Signal integrity?
What is the length of your ribbon cable? A long cable will cause
your beautiful square signal coming out the driver looking bad at the
load.
And what is clock rate of the transmission into the cable? Fast transmission might induce signal integrity issues such as cross-talk between the lines.
Try to add a GND line in between every signal in your ribbon cable.
It's the speed that tells you if your cable is long or not. (and more precisely, it's the rising/falling time of the signal, not its toggling frequency... )
Power integrity?
If you have only one small wire for Vcc, this could create a voltage drop that is big enough to disturb your chip.
If you have only one GND for the power and all the signals, this might cause the same issue.
Did you put enough decoupling caps at the power pins of you chip? (both chips)
As @Ben Jackson suggested: Is Vcc properly connected?
And last (but not least):
- Software: Are you sure about the code you use to check your hardware? You might be looking for a hardware bug, when it's your test software that has a bug.
This is just guesses, but related to my experience, it's highly probable that the issue is here somewhere.
Your main considerations should be as follows for your cable:
- Flex life, i.e. how long the cable can be flexed before it breaks
(very important for your application)
- Shielding/grounding for the composite video portion and other low-level signals
- Impedance for signals with content above 16 MHz (see calculation below)
For a cable run of 4 feet and assuming a typical velocity of propagation of ~66% for the cable, the one-tenth wavelength rule says you can ignore transmission line effects below about 16 MHz. I don't know enough about the frequency content of your I2C lines to give a recommendation there, but analog baseband composite video will have frequency content below 8 MHz in all regions of the world.
If you decide to go with a Cat 5E Ethernet product, try to find one designed for continuous flexing. Those types of products are used in industrial robotics products.
For the composite video portion, you have two choices:
- Make a composite cable (cable made up of multiple elements) yourself with a coaxial element and your Ethernet element.
- Use a 100-ohm twisted pair from the Ethernet with a balun.
If you decide to make a composite cable yourself, find a 75-ohm video product designed for continuous flexing (long flex life). Combine it with the Ethernet element in a braid.
To maximize your flexibility and the flex life of your finished product, you want to use flexible materials (soft plastic and many strands of copper) and construct things in such a way as to minimize the twisting and untwisting of the internal elements.
I'm a former applications engineer for a wire and cable company. Here is a link to a general-audience presentation about wire and cable: http://www.belden.com/resourcecenter/cablebasics/upload/Cable-101.pdf
And here is a link to industrial products in a wire and cable catalog: https://www.belden.com/docs/upload/Cabling-Solutions-for-Industrial-Applications.pdf
This 2002 book is also a good overview of the audio/video cable industry, even if it is somewhat outdated:
http://www.amazon.com/Audio-Video-Installers-Pocket-Reference/dp/0071386211
Best Answer
It's the same stuff as other electrical cable insulation. And just as that, the structure of the stuff changes, plasticizers evaporate or decay, and the thing gets brittle. So, probably, vinyl.
Good news is that if it's stiff, the plasticizer is largely gone, so now you have stiff, brittle PVC that doesn't off-gas anymore. Also, you'd want to reach the EU reports that were written before the REACH directive, which now severely limits the use of many plasticizers that were popular back-in-the-day. It's not likely that you or anyone in your household is under acute risk of anything due to having lived in a house with a coax cable.
Downside of not containing any plasticizers anymore is that it's brittle and won't fulfill its role as isolator anymore. Also, depending on the construction and use of the cable, it might have had a barrier function against water vapor, which tends to damage the dielectric (the white stuff between inner and outer conductor, not the stuff outside the outer conductor of your coax).
It probably strikes you more on coax cables than on the usual power cords because it's a rather thin layer directly on a thick-diameter metal core, as opposed to the 2 to 5 mm of total polymer until you strike the copper of a power cord.