PURPOSE:To easily judges trouble contents originating from a common bus by monitoring the time from the generation of a request for the right to control the common bus to the reception of a transfer answer signal, and releasing a microcomputer from being in a state WAIT and performing interruption processing operation. CONSTITUTION:The microcomputer 10 enters the state WAIT after sending the request for the right to control the common bus to a bus master. A trouble diagnostic program that the microcomputer 10 uses in interrruption operation is stored in internal memory 23. The timer from the generation of the request for the right to control the common bus to the reception of the transfer answer signal is monitored by a timer circuit 17, and when this timer circuit 17 detects the monitored time being over, the microcomputer 10 is allowed to exit from the state WAIT and the interruption processing operation is performed.
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