You want to keep cables away from magnetic sources about as far as required not to see an appreciable deflection on the compass. If you start with good, shielded equipment that has passed the FCC level B emission standard then you are off to a great start. Consistently use shielded cables, apply filtering and ferrite toroids to all cabling entering or leaving your computer equipment. Finally, consider a good EMI ground and Faraday cages. All Rights Reserved.
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This is a must-read for anyone wishing to keep their bits to themselves. Discussions of testing and validation methods are not discussed in the unclassified documents. U.S. Dept. of Defense, July 1, 1981. May 2, 1980. Specification NSA No.