Some GPIO connector plugs for Raspberry PIs are made for 26 pin GPIO rather than the more common 40 pins currently used. Fortunately on the 40 pin GPIO the first 26 pins are identical to the earlier versions, so homemade solutions can be used.

To get started on such solutions, Ecosia or another search engine is your friend. Inspired by the post Raspberry Pi GPIO Expansion Cable from a Used IDE Cable provided a source of inspiration, for creating a homemade connector cable. Most important detail from Tom Hardgrave’s (not sure if he’s the original author, the original post is without an author 😞) post is noting this part:

The 40 ribbon cables can be found in older workstation computers - especially if they are old enough to have a DVD/CD-rom drive. The 40 pin cable fits the GPIO port of the Pi B+ (or newer). On the breadboard side, there’s a little work needed.

The connector is built for the 26-pin connector of the older Pi models. My solution was to simply cut off one end of the IDE cable, and remove one of the connectors of the 26-pin cable. This can be done with a screwdriver and a bit of force – be careful though not to break the “knives” for connecting to the wires inside the cable.

When one gets the cable cut at one end (keeping one 40-pin connector for connecting to the Pi) and the 26 pin connector from the other cable, one needs to decide how to turn cable in the Pi.

It’s easier working if you get the 16 wires you won’t be using out of the way. This can be done using a knife to cut in between cable 26 and 27. Then the 26-pin connector can be inserted into the remaining cable, carefully using a screwdriver to make sure that the connecting “knives” go all the way through the wire, thus getting a connection on all wires.

Being paranoid or a good tester (I wonder which), testing that all the pins connect correctly can be done using a multimeter. It’s also possible to measure directly on the connector inserted into the breadboard — the pins might be remapped in the connector.

For personal use, I created a spreadsheet with the mappings. In the document there’s also links to the sources.

The final result is a cable that connects the 40 pin GPIO of the Raspberry Pi B+ and the breadboard using the simple connector chip. The setup took a fair amount of time, and it works like a charm.