Collision detection and avoidance wiring


YASPIE protocol, basically bit banging one's and zero's to the basement arduino uno from wall socket arduino micro's. Using a signal high wire (pullup) to 5V. The micro in the wall pulls the signal wire down for a 1 through a BC547, it's base is connected to pin15 (A1) (used as digital OUTPUT pin):

see A1.png

I am now in the phase were I need to check if the signal wire is already occupied (used): the collision avoidance and detection part. The logic is, IMHO, quite simple cause:
A1 is HIGH, signal wire will ALWAYS be LOW, as it is grounded, so no need to check that
A1 is LOW, signal wire should be HIGH, but there might be another Micro pulling the wire down.
Putting the signal wire into the base of a BC547, the collector connected to pin 14 (A2 used as digital INPUT PULLUP), when A1 == 0, A2 should be 0 if not, there is a collision ..

see A1+A2.png

I just wonder if I can simply do this or if I am overlooking something really significantly important

All your transistor circuits are missing base resistors to limit the current.

The micro in the wall pulls the signal wire down for a 1 through a BC547
Doesn't look that way to me.
Perhaps it would be better to explain/draw what it is you are trying to achieve.
Opto's might be an alternative worth thinking about, still need resistors.
Advantage may be when you start adding transistors in certain configurations.