Abstract: An automatic self-test apparatus for use in a digital multiplexed communication system and self-testing idle circuitry coupled to idle telephone lines. The apparatus utilizes a tone generator which generates digital words describing an analog test signal and a detector which detects digital data on the internal data bus which describes this self-test analog signal. The data from the tone generator is coupled to the circuitry connected to the idle line through a time division multiplexer under the control of a controller consisting of a programmed microprocessor. The microprocessor controls when the transfer is made and to which telephone line it is directed. This control is accomplished by writing a control word to a multiplexer naming the source and destination for the transfer and the time slot in which it is to occur. When the circuitry connected to the idle line functions properly, the controller proceeds to the next idle line. If a malfunction is detected, the controller sets an alarm.