In recent years, computers have come to play increasingly more important roles. In consequence, the reliability of the computers have come to be eagerly requested. As the causes of malfunctions which spoil the reliability of the computers, there are mentioned permanent failure of hardware, bugs in software, transient data errors ascribable to electrical noise and the irradiation of radiation, etc.
For heightening the reliability of a system, there is employed a method wherein subsystems constituting the system are made redundant and multiplied. Regarding methods of handling the outputs of the multiplexed subsystems, "A Highly Efficient Redundancy Scheme: Self-Purging Redundancy", IEEE Transactions on Computers, Vol. C-25, No. 6, June 1976, written by Mr. Jacques Losq, states in FIGS. 2 and 3 and its explanation on page 570 a method of making decisions by majority voter and a method of changing-over outputs on the basis of the cross-diagnostic results of the subsystems.
Referring to the prior art, the outputs of redundant subsystems are input to an elementary switch. In the elementary switch, only the input signals which coincide with the output signal of the majority voter is output to the voter. The voter produces the system output by majority voting of the output signals of the elementary switch. In the prior art mentioned above, the method based on the decision of majority voting has had a drawback that the majority voter can not output a normal signal unless two or more subsystems or modules are normal as disclosed in paragraph one on the right column on page 570 thereof. Namely, when a breakdown occurred in the redundant subsystems and the number of normal subsystems is reduced to one, the system is never operated normally.