> Nope.  I can't.  The RB outputs here control the columns of 7-seg LED
> displays, which is why they go to the MOSFET gates.  If I just shorted a
> switch between an RB output and ground, then the output will be forced
> low all the time when the switch is pressed.

I didn't realize the B ports were the outputs and the A port the input.
In that case the mechanical switch is fine.

However, the NPN should be a PNP with emitter to the RB line, collector to
the RA line, and base to the level being sensed via a 10Kohm or so
resistor.  The transistor can pull the RA line high only when its RB line
is high.  This does invert the logic for sensing the line from the way you
had it, but that's a minor software change.


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