The instant you have the same output connected on more than one slave box, it provides a direct connection between the different box select lines, via the external wiring.
Wiring and terminals are not "directional" - power will flow either way through any connection.
Adding diodes makes the individual outputs directional; power can flow out to the load, but not back in to affect other channels.
It needs a separate diode on each output at each D socket.
Wiring and terminals are not "directional" - power will flow either way through any connection.
Adding diodes makes the individual outputs directional; power can flow out to the load, but not back in to affect other channels.
It needs a separate diode on each output at each D socket.