Importance

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Abstract

A programmable controller includes a number of equipment racks having slots which receive a processor module and a plurality of input/output modules. Each rack has a backplane which includes a number of buses for electrically interconnecting the modules therein, one of these buses carries slot address signals identifying the slot containing an input/output module which the processor module seeks to access. The backplanes of the racks are connected together in a daisy chain. Each backplane has a circuit which responds to the addresses of that rack's slots by producing enable signals for the module in the corresponding slot. This backplane circuit also subtracts the number of slots in its rack from the slot address and passes the result to the next rack in the daisy chain. By this justification of the slot address as it is passed down the daisy chain, each rack address decoder circuit can be hard wired to respond to slot addresses between zero and X, where X is the number of slots in that rack; regardless of the rack's relative position within the daisy chain and the range of virtual addresses for its slots.

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First Claim