They have two I2C connectors on one side and the programming connector on the other side. The I2C connectors allow daisy chaining either with fixed-length PCBs or with flat cables.
The daughterboards connect on the two single row connectors on the long sides. The mounting screws on 3 corners only are used to secure against placing the daughter board the wrong way round.
The following figure shows the pinning in case of test needs:
A test board routes the FPGA motherboard connector pins from one to the next. The path also includes the SCL and SDA pins of the bus connector. The path extremities are located on spare I/Os of the JTAG programming connector. Each pair of pins are also connected to a LED, which shows to what extent the signal path is functional.
A square wave is sent from one extermity to the other, thus having half of the pins working as outputs and the other half working as inputs. A switch allows to invert the path's direction, thus changing the I/O mode of all the pins.
The system is powered via a circular connector which connects to a 3.3 V regulator. 5 V is a good power voltage candidate.
The RS232 to USB board can be used as an FPGA mezzanine with only the FTDI USB to RS232 chip mounted.