mmm, well there's many different method from the opto coupler, to the funky MAX 232, to the current loop, to the simple 'just extend your wires - but do a software compensation method' and finally, use 2 i/o : one to send a query to the button, and the other to read it.

Which one is the best? i think this might be different from an environment to another. (HV or not)