Communication between multiple Rainbowduinos

I’ve been trying to find a way to have multiple Rainbowduinos communicate when chained together. My ultimate goal is to have a chain of these reacting to a single sensor that is connected to the 1st Rainbowduino, which sends messages to the others. I can get it to work using simple serial communication if I connect the 5v, GND, and 1 tx > rx line manually between Rainbowduinos (but backwards… see second image below). If I connect the Rainbowduinos together using their built-in male > female header pins the serial communication does not work. I also tested this direction using only the 3 necessary jumper wires, but it still doesn’t work. Although if I connect the Female to Female headers via jumper wires it works fine. Of course this is not an ideal setup as you can’t seamlessly chain the rainbowduinos together this way.

I also briefly looked into using I2C but according to some forum posts I found here that has been disabled in the v3.0 firmware? (sorry can’t post links because the spam bot blocks me…)

Anyone have any hints as to what is going on here or why I cannot get communication to work using the built-in male > female header pins? Is there anyway to get communication between these when chained together?

I have some images but I also cannot post those because of the spam bot… Hope my description makes sense.

Ditto

Anyone have any hints as to what is going on here or why I cannot get communication to work using the built-in male > female header pins? Is there anyway to get communication between these when chained together?