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D0 D7 RD WR A0 A1 CS. We keep sending walking zeros: 111 0 - 11 0 1 - 1 0 11 - 0 111. Now we sense the change by inputting data from Port A. This combination can be used to look up what button this represents i.e. A.

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**D0**D7 RD WR A0 A1 CS We keep sending walking zeros:1110-1101-1011-0111 Now we sense the change by inputting data from Port A. This combination can be used to look up what button this represents i.e. A. We output a walking Zero through Port B and keep reading until we sense a change in Port A values. Now we have a combination of what we output to Port B when we sensed a change in Port A. We sensed a changed in Port A to 1110 Assume a button is pressed! Assuming 82C55 is enabled. We program Port A to input data and Port B to output data. No change! Send data to Port B 1110 82C55 PA0 PA7 PB0 PB7 PC0 PC7 D7-D0 1 0 0 1 IORC IOWC A0 A1 Vcc 0 1 1 0 1 0 11 0 1 11 1 1 01 1 1 10

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