The 4 Module Motherboard grew to 51 x 100mm as I decided to add the power connectors (seen on right above) outside the module area. You can also see two 5V regulators on the motherboard. Each SPX3819 deliver 500mA and I can feed of 6-8V or 12V with jumpers, but I could also add a small watchdog with 4 x SPX3819 and capability to switch power on/off on each module. Regardless this motherboard is a proto, we can modify it later.
I might create a new version later with a watchdog and power on/off relays on the side – space under will be to tight, but I can switch on/off 5V using 4 x SPX3819 regulators. For 8V, 12V and 24V I need proper relays and drivers for these. Power lanes on the board is a bit tricky as well. I benefit from using 4 layers here, but I am not sure about high speed RS485 and CAN-FD from board to board. We will find out. CAN-FD and RS485 connectors can be added to the other end for 1 module or 2 module motherboards. I need to make the 1-module motherboard to support stand-alone modules as a minimum. Not sure about 2-module yet, but I do want larger motherboards.