PURPOSE: To easily check whether or not the connection of a cable is correct when the cable is used for interconnecting devices.

CONSTITUTION: Tri-state output gates 11-14 are connected respectively in series with plural input/output lines 31 in a high-order device 100 connected via cables 26A-26D. Similarly, tri-state output gates 24-25 are connected in series respectively with an input/ /output line in a low-order device 200. While electronic devices are interconnected by the cables, when the connection state of the cable is tested, the tri-state output gates 21-25 are turned on and the connection of the cables 26A-26D is all correct, an LED lamp 5 is lighted. When the test is conducted, all the tri-state output gates 11-18 are turned off.