A triple modular redundancy (TMR) system incorporates majority voting to achieve fault tolerance, although not all single faults can be tolerated due to the possibility of voter failure. A totally self-checking (TSC) circuit is introduced to monitor the TMR system; it receives the outputs of each module and that of the voter to generate three system statuses, and three module status outputs. The TSC± TMR system has the capability to detect all single fault occurrences inclusive of the checking circuitry, thus achieving optimum availability and maintainability regarding single fault occurrences. Morphic Boolean algebra forms the basis of the TSC fault detecting circuit design. Morphic building blocks constitute the newly designed checking circuit.