Character Parity

Parity is an error detection technique which is typically used on asynchronous links. It is used to verify the integrity of individual characters (or bytes) within the transmitted stream. When used, each character is protected by a single parity bit which is the logical exclusive or of all the bits in the character. Two types of parity are used: even parity where a 1-parity-bit is sent if there are an even number of 1-bits and odd which is the opposite (i.e. a 1-parity-bit is sent when there are an odd number of 1-bits).

Parity bit added to a transmitted character at the transmitter and checked at the receiver

At the receiver the parity is recalculated using either an exclusive-or adder or a Finite State Machine (FSM). When the calculated parity from the received character does not match the value of the received parity bit, then a parity error is said to have occurred, and the character is normally discarded. This parity check detects any number of odd errors but passes any number of even errors without detecting an error.

A more powerful error detection techniques is normally used in synchronous communication (e.g. a Cyclic Redundancy Check (CRC)).

Gorry Fairhurst - Date: 01/10/2001