PURPOSE:To facilitate the diagnosis of the fault of a shift bus of its own and to shorten a recovering time, by providing the shift bus consisting of the cascade connection of plural diagnosis blocks, and the shift bus consisting of a storage cell at the rearmost part of the diagnosis block. CONSTITUTION:In a maintenance exchange unit 1a, a first shift bus is constituted by cascase-connecting the storage cells 1a-2-1a-p, and a second shift bus is constituted of only the storage cell 1a-q, and each shift bus is switched through a shift bus selection circuit 1a-1, etc. And when the same data is scanned in the shift buses 1-(n), a second shift bus at every maintenance exchange unit is selected, and the content of each storage cell is read out, and the diagnosis by every maintenance exchange unit is performed. In other words, by providing the second shift bus consisting of only the storage cell 1a-q, and checking the content of it, it is possible to easily diagnose the fault of the shift bus in the unit 1a, and to shorten the recovering time.
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