Converting an old DMU 50, where to start?

other way round. the 7i94 is the "master" with the ethernet port.

Mesa can provide the connecting cable for that or you can make up your own. The pin out is in the manuals. 

I use a ribbon cable and I mount the boards close together. The 26th pin on the idc is not used.

Hello, I would like to know if you managed to do this retrofit without changing the original drivers ? Could you update us ?