HDMI cables are NOT all the same!

Sometimes very cheap or thin HDMI cables do not work with the Raspberry Pi Camera extension kit I made. Even if a camera is correctly plugged at startup, the Rpi cannot detect the camera and output an error message. In this short post I will explain why and show a quick fix.

First a normal HDMI cable with 19 lines. This is the most common design. There are four data buses (one is hidden behind in the pictures) with 3 cables in each, a GND line and a GND shield. The HDMI connectors are also put to GND. The Raspberry Pi Camera extension kit works with this kind of cable.

Now, this is a smaller diameter (it was Micro HDMI before I cut it) cable with only 15 threads. This cable has 5 data shields but there are no GND lines in them! There is not GND shield, just a wrapping that was cut before taking the picture. The HDMI connectors are put to GND. This cable does not work out of the box with the Raspberry Pi Camera extension kit.

The trick to make the second cable work with the kit is to connect with a short piece of wire and solder the GND on the PCB and the HDMI connector. Of course this must be done on both sides of the cable. Et voila!
Even if there are no data shields, the quality of the image from the camera does not seem to be affected.

 If at this stage if there is no image from the camera it is time to ask me an exchange.

Note that I could have connected the HDMI connector to GND by default, I decided against this. For safety reason I try to avoid connected metallic parts floating around.  

The Raspberry Pi Camera Extension Cable

The Raspberry Pi camera is wonderful;  video and still images are high quality and the functionality of the camera can be controlled with shell commands or using the Picamera Python module.

The flat ribbon cable that connects the Pi to its camera is fine for short distance but it is hard to find longer versions (Adafruit has a complete line-up), and it is fragile and sometime hard to place in a design. I needed a longer and stronger cable with solid connections for my designs. For example, I want to use the Pi camera in my garden or close to heavy machinery.
 
Searching the web I did not find anything usable for my applications. However a thread on the Raspberry Pi forum gave me the idea to try using an HDMI cable for extension. Surely an HDMI cable would be convenient, it can be purchased at a low price in local stores in length up to five meters.


By comparing the diagram of the Pi Camera connector with an HDMI connector one can see that both have four data buses made by pairs of cables with a ground shield.

Please read Peter Vis's very informative and detailed article on the Raspberry Pi CSI camera module and connection for a complete description of the bus.


The design I made put the right cable at the right place to transform the flat ribbon into a round cable. There are four pins left since HDMI has 19 pins, these may become handy to use a few sensors close to the camera.

I have tested successfully simple 5 meter HDMI cables, cheap or expensive cables work the same. The video signal should not show any degradation until a certain distance (yet to be found). Passed the limit the video signal will be lost entirely.

I made a kit available on Tindie, so far it sells very well! It looks like I wasn't the only one in need of something like this.