PURPOSE:To simply and quickly check the normal operation of a bus arbiter by selecting the same module in each times, confirming normal arbitration operation and detecting a failure. CONSTITUTION:A module requesting bus using right outputs arbitration numbers belonging to a code '1' to lines an0 and an6 and sets up a Comp signal and the inverse of Comp signal respectively to '1' and '0'. A module requesting in the next arbitration outputs arbitration number belonging to a code '2' to lines an0 to an6 and set up the Comp signal and the inverse of Comp signal respectively to '0' and '1'. As the result of two arbitrations, a module having respective outputs (1,0), (0,1) is regarded as a module succeeding in bus using right acquisition. The absence of the module for acquiring the bus using right indicates the existence of a failure on the arbiter of a bus line. Consequently, the abnormality of the bus line can be detected in addition to the detection of abnormality on the arbiter and the normal operation of arbitration can be confirmed.
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