Re: CAN cables
Hi all,
This is my first year in the FRC program. So far it has been a lot of fun doing this with my son.
I have a question on CAN implementation. I have searched this forum, but I cannot find a definitive answer. From what I have researched the CAN bus implemented for 1Mb/s is a two wire balanced differential network with a characteristic impedance of 120 ohms. But what is supplied in the KOP, and what is sold by many FRC authorized vendors, are four conductor flat modular cables.
So I am confused somewhat. The CAN spec for the type of network being used is 120 ohms, yet 120 ohm two conductor cable is virtually unattainable. A ground is not required to operate the bus as designed. Having a shielded cable and using the drain wire to go back to one side of a node and connect to ground only there makes sense. Connecting a ground wire along the bus to all nodes does not (ground loops).
We are using the 2CAN so running a ground to it, is not necessary as well seems a little dangerous. I would prefer to keep it isolated from any possible spikes that could arise.
Am I missing something here? This system was designed to only require two conductors for a high, low differential and work in extremely harsh electrical systems (cars). I am scratching my brain on this, I can only guess as to how much most HS students are understanding it.
Also i am unsure of which value for terminations. I have read in TI documents a recommended 100 ohm termination. However, the 2CAN has 120 ohm (following CAN bus spec). So I am recommending we terminate the other end of the bus with 120 ohms. However, if the physical media is not characteristic 120 ohms, and unshielded as well, I question whether 20 ohms of difference at termination will have any real impact to reducing reflections or keeping common mode from be a problem.
My years of working with coax in the CATV industry tells me to use 120 ohm shielded two conductor cable. The AWG would need to be 24/26 to be used in 6p4c RJ plugs. Also to terminate with 120 ohm or split across both wires and use a cap to filter noise. Again, this is my first year doing so if someone with greater knowledge of CAN bus can offer some advise if I am off base I would appreciate it. Thanks.
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