I finally set aside the time to add the components to the backplane (it's better if one is in the right mood when there are so many solder points). The 5 din 41612 connectors provide power/gnd to the attached cards as well as a bus of 21 differential pairs and 20 single ended lines. There is no termination on the backplane itself. One the top is a microcontroller (Microchip pic 18F64K22) that connects to some of the single ended bus lines (for i2c and spi and msten), a single unique line to each connector (for spi select and i2c configuration remapping) and a series of indicator leds for status information and a low voltage serial interface for debugging. I haven't used this particular one before but it should be ideal for this board.