1. We have a BUS/USB COMMs board to handle communications over the bus and to/from USB. The main function of this board is to control the bus and handle communications over the bus. This would be a Cyclone II - Cypress FX2 based board like what Leon is designing. It could take on other roles as LE space permits.
Do you really need much more than the Cypress USB chip for this? What role does the FPGA play?
2. Since the Cyclone II can do LVDS, maybe we should be looking at having only LVDS signals on the motherboard bus. This would require that we either use a LVDS capable FPGA/CPLD or a LVDS serializer/deserializer chip on each plug in board to talk to this bus.
I suggest that you have some group of backplane pins labeled "user1..userx" so you don't restrict things to only LVDS. Alternately, just have a group of perhaps 8 signal pairs defined as LVDS.
You might have some slower signals running around like I2C, SPI, CAN, JTAG (please to consider the ability to use a JTAG chain across the backplane), some open collector lines for a wire-ored way to send interrupt requests around, etc. By limiting the board to only 100mm square, you are ensuring that most systems will be comprised of multiple boards. No one has articulated what is being designed yet -- just some flexible hardware for fun. That being the case, you probably want to leave your options open.
Stirring the pot, Lyle KK7P