All-Magnetic Logic

Multiaperture devices, or MADs, are magnetic ferrite elements of complex shape, interconnected solely by winding of copper wire. Hew Crane began their development, working from the notion of controlling the direction of bit flow in adaptations of magnetic ferrite memory cores to achieve complete logic capability. These logic circuits were essentially indestructible, in contrast to vacuum tubes and transistors, and they drew no power when not in use.

SRI's technology was commercialized by AMP, Inc. MADs were used in a control system for the Canadian National Railroad Hump Yard in Toronto because semiconductors could not withstand lightning surges. They controlled a portion of the New York subway lines to assure safe operation against electrical transients. Lockheed and Ford Aeronautics used them in a secure access decoder so that no unauthorized agency could access or control intelligence satellites. They were used in the B-52 bomber for the IFF (identification friend or foe) system. The Minuteman launch control system used them for immunity to electromagnetic pulses from nuclear detonations. Television stations used MADs for the audio part of video switching. Patents from the SRI work went to Burroughs (16 patents) and AMP, Inc. (6 patents).

Crane began the work the mid-1950s at RCA Laboratories (now Sarnoff Corporation, a subsidiary of SRI International) and continued after he joined SRI in 1956. He introduced the basic all-magnetic logic approach at the Fall Joint Computer Conference in 1959. In 1961, the SRI magnetics group demonstrated an Air Force-funded multiaperture logic system that was the world's first, and only, all-magnetic computer.

In 1969, McGraw-Hill published Digital Magnetic Logic by Crane, Dave Bennion, and David Nitzan. What prevented the all-magnetic logic from developing a permanent niche in the computer market was the advent of the integrated circuit and the silicon chip. However, MAD-like units may yet be needed for long space flights or remote installations where maintenance and replacement are difficult.