All my logic gates have two output ports, the positive and the
negative result. So for example my "OR" gate can act as both an OR
and as a NOR gate.
In fact my first design didn't have just dual output ports but
dual LEDs as well. A red one for when we have a "1" output, and a
blue one for the "0" output. But after looking at it for a while
it seemed to me it might be more confusing than illuminating so I
dropped that idea. (I did then briefly consider a single bi-colour
LED, but didn't think that would be any better).
The main reason for dual outputs was that it halves the number of
different board types I thought I would have to have built. At my
low volumes most of the cost of my pcbs is in the tooling rather
than how many pcbs I actually have manufactured.
I also thought there might be some occasions when the logic
actually wanted both the positive and negative results so having
one board outputting both saves some space. (I did use this
feature a few times, but it's not a huge number).
I think I've made the wrong decision here.
The negative result is only used for a small proportion of the
board types (the smallest and simplest). So it did save some money
on board manufacture but not a huge amount, and the extra (rarely
used) terminal adds some cost.
And the dual output adds clutter and reduces clarity.
So if I had my time again I think I would have distinct xxx and