Parity checking

Abstract

You are probably already familiar with the idea of parity checking as a means of error detection. This technique has been built into many lowspeed serial communications links for decades and provides a good starting place for us. In its simplest form the parity check uses one extra bit per character, where traditionally a character is a five-, seven- or eight-bit word. The two ends of the communications link must first be agreed upon the type of parity used, such as no parity, odd parity or even parity. For example, if even parity is used, then the extra parity bit is set so that the total number of bits in the character plus parity that are set to one (the weight of the message) is even. With odd parity the weight of the character plus parity is arranged to be odd. Some examples are shown in Table 2.1 below. Column E contains the even parity bits while column O shows the odd parity bits.