I’ve made a key discovery with how the control unit and top panel communicate. As the control unit has connectios for 3 main panels, pin5 goes low on each port in turn so each top panel knows when it should be active.

This explains why we saw the control unit send out it’s data 3 times, but then only a reply once. It also makes knowing when the data is actually the start of a new message much easier as you just discard data unless panel select is LOW. This is espcially useful as unlike the newer BP series, there is no explict start of message in the protocol to indicate the start of a new frame

I’ve written a simple test sketch to prove this theory and and the code is much neater Pin5test