Hans:
You pose a very intersting idea. I'll have to sketch that out and see what it looks like. It may be an excellent scheme seeing as I already have two diodes per input. Maybe this is a way to return to the concept of two pins per input out to the connector [oops cant get that many pins on a DB-25 :-) ] and let the user pick the polarity by the external wiring arrangement.

I'll post back if anything gels along these lines.
Michael Karas