Electronic Design

Redundancy Promises High Reliability

Reliability demands of military electronics are forcing development of advanced concepts of redundant circuit design. Majority voting, network, and adaptive concepts are commanding the interest of the armed services, now saddled with the "put in two of everything" approach to redundant design.

Typical of this approach, which only doubles reliability, is the SAGE FSQ-8 air defense system, where duplicate $12-million computers are used. Similarly, BMEWS plays it safe with two 7090 computers. The pitfalls in this approach to reliability were convincingly demonstrated by last summer's Courier communications satellite, which failed aloft when a selection mechanism between two of its redundant systems failed. Parallel redundant units are certainly feasible in ground systems, where defective equipment can be serviced while standby units take over. In space or aloft, the concept and the equipment both fail.

The Air Force, quite aware of these shortcomings, is encouraging development of more sophisticated redundancy concepts. Majority voting techniques and network redundancy, such as the Quad concept developed by Magnavox Research Laboratories, typify current approaches to the problem. A recent contract from the Rome Air Development Center to Aeronautical Radio, Inc. (ARINC), Washington, D.C., calls for design of a "hypothetical computer for a hypothetical satellite" based on majority-voting techniques.

In this type of redundancy, the outputs of several circuits performing the same function are scanned by a logic system that transmits a signal representing a majority of the outputs present. Since an odd number of circuits is commonly used, total failure is avoided until more than half of the individual circuits fail.

An unusual and rather fully developed technique of achieving ultra-reliable circuitry through network redundancy is the Quad principle being proposed by Magnavox. Each Quad has a characteristic terminal impedance identical to the impedance of the equivalent single component. Failure of one or more components through short-circuit or open circuit will alter the terminal impedance of the Quad. Through proper design, however, the circuit can survive a wide variation of impedance among its individual quadded components. (electronic design, Feb. 15, 1961, p. 4)

In the early 1960s, as confidence grew in the new technology, it became clear that larger and larger systems were not only practical, but also were necessary to handle the growing demands of cold-war defense and space exploration. The systems were bigger and more likely to fail, and redundancy appeared to be the most practical solution for reliable operations.

See associated figure.

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