Start, stop, and mark bits

The start and stop bits are used in asynchronous communication as a means of timing or synchronizing the data characters being transmitted.

Without the use of these bits, the sending and receiving systems will not know where one character ends and another begins.

Another bit used to separate data characters during transmission is the mark (or idle) RS bit. This bit, a binary 1, is transmitted when the communication line is idle and no characters are being sent or received.

When a start bit (binary 0) is received by the system, it is understood that a fixed number character bit (determined by the bits per character parameter), and even a parity bit (determined by the parity parameter), follows the start bit. Then a stop bit (binary 1) is received by the system. In the following example, the parity bit is present, and the bits per character is 7.