Discuss how to competently carry out diagnostics and repair of engine management systems
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Joined: Mon Jun 18, 2018 11:59 pm


Post by zap » Fri Apr 26, 2019 2:16 am

So, I had a 17 Audi Q7 come in today with communication codes. I attempted to scope the network lines, and this is what I got:

flexray.png (79.45 KiB) Viewed 332 times

The picture above is with the sampling rate maxed out at 5Ms. I tried switching to a single channel and cranking it up to 12.5Ms. Below is what I got:

max sampling.png
max sampling.png (69.54 KiB) Viewed 332 times

Still doesn't look right. After doing some research, I learned that this system on this vehicle uses Flexray. Just for the hell of it, I hooked up the shop's Picoscope.

pico flexray capture.png
pico flexray capture.png (162.18 KiB) Viewed 332 times

It looks slightly better, but still not great. After doing some reading, I found that special test leads are required for getting a good signal with high speed signals like this. Here are the Pico leads: https://www.autonerdz.com/Blue%20Site/t ... 5large.jpg

I have two questions. First, it seems like Flexray is pretty close to maxing out the capabilities of the Autoscope. With the proper leads, can the Autoscope get a good pattern on signals of this speed?

If so, does Autoscope sell an equivalent test lead? If not, could I use the Pico leads with an adapter?
17 Q7 flexray3.mwf
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17 Q7 flexray4.mwf
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Posts: 14
Joined: Sun Mar 01, 2015 10:35 pm

Re: Flexray

Post by Kelvin » Sat Apr 27, 2019 5:41 am

Yes, you can use the PIN3-BNC adapter together with the high-frequency oscilloscope probe, the distortion from the parasitic capacitance of the cable will be minimal.

Here, the pulse repetition frequency is 5 MHz, this is theoretically the upper possible limit of the bandwidth, which could be measured at a signal sampling frequency of 12.5 MHz.
In practice, the upper limit for USB Autoscope IV is 2 MHz.

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