PURPOSE:To discriminate whether a terminal is in use or a monitor has trouble by providing a coincidence circuit which collates a scan reading signal which is sent to monitor the using condition of the terminal with the detected results of the scan whose condition is monitored. CONSTITUTION:A scan result signal, which is sent from a subscriber's circuit group 4, is supplied to a terminal 12b of a scan coincidence circuit 12. The scan signal, which is supplied to the circuit group 4, is supplied to a terminal 12a of the circuit 12, this signal is delayed by a shift register 12e, and the signal is supplied to a EOR circuit 12f. At this time, the circuit 12f does not generate an output signal by setting the delaying time of the register 12e, a register 12g and a buffer 12h do not also generate the output signal, and a display function 13 does not light up. Therefore, when only the scan display function, which is not shown in the figure, lights up, a terminal is in use, and in addition, when the function 13 also lights up, you can discriminate that a circuit group 4 has trouble.
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