Abstract: A testable physically isolated control circuit includes a first unit 10, a second unit 40, and an optical coupler 30 to couple units 10 and 40 together. The functional portion 10a of the first unit includes an input terminal 11 and an oscillator 12 for controlling a first transmitter 17. Signals from the first transmitter are received by a receiver 44 in unit 40 to control the relay 48. Each unit 10 and 40 also includes a testing portion 10b and 40b for monitoring the condition of the functional portions. The testing portion 40b detects the state of the receiver 44 and the relay 48 and transmits this information via transmitter 65 to a receiver 22 in first unit 10. This receiver 22 controls input terminals 29 and 15 to provide control information concerning the proper functioning of the circuit.