H Mark

Just got round to reading your last post. Congratulations !! This result was in no short measure down to your logical and methodical approach and sheer dogged persistence. well done.

I have been meaning to comment on wrapping a copper screen around the CAN BUS cables. With great respect to other posters this is something you should definitely not do. At the risk boring every one I will try to explain why.

There are two types of signal transmission lines balanced and unbalanced, an example of an unbalanced line is as follows. The transmitting end has one cable connected to ground, the receiving end also has one cable connected to ground, so you only need one signal wire connected between them. A bit like a headlamp bulb only needs one wire, the return is through ground.

In an unbalanced type lets say the transmitter sends a signal by increasing the signal wires voltage to one volt, this travels down the wire and arrives at the receiver pretty much at one volt. All is well however, if the signal wire passes close to an electric motor or ignition cable a voltage could be induced within it lets say it does and its one volt. This arrives at the receiver, which thinks it's a signal and so we have interference. To stop this happening the signal wire is enclosed within a woven copper screen all along its length and the screen is connected to earth. The interfering signal does not get past the screen and goes harmlessly to earth.

The second type of transmission line is a Balanced Line as in the case of CAN BUS. With this type there are two wires neither of which are connected to ground and the receiver responds to the DIFFERENCE between these two wires. In a balanced line, with no signal, both wires sit at the same nominal voltage. Lets say it is 4 volts, to send a one volt signal the transmitter moves one line up to 4.5 volts and the other line down to 3.5 volts. The receiver responds to the DIFFERENCE i.e. 1 volt. These two wires are twisted together so if they run close to a motor or ignition cable both of them get the same interference signal. So with this type of line with a one volt interference signal they would BOTH rise by 1 volt i.e. to 5 volts. The receiver sees no difference and therefore does not respond to the interfering signal. Hence no screening needed.

Producing a transmission line by screening one conductor (unbalanced ) or twisting two conductors ( balanced ) produces something called a transmission lines characteristic impedance, which in the case of CAN BUS is 120 ohms. It is important that the BUS is terminated at both ends with 120 ohms to match the characteristic impedance. If this is not done then reflections along the line can occur causing data corruption.

So, if you are still with me, if you wrap a copper screen around a CAN BUS line you will change its characteristic impedance and it will no longer be 120 ohms and therefore not properly terminated by the 120 ohms resistors that are fitted either end of it and that can cause reflections so you could end up worse off

Sorry for the long post, both my adult daughters have said to me “Dad, when you give me one of your explanations would you mind if I record it on my mobile phone, so that if I am having a sleepless night I can play it, I will be gone in two minutes”.

Good Luck
Regards Tony