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I've got a twisted pair cable that I'd like to use for connecting distributed systems that use 1Mbps CAN network. The problem is that I don't have a clue what's the impedance of this cable and I know that these systems have 150ohm terminating resistors. How can I evaluate the cable impedance to verify if it matches this network?

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May be worth noting that the CAN spec (ISO 11898) states the standard resistor is 120ohm – Joshpbarron Feb 25 at 14:12
@Joshpbarron Tks, but there are variations, I've seen 100 to 180 ohms throughout different industries – PDuarte Feb 25 at 15:51
up vote 13 down vote accepted

Do some time domain reflectometry, a useful video to this process is probably one from w2aew. He shows it with a coax, but it should work with anything that has a somewhat consistent impedance.

In short:

  • Inject a pulse with a short risetime into the cable
  • terminate the cable with a potentiometer that has the range of suspected impedance (like 200Ω)
  • Watch the pulse being reflected on the scope. Adjust the potentiometer until you have minimized the reflection
  • Your potentiometer is now approximately the impedance of the cable
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Is this for a one-off project or for "real-production" use?

For one-offs, which aren't pushing the limits (eg of bus length) it'll probably work fine. CAN is very tolerant of all sorts of reductions in signal quality, and even 1Mbps is not really in "transmission line" territory. As long as you have the terminators at the end of the bus it'll probably work.

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