Parity Bit - History

History

A "parity track" was present on the first magnetic tape data storage in 1951. Parity in this form, applied across multiple parallel signals, is known as a transverse redundancy check. This can be combined with parity computed over multiple bits sent on a single signal, a longitudinal redundancy check. In a parallel bus, there is one longitudinal redundancy check bit per parallel signal.

Parity was also used on at least some paper-tape (punched tape) data entry systems (which preceded magnetic tape systems). On the systems sold by British company ICL (formerly ICT) the 1-inch-wide (25 mm) paper tape had 8 hole positions running across it, with the 8th being for parity. 7 positions were used for the data, e.g., 7-bit ASCII. The 8th position had a hole punched in it depending on the number of data holes punched.

For a contrary view, Seymour Cray, premier designer of supercomputers, held parity designs in contempt. He felt it showed poor design—if you designed your transmission paths to be reliable, you would not have to waste resources on parity. His famous quote on this (circa 1963) was "Parity is for farmers" (after the use of the term "parity" in the New Deal). After he later included parity bits on the CDC 7600, Cray reputedly said that "I learned that a lot of farmers buy computers."