Measure CAN bus

Hello,

Disclaimer : I am a beginner in electronics :grinning:.

I want to measure the difference between CAN H and CAN L onthe CAN bus with my Logic analyzer.
So, I plugged CAN H on pin 1 and CAN L on GND (associated to pin 1), so that Channel 1 will have CAN H - CAN L as voltage.
Unfortunately, I am not able to see anything.

Could you explain me why ?
Best,
A.F

@bebedulac Thanks for writing in about this. In short, we do not recommend connecting our Logic’s GND pin to anything other than GND on your DUT. This is because, if your DUT somehow shares the same GND bus as the PC that your Logic device is connected to, a short to GND may occur (i.e. CAN L will be shorted to GND).

Our recommendations for hooking up to a CAN bus is described in the support article below:

If you run into any issues with that, feel free to let me know which specific Saleae Logic model you own, as well as a copy of your capture file (in .sal file format).

Thank you @timreyes for your reply !
My DUT is powered by an external generator. I am using a Logic 16 Pro analyzer.

I connected one pin on CAN L and the other on the GND of my DUT. However I am not able to see any valuable signal (only noise I think, see capture). I am sure, there is can activity, as I see it on Wireshark with my usb-can dongle.
Session 1.sal (437.6 KB)
How can we explain that ?

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It’s hard, there’s no getting around that. Measuring CAN with a logic analyzer is going to be hit or miss. Having said that, your capture looks awful (sorry, no other way to really say that…) It’s absolutely wonderful that you have the analog capture as well. That’s immensely helpful! The analog capture looks nothing at all like CAN. There are a few big problems.

  1. CAN is a digital signal so there should be hard, sharp transitions. You don’t have that.
  2. CAN should float around 2-2.5v above ground. So far, so good. You are floating at about 2.5v which is correct. However, CAN-L should pull down by a full volt when it goes low. So, around 1.5v in your case. Instead you’re only getting 0.45v of drop which may not quite be enough.
  3. The timing of the peaks doesn’t seem all that consistent to me.

Was this capture with your Logic done with the DUT and all connected hardware fully powered up and connected? Was the capture device you use with Wireshark connected? I’m seriously leaning toward “you have no termination resistors on this CAN bus”. Or, for some reason you just are not getting a good ground. That analog signal is just not suitably like a CAN signal for the digital signal to have any hope of making sense.

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Hello @Collin,
Thank your for these precise information.
You were right, I did not have the right resistors.
Thank you :wink: