A system for monitoring multiple power supply signals with respect to a threshold has multiple inputs for receiving input power supply signals. A processing circuit of the system produces an output signal at a first level when the power supply signals have a prescribed relationship to a threshold value, and produces the output signal at a second level when at least one of the input power supply signals does not have the prescribed relationship to the threshold value. The processing circuit is configured for selecting the first level corresponding to a level of an input power supply signal at a pre-selected one of the inputs.