Rev B of the raspberry Pi ships with a 26 pin GPIO port and my first dilemma after bringing up my Pi was, how was I going to break out the PI’s GPIO pins so that I could easily access them for design work? I searched on-line and found several solutions but all of them already did too much. I did not want someone else’s interface to their set of hardware and I did not want to use someone else’s buffer design. Then while searching through a drawer of parts, I had in my hand a solution – an old unused 40 pin IDE hard drive cable. I could plug one of the existing cable headers onto my Pi GPIO port and plug a custom header into my powered breadboard.
For more info. please visit http://www.raspberryproject.com/a-gpio-expansion-cable-from-a-used-hd-cable/
Notes: There is some bad information floating around about the Raspberry Pi GPIO and a IDE cable.
1. Some people are claiming that using this cable will damage the GPIO port because per the IDE standard, some of the pins are shorted together. It's true that some of the pins are shorted together but this is at the IDE hardware & not in the cable. The cable is just wired straight through.
2. Some people are claiming that using this cable will damage the GPIO port because every second conductor is grounded. This is only true with the newer IDE standard that uses a 80 pin ribbon cable. A cable built under the older IDE standard is perfectly safe and it's easy to tell which one you have - just count the wires (ridges) in the cable. The cable you want to use will have only 40 wires, not 80 wires.