The friendship module is the backplane of any HGG groundstation. This is basically the board where the extension cards are plugged in to. It is the backplane for the bus used by the modules to carry measured data, timing information and all other things over. Thus friendship connects everything.

For simplicity the cards are connected over a low speed data bus that will carry a protocol named Magic. This bus will use RS485 for communication since it can be used with most microcontrollers and there are existing cores for it for FPGA and CPLD. The slow speed data bus is mandatory to support for all modules. Modules must be able to cope with a system that only supports the slow speed bus. A high speed data bus is currently not supported and subject to later development. About 32 pins in the connectors have been reserved for that purpose.

The connectors used for the module cards are physically (but not electrically) PCI-Express x4 Slots. Those are reasonably cheap and generally available at electronic shops.

One of the ports is special, it is the port for the busmaster. The busmaster controls the stuff that happens on the bus and is also controlling the bus protocol. The busmaster port has an additional set of pins at the front of the PCIe slot. Thus not more than 1 busmaster can be present on the bus.

Only few things should happen on the backplane board. It is mainly a bus system and nothing more. So there's just the slots and a few components on that board.

The design for PCIe x4 slots and Cards can be found in the HGG git repository on github (direct link: https://github.com/shackspace/hgg/tree/master/eagle). It contains an eagle library we use in our designs. Currently only one side of the PCIe slots is used, the other pins are reserved for extensions (like for example a high speed bus).

The bus looks like this in the schematic: