An independently based diagnostic system tests the execution of a processor. The processor is arranged to provide a diagnostic output that provides a pre-determined time-variant signal. The independently based diagnostic system has an independent basis from which to evaluate the pre-determined time-variant signal. The independent basis can be, for example, an independent time base that is separately generated from the processor time base used to clock the processor and/or an independent voltage source that is separate from the processor power supply. The independently based diagnostic system provides progressive notifications of the results of successive evaluations of the pre-determined time-variant signal.