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 duplicates the Raspberry Pi header and keeps the same pinout. 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.
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.