Feature request: Trigger from maths channel?

[davidwag]

Hi,

Is it possible to trigger from a maths channel?

Today a colleague was looking for a fault on a CAN Bus system where one channel would intermittently drop out. He was hoping to be able to use single trigger to capture the event. Obviously you can set up a A+B maths channel that would basically show a consistent voltage until either CAN High or CAN low plays up, at this point I was hoping to trigger a capture of the glitch from voltage change on the maths channel.

Is it possible to do this?

Thanks

David

[Fat Freddy]

Hi

Have you tried using one of the trigger options? I would certainly be trying the 'Logic' option. After all that is what you are trying to capture.
And this would be a great opportunity to get a better hands on of that trigger option.
HTH
FF

[FioranoCars]

Hi David
I agree with you about the ability to use Maths Channels for Triggers (or allowing maths within the Trigger options, but using a Maths Channel is easier and reduces the risk of the Trigger and Maths Channel being designed subtly differently by mistake).

In you specific application this might be especially important if it's an intermittent issue, where huge volumes of data could otherwise be generated. We had a similar issue with a dual potentiometer, where maths triggering would have saved lots of time, for us the signal never exceeded or fell outside the min max bounds, it was the incongruence between the two that we needed to monitor/capture.

Is the signal you can see exceeding the normal logic levels of the CanBus? Then a simple rising or falling trigger might work?
Otherwise you might, with trial and error, ... and a few false positives, be able to set the pulse width window or advanced edge to do this? If you have a capture you can share, then it might be easier to see what options could be made to work or even be reliable?

+1 for Maths Channel Trigger to be added to the Feature Request list

Good luck

Richard Lukins
FioranoCars.com

PS I had some issues using triggers last week, for an old parasitic drain test template (used it for years with no issues, probably over 500 times) - triggering on door locks locking, 16amps showing on screen, reduced trigger from 10amps to 5,3,1 (simple rising edge) but still refused to trigger! We've had lots of issues with this laptop (Win7x64) using Pico (image preview etc) so we're going to do a deep uninstall, re-install and retest when we get a moment, but still at a loss as to why the trigger did not work? Just thought I'd add in case anyone else has had an issue using an old template or with triggers in general?

[Fat Freddy]

Richard

Have you tried the 'logic' option?

Not that I have. But since CAN is all about logic. I wonder.

[FioranoCars]

Hi FF
I think logic is only available on certain models of scope, but not having a scope here, I'm using the "Demo" scope and that does not show the advanced triggers (Logic, dropout, runt), but this video might help?
Advanced Trigger options - Logic

It's not my video or anything, just something I found ...

Worse, I don't think our early 3xxx scope has these options, will need to check!

Hey, Pico Support ...
Maybe the "Demo Scope" could be upgraded, so that all the features can be seen ?

Many thanks
Richard

[Martyn]

Logic triggers require 2 or more channels to be active, if you only have 1 channel active in demo mode you will not see the option.

[FioranoCars]

thanks Martyn
Stupid me, should have simulated a real situation!!
In a hurry and I was not checking my thoughts ...

thanks
Richard

[davidwag]

The problem was when this was faulting, one of the CAN lines would just float and not be at a logic level.

Was hoping to just trigger when A+B was not at a set level.

How would you do this with logic trigger?

Steve Smith any ideas?

David Wagstaff

[Steve Smith]

Hello and sorry for the late reply surrounding such triggering.

Triggering on a math channel sounds like a nice addition to the software and rest assured I will add this as a New Feature Request here at Pico.

The issue with CAN triggering is that we often do not know what to expect or when to expect it!
Serial data may reveal “U” codes (if we are lucky) that “point the finger” but nothing “Physical” in the way of real-time captured data.

Capturing a momentary interruption of the CAN Network is a nightmare but if successful, holds the key to those mysterious “U” codes that stack up and create havoc with on-board systems.

The solution is via a math channel in conjunction with a Mask

With CAN HI and LO signals (CAN Hi Channel A CAN Lo Channel B) we can use the built in math channel A+B, so giving us a near straight line at 5 V (assuming we have the correct probes and sample rate for high speed captures)

Open the Communication Network Guided Test from the Automotive menu within PicoScope and load the example waveform CAN L & H which is “clean” generated CAN signal
Automotive > Communication Network > CAN L & H (Increase the number of samples from the loaded 200 kS to 1MS)

Apply A+B math channel to the loaded waveform
Tools > Math Channels and tick the box adjacent to A+B under the “Built in” math channel list

Hide channels A and B
Right click on screen, click on Channels and tick the boxes next to A and B so only the math channel A+B is visible (Straight line at 5 V)

Amend the scale for the A+B math channel to x2.0 and just the scale offset to -25.00%
Alternatively scroll down on the scale so you have straight line across the centre of your scope screen at 5 V (0 V at the bottom of the screen and 10 V at the top)

Now click on Tools > Masks and Add masks and click Generate

Enter the X and Y Offset (X Offset 470.0 us Y Offset 1 V) click on Generate > Apply and OK
You may amend the X and Y Offset of your mask to allow for an increase or decrease in error detection

You should now have a mask drawn either side of your math channel allowing for a positive or negative voltage error of 1 V either side of your 5 V signal across the entire width of the screen

You can now use the Alarms function of PicoScope to indicate when a signal intrudes into your Mask (indicting a CAN signal error)

Click on Tools > Alarms and in the Event Box click the down arrow and check the box Mask Fail > Apply and OK.

When your CAN signal intrudes into the Mask, a beep will now sound and a Mask Failure will be recorded in the A+B measurement table (Indicating the number of errors during your CAN signal testing)

You can choose what you would like to happen when your signal intrudes into the Mask.

Click on Tools > Alarms > Add and click the down arrow in the Action box. Choose the response from PicoScope when the Mask is intruded upon. These include “Play Sound” and “Stop Capture” to name just a few.

With the scope connected to your CAN signals (Hi and Lo) click on the Start Button and carry out the relevant road-test or harness wiggle test in order to reproduce the error and capture the signal intruding into the Mask

I have attached a short video below that will help with the setup and configuration (Apologies for the sound)

CAN TRIGGERING.mp4

PicoScope Mask and Alarm set up

(7 MiB) Downloaded 585 times

Take care…….Steve

[FioranoCars]

Hi Steve
Thanks for the tutorial, one thing that I would like to ask you to add to the wish list, seems we're talking can-bus, is the packet being viewed to be high-lighted, just like in the table at the bottom of the screen, maybe using the same colour ass a bold border to the packet in the signals window? or maybe a true highlight of the "diamond" ? see picture?

high-light the packet

secondly, could the "selection" of the packet change the table to bring it's data and highlight marker to it. Not changing the hoover, but for selection actually changing the table display?

Many thanks
Richard Lukins
FioranoCars.com