The M500M Monitor Modules from System Sensor provide a two-wire, or fault-tolerant, initiating circuit for monitoring and operating contact fire alarm and supervisory devices or security devices. The module can normally open contact fire alarm and supervisory devices or either normally open or normally closed security devices. The M500M Monitor Module is intended for use in intelligent, two-wire systems, where the individual address of each module is selected using the built-in rotary decade switches.