Jameco Electronics Rabbit 3000 - Manual (Page 60)

User’s Manual 51
4.2 Open-Drain Outputs Used for Key Scan
The Parallel Port D outputs can be individually programmed to be open drain. This is use-
ful for scanning a switch matrix, as shown in Figure 4-2. A row is driven low, then the col-
umns are scanned for a low input line, which indicates a key is closed. This is repeated for
each row. The advantage of using open-drain outputs is that if two keys in the same col-
umn are depressed, there will not be a fight between a driver driving the line high and
another driver driving it low.
Figure 4-2. Using Open-Drain Outputs for Key Scan
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o.d.
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