclopedia

Jump to: navigation, search

Asynchronous serial communication describes an asynchronous, serial transmission protocol in which a start signal is sent prior to each byte, character or code word and a stop signal is sent after each code word. The start signal serves to prepare the receiving mechanism for the reception and registration of a symbol and the stop signal serves to bring the receiving mechanism to rest in preparation for the reception of the next symbol. A common kind of start-stop transmission is ASCII over RS-232, for example for use in teletypewriter operation.

In the diagram, a start bit is sent, followed by eight data bits, no parity bit and one "stop bit", for a 10-bit character frame. The number of data and formatting bits, and the transmission speed, 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 in synchronism.