PURPOSE: To test easily and correctly a troubled communication controller by providing a terminal for connecting a tester to a switching means for switching in-use and standby communication controllers.

CONSTITUTION: Where the communications controllers 1a and 1b are living or in a standby state, respectively, they are connected to a modulator/ demodulator 3 through a changover switch 16 and the tester through a changeover switch 6b, respectively. It is assumed that the controller 1a is troubled and inoperable. At such a time, a changeover lever A interlocks the switches 6a and 6b to switch them downward simultaneously. Thus the controllers 1a and 1b are connected to the tester 4 and th modulator/demodulator 3 through the switches 1b and 1a, respectively. As a result, the controller 1b is present, while the controller 1a can be tested by the tester 4.