Abstract: A fail-safe microprocessor-based system and method permits the controlling and monitoring of electrical devices for use in fail-safe interfacing of electrical devices to microprocessor-based control equipment in applications that are highly safety-critical. Such devices as relays, lamps, motors, contactors, pushbuttons, limit switches, solenoids and the like are monitored and controlled using codes. These functions are performed by standard, “off-the-shelf” microprocessor-based systems that are not ordinarily employed in or configured for such safety-critical applications. The fail-safe system provides for sufficiently fail-safe checking for such systems to be rendered highly reliable.