Abstract: A fire alarm system in which a large number of terminal units, comprising detectors and interface circuits, at a variety of locations are coupled or connected in separate, independent loops of the system, each of the scattered terminal units being addressable from a central control panel. The arrangement is such that serial synchronous power transmission and data communication is effectuated over the individual loops of the system. The improvement resides in providing continuity-isolation testing for a Class A wiring scheme involved in the loops. By Class A operation is meant that there are two separate channels involved in each loop. A continuity test is sequentially performed on both channels of the loop such that discontinuities can be precisely located.