All 26 GPIOs are buffered with bi-directional voltage translators to protect the RPi when experimenting with new circuits. There are two different PCBs, a normal and a T-board version. In both cases, the complete circuit has been designed using four TXS0108E ICs by Texas Instruments.
This version (PCB 150719-1) is designed to connect any PCB safely to an RPi. It has been laid out in this format to leave as much useable space as possible on the breadboard. If connected by way of a ribbon cable K1 should be a normal 2x20 pin header. It is also possible to use an extra high female 2x20 stacking header for K1, and to connect the PCB directly onto the Raspberry Pi, just like most HATs. Note that for the T-Board this probably isn’t such a good idea. The breadboard would then have to be placed at exactly the right height to make a good connection. The only advantage with this method is that you could still add a HAT to the Raspberry Pi.
A ribbon cable can connect the output of the buffer to the external circuit. The use of 2x20 sockets (bottom side) is also possible. Connecting the RPi side of the PCBs can be done by a short 40-way ribbon cable with two 2x20 connectors if K1 is an ordinary 2x20 pin header.