PURPOSE: To enable the detection of a cause for a short time by feeding a pulse signal to a cable to determine the presence of an output of a circuit for detection of a reflected wave thereof.
CONSTITUTION: When the communication is disabled for some reasons or another, it is halted while an electromotive signal of a pulse generation circuit 2 is applied to a terminal 3. On the electromotive signal, a pulse signal is generated and fed to a cable 12. When there is any disconnection in the cable 12, a positive reflection is caused at the disconnected point, a diode 6 conducts to let current flow and a differential amplifier 9 is operated to produce an output at a terminal 10. When there is no disconnection in the cable, no reflected pulse is returned and hence no output is produced at the terminal 10 of the differential amplifier 9. Therefore, when no output is obtained at the terminal 10, causes other than disconnection are identified, which enables the detection of causes for a short time.