Abstract: An alarm system is described wherein both an open connection between an alarm monitor and an alarmed unit or a short circuit across the connection between the monitor and the unit can be detected. A controllable, normally open test device is placed in parallel with a normally open alarm contact. If current flows when the test device is open, either a short circuit or an alarm condition exists. If current does not flow when the test device is closed, an open connection between the monitor and the unit exists. Advantageously, the integrity of connections between alarm monitors and alarmed units is enhanced.

Abstract: A circuit for selectively lighting one of a pair of light-emitting diodes (LEDs) arranged in a parallel, back-to-back, circuit configuration also known as an anti-parallel circuit configuration. One side of the configuration is connected with ground connecting means through a first photon-emitting device which device is optically coupled to a first photon-responsive device connected between the other side of the configuration and a current source. The other side of the configuration is also connected with a ground connecting means through a second photon-emitting device which device is optically coupled to a second photon-responsive device connected between the first side of the configuration and the current source.