All-Magnetic Logic Computing

SRI's all-magnetic logic computer prototype

In computing, multiaperture devices, or MADs, are magnetic ferrite elements of complex shape, interconnected solely by winded copper wire. In the 1950s, SRI's Hewitt Crane began developing computer circuitry with MADs, with the goal of achieving complete logic capability by controlling the direction of bit flow in adaptations of magnetic ferrite memory cores. These logic circuits were essentially indestructible, in contrast to vacuum tubes and transistors, and they did not draw power when not in use. Although the advent of the integrated circuit and the silicon chip prevented all-magnetic logic from occupying a permanent niche in the computer market, MAD-like units may yet be needed for long space flights or remote installations where maintenance and replacement are difficult.

Crane began his work on all-magnetic logic at RCA Laboratories, which is now part of SRI, and continued after he joined SRI in 1956. He introduced the basic all-magnetic logic approach at the Fall Joint Computer Conference in 1959. Two years later, an SRI magnetics group demonstrated an Air Force-funded multiaperture logic system that was the world's first, and only, all-magnetic computer. In 1969, Crane's book Digital Magnetic Logic was published by McGraw-Hill. Co-authors included former SRI staff members Dave Bennion and David Nitzan.

SRI's technology was commercialized by Accelerated Memory Production (AMP). MADs were used in a control system for the Canadian National Railroad Hump Yard in Toronto, because semiconductors could not withstand lightning surges. MADs also controlled a portion of the New York subway lines to ensure safe operation against electrical transients. Lockheed and Ford Aeronautics used MADs in a secure access decoder to prevent unauthorized agencies from accessing or controlling intelligence satellites. MADs were also used for the identification friend-or-foe system in the B-52 bomber. The Minuteman launch control system used them for immunity to electromagnetic pulses caused by nuclear detonations. Television stations used MADs for the audio part of video switching. Sixteen patents resulting from this SRI work went to Burroughs, and six went to AMP.

A prototype version of the all-magnetic logic computer was donated to the archives of the Computer History Museum in Mountain View, California.