55 shuts off the power constant voltage vcc for the can driver 52 when a constant voltage failure system shut signal or other failure signal in the faulty unit 50 is detected by the failure check means 53 in the present embodiment the failure check means 53 also outputs a high level failure detection signal the transistor switching circuit 54 then performs a switching operation so that the high side driver 55 which supplies power to the can driver 52 changes its state so as to stop the power supply to the can driver 52 as a result the can driver 52 stays in a no signal output state the other unit 90 which is connected to the communication bus 100 detects this state and then performs a fail safe process in the same manner as with the first embodiment shown in fig5 in the second embodiment the cost is relatively high because the high side driver 55 turns off the can driver 52 instead of disconnecting the line between the can controller cpu 51 and can driver 52 however it is not necessary to insert a resistor or the like into a serial communication line between the can controller cpu 51 and can driver 52 therefore no design changes need be made to compensate for a decreased communication speed or the like the present embodiment supports high grade specifications for high speed communications 0059 fig7 illustrates a third embodiment which disconnects the communication bus can bus 100 outside the faulty unit 50 in the present embodiment the communication bus 100 is provided with a relay switch 61 the relay switch 61 opens.