Yesterday, in my review of the LEDBorg, I wrote about how it can be inconvenient to connect two devices to the Raspberry Pi GPIO header at the same time, especially when one of them has a header which covers all the pins.
I mostly write and run my own bare-metal software on the Raspberry Pi, and as I have not got anywhere near making the major I/O (USB, ethernet, etc.) work, my only option for communicating with the Pi seems to be via the on-board mini-UART serial interface. I have that talking to a terminal emulator, and it makes it much easier to get diagnostic information so I can see what’s going on.
There’s a big problem, though. The mini-UART uses pins on the same GPIO header as (for example) the LEDBorg. This makes it practically impossible to use both at once. This means it’s very tricky for me to work out what is wrong with my experiments in interrupt-driven PWM (No blog post on that, yet – it is stubbornly refusing to work for me). The GPIO header on the Raspberry Pi has 26 pins which point straight up, and the serial port connector I use has four in-line sockets which slip end-on to the pins. Without somehow duplicating the header pins I can see no way of connecting both the UART cable and the LEDBorg at the same time.
After thinking about this for a while, It occurred to me that there ought to be some sort of connectors which clip on to the pins sideways, rather than downwards. That way, a mixture of sideways clips, headers and in-line single sockets can be used, to allow any combination of devices to be connected at once. Obviously it would not make sense to connect two devices which used the same pins, but this is not the case with the LEDBorg and the mini-UART.
Unfortunately, I have no idea what such connectors might be called, and my web searching has so far turned up nothing useful. I have drawn a sketch of the kind of thing I am looking for – does anyone have any suggestions what they are called, or where I might get such things?
Many thanks in advance to anyone who replies!
Or you could use http://www.hobbytronics.co.uk/raspberry-pi/rpi-stackable_header-long and some stripboard to ‘escape’ the signals from between the Raspi header and the LEDBorg – not forgetting to make track-breaks between the two rows of course!
Or the more fiddly alternative (but simpler if you have no soldering equipment) would be to use male-female jumper cables to connect the LEDBorg to your Raspi, only connecting just the neccesary pins.
Or (most difficult option) using a dremel and some solder wick, you could cut off and unsolder the LEDBorg’s current connector and replace it with a stackable connector ;-)
I toyed with the idea of the stripboard thing, and I may yet find myself making one. It seems a reasonably generic solution. I recently got some male/female jumper cables so the second option is a possibility now too. A further option might even be to take the GPIO header out to a “backplane”-style board which supports connecting multiple boards at once. Of course, they would need to be compatible boards, and not tread on each other’s toes as far as using pins goes.
I can’t imagine that I would want to take a dremel to a LEDBorg, though. There are considerably cheaper ways of DIY connecting an LED to a Pi! What makes the LEDBorg a nice product is its neatness. It sits unobtrusively inside all the Pi cases I have and is easily added and removed
Just been messing about in GIMP with your idea of a “backplane” and if you had the skills / facilities to make a double-sided PCB (I don’t!) you could make something like this mockup
http://www.andrewscheller.co.uk/P1-backplane.png
The thick red lines would be on the back of the board for power and ground, and the thin blue lines would be on the top of the board for signals. Simply solder a stacking-header into one of the slots and normal pin-headers in the other two slots :-)
Disclaimer: I’m *not* a PCB designer, so I’ve no idea what impact such a simple design would have on noise / signal integrity (especially when using high-speed signals).
Heh heh, I’ve just spotted that someone’s actually made something almost exactly like the “backplane” idea I had and named it PiRack.
http://www.element14.com/community/groups/raspberry-pi-accessories
Hah, just spotted something even more similar to my backplane idea!
http://www.piborg.com/triborg
Great minds think alike and all that ;-)
Thanks, I’ve ordered one of those now. Especially Interesting that it is from the people who made the LEDBorg that prompted this blog post in the first place!