Hi Nico,

I work with an 80c31 system that has a master cpu board that turns on "daughter" boards that also have 80c31 when it needs to talk to them. 74hc138 is used to turn them on. 8255 could be used as well. Transistor are used to "couple" the Txd and Rxd lines to and from the cpu. Also and AND gate for one of the lines. I will not totally explain how the circuits are put together because of "proprietary" reasons. But what I have told seems to be known to others. Hope this helps.

TTYL,

Dave Kaleta