Periodic signals have a known behavior, and deviations in their waveforms may indicate failures in the signal source. Monitoring these signals can be a valuable technique in detecting both hardware and software failures in a computer. Schemes previously used to check for errors in these signals are reviewed and evaluated. These circuits, however, share a common weakness in that they are susceptible to undetectable internal faults. Described here is a self-checking periodic-signal checker; that is, the checker is capable of indicating the occurrence of any error in the input signal and failures in the checker itself. Applications of the checker in aspects of computer operation as well as a fail-safe circuit for communicating the detection of an error to an external observer are presented.