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HowTo: How To Build A Parallel Port Controller Box
( Hill Robertson)
Step 8 : General Control
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| | Here is the top left of the schematic so you can see how the signaling works. As you can see, there are two major databuses in the schematic. The first is indicated by the blue and green connections between the parallel port, the first flip flop (374) chip and all of the decoder chips (138). The second is indicated by the blue connections between the output of the first flip flop (374) and the input of all other flip flops.
This first databus is used to transfer the data for each box and select which 8 circuit box receives the byte of data. The second databus puts the byte of circuit box data to the inputs of all flip flops. The control bits are used as enables (or chip select) for the flip flop data buffer and the decoders.
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