Abstract: A communication line diagnostic unit (10, 10') that allows telephone central office equipment to perform a test of a communication line and determine the ability of the line as a communication medium is shown. The device is completely passive and transparent during normal operation and responds only to a unique trigger signal generated from the central office verification controller (VC). The unit is powered from a capacitor storage element (C1) charged by the communication line through an AC coupling network (pins 11, 12 and 8) as well as through a DC network comprising voltage regulator device (Q2; Q23; Q24, Q25) when the unit is awakened. Upon receipt of the trigger signal the device responds as by disconnecting the subscriber apparatus and then providing certain electrical terminations through a single solid state switch (Q10) to allow for identification, loop resistance, noise and frequency response measurements.