in an apparatus having a main control section and sub - control sections connected to the control section through communication lines for executing desired operations by means of mutual intercommunication with the control section through the communication lines , first and second tri - state buffers are provided between the communication lines and the main control section and between the communication lines and the sub - control section respectively . the tri - state buffers short the communications lines by respective test signals output from the main control sections . when the test signal is not retransmitted to the control section through the communication lines shorted by the second tri - state buffer , anomalous operation of the sub - control section is detected . when the test sinal is not retransmitted to the control section through the communication lines shorted by the first tri - state buffer , anomalous operation of the main control section is detected .