PURPOSE: To ensure the detection at an early stage for the faulty area by connecting the non-busy channels to the automatic test equipment in sequence under the operation of PCM exchange.

CONSTITUTION: The test signal given from point x of test equipment TECT is supplied to decorder DEC1 via common line switch HWSW, and the decoding signal is supplied to point q of TEST via hybrid H and prescrived route R3 of switch steel SDSW. The input signal is returned directly through TEST and then supplied to H via point q and route R3 again to be next supplied to point y of TEST via encoder COD1 and HWSW. TEST gives a test to the signal transmitted from point x as well as to the signal supplied to point y. In this case, the signal produced by sampling, quantizes and encodes the DSB wave of 100% modulation degree is used for the test signal. Furtheremore, the frequency is selected for the modulation wave and the carrier wave so that both the upper and lower side band waves of the modulation wave may be within the pass band of the telephone signal and also be close the frequency of the upper and lower limit of the band.