An assessment of the value of triplicated redundancy in digital systems
Abstract The use of redundant components has been suggested as a method of improving both the manufacturing yields of “integrated” electronic devices and the reliability of operation of electronic digital systems. One method is to employ three copies of the equipment and take a “vote” on the output. Provided only one system is in error, the correct output will be maintained, but two systems in error will produce an erroneous result. The basic idea can be improved by undertaking voting at more than one place in the system so that a multiplicity of faults may (with calculable probability) be tolerated. The number of stages of voting can be optimized with respect to parameters which include the reliability of individual components and the size of the system. A further improvement can be achieved by triplicating the voting circuits themselves. This latter system is more reliable than the former but requires the use of six times as many components as the equivalent non-redundant system. Significant improvements in yield and reliability over that obtained with the non-redundant system, while maintaining reasonable levels of yield and reliability are only obtained with very large systems employing very good components.