Powertrain CAN Bus

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Powertrain CAN Bus

Postby Jabsam » Tue May 24, 2011 7:20 am

Hi all,

I tested powertrain CAN Bus signal of a Volkswagen Magotan. And found that the waveform is a little interesting.I attached the file(Magotan CAN Bus.psdata) to you all. Hope that could somebody make some explanation for me.

What I interest in is why the waveform is not a strict Rectangular waveform.Is that noise ? Where it come from ? Can we avoid it ?

And I think the waveform hint some faulty.The amplitude of the two trace (blue and red ) is not equal.

By the way, I tested the CAN Bus with VW original tester VAS5051B. The result is not very same with the PicoScope's. Please find the attached file (VAS5051B.jpg)

Cheers.
Attachments
VAS5051B.jpg
Magotan CAN Bus.psdata
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Jabsam
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Re: Powertrain CAN Bus

Postby Alan » Wed May 25, 2011 5:49 am

Hi,

First of all the waveform captured by the VAS5051B looks as if it does not have enough samples across the screen to see the waveform properly. From memory the sampling rate of this unit is 5MS/s in single channel mode and 2.5MS/s in dual channel - thats not enough for CAN bus to be viewed properly.

One question - are there any symptoms / fault codes or are you just looking at the CAN bus for fun :D

The spikes / noise (ringing on the signal edges) are fairly normal when CAN bus signals are sent down long lengths of wires and / or you have lots of different CAN bus modules on the network. Its one of those cases where the power of the PicoScope shows you a signal in so much detail that it can look different to other test equipment.

CAN bus is a differential signal (each wire does the opposite of the other) and this helps reject noise and increases the amplitude of the signal. For your waveform try going to Tools / Maths channels and click on the A-B button - the resulting waveform is closer to what each CAN bus module will see internally when the two differential signals are combined into a single signal to process (less noise, more amplitude).

Channel A looks normal (CAN high) and the signal jumps from 2.5V to 3.5V.

I am not so sure about channel B (CAN low), I would expect it to switch between 1.5V and 2.5V. The 2.5V looks OK, but the lower part of the waveform looks odd to me - for the first half of the waveform its about 1.7V and then its 1.2V. Had one half been normal at 1.5V and the other been say 1.2V then my guess would be that a module on the network was loading the network when transmitting.

Given that the voltage is wrong on both halves of the waveform I am not sure - its got enough signal to work OK so its possible that this is just how they look on these vehicles. It might help to capture a few more waveforms over a longer time period so we can zoom in and look in more detail. Try 1 or 10ms/div and wind the memory up to make sure you are still sampling at at least 20MS/s.

If anyone else has experience on these CAN systems please feel free to enlighten us!
Alan
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