Buffer screen help

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Buffer screen help

Postby blake » Wed Aug 07, 2013 9:18 pm

Hello everyone,

I am a beginner with the pico 4 channel scope and I have been having trouble with the amount of buffer screens that i can go back and review. The scope seems to record up to 31 screens and then earlier data is erased. I went into the tools section and changed the buffers up to 800 and the scope still stops at 31 screens. How do i fix this? If I am not directly watching the scope and hitting spacebar when a fault happens I am going to miss the fault. I was fortunate to be watching the scope when a cam sensor signal dropped out causing the engine to stall and set a false crank sensor code. My point is I shouldn't have to run to the scope and hit spacebar when the engine stalls or when a fault occurs. Also updating the software did not fix this problem.

Help is greatly appreciated,
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Re: Buffer screen help

Postby Liteace » Thu Aug 08, 2013 12:14 am

Just change the collection time base, if you stick it on the max 5000s/div you have 13 hours 53 mins on just 1 screen, then you wont have to run anywhere but you wont capture a lot with that, so 1 s/div will give you 10 secs per screen x 32 / 60 is just a little 5 mins
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Re: Buffer screen help

Postby blake » Thu Aug 08, 2013 12:40 am

So what your saying is that even though 1 second per division is too slow to see a correct ignition coil or injector waveform which is usually viewed at 1 or 2 miliseconds per division, I can go back and use the windowed zoom in feature and get an accurate waveform? Basically a longer time base is better for catching faults and giving you more time to review and then go back in and zoom in to get the detail that you need, am I understanding correctly?

Thanks,
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Re: Buffer screen help

Postby Alan » Thu Aug 08, 2013 4:54 am

Hi,

The number of waveforms you can capture in the buffer depends on the number of samples collected in each waveform. You have a total buffer memory of 32,000,000 samples so can collect say 32 waveforms of 1,000,000 samples or 3200 of 10,000 etc (actually it will be 31 or 3199 as one is reserved for the next capture).

This way of working is best if you are using a trigger - you can for example have a 1000 ignition waveforms captured. In your case you might have to use an advanced trigger to capture only when the crank signal drops out for a period of time (pulse width).

The alternative approach is to capture everything on a long time base, stop the scope when the problem occurs then zoom in to see the problem. This is an option with the PicoScope due to the large number of samples you can capture and is often the best approach if you do not yet know the problem so do not know what to trigger on.

To do this for the crank signal, dial up the max memory to at least 100MS (100,000,000 samples) and then you can select a timebase of say 10 or 100 sec / div. Run until the problem occurs, stop the scope and then as you suggest use the zoom to see what happened.

Both approaches are valid depending on what you are trying to do. Not that at timebases longer than 100ms/div the scope will stop using its internal 32M memory and switch to a "USB streaming" mode which allows up to 100,000,000 samples to be collected - hence the reason I said increase the memory to > 100MS. 100,000,000 samples spread over 10000 seconds (100sec/div) allows for 10,000 samples every second which should be enough to capture the crank signal (unless the engine is at 6000 rpm).

Sorry if there is a lot of maths in the answer!
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Re: Buffer screen help

Postby blake » Thu Aug 08, 2013 8:26 pm

I'm still not understanding why that I cannot use the correct timebase to capture a signal and have the scope continuously record. My question is not about samples but about the number of screens that I can go back and review. If I am scoping an ignition coil or injector at 1 millisecond per division, 32 screens pass very quickly and if your not watching the scope and hitting spacebar you will miss the fault you are looking for.
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Re: Buffer screen help

Postby Robski » Thu Aug 08, 2013 8:55 pm

My question is not about samples but about the number of screens that I can go back and review

This has been explained to you.

It sounds like a course is in order for you ??

If you lower your sample rate the buffer count will go up, a seasoned Picoscope user knows how to exactly manipulate the settings to gain the information on screen reqd via the properties tab, enjoy the course it will do you good ;)

What settings did you have & sample rate ??
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Re: Buffer screen help

Postby Fat Freddy » Fri Aug 09, 2013 12:02 am

My question is not about samples


Unfortunatly it is all tied togeather. Sample rate, samples, buffer size, number of screens are all related.

I cannot use the correct timebase to capture a signal and have the scope continuously record.


You can. But you can't expect a scope to be sampling at a rate of 1us at 1milliseconds a division and let it run overnight. Edit: And expect it save all those screens.

If I am scoping an ignition coil or injector at 1 millisecond per division,


For an intermitant fault that is an incorrect set up and the chance of hitting the space bar before the capture is lost is unlikely. You can still get 1 Mssec (for ignition) but use 10 sec a division. Thats 3 screens or 30 seconds of capture time, using all four channels. I have only used this as an example as this is my current set up for the problem I'm chasing.
If I'm correct the pico has two ADC's for the four channels. So it also depends on how many channels you are using.

Each set up has to be tailored to the specific fault you are looking for.

There is only a certain number of samples any scope will hold in its buffer. And the Pico is a giant for buffer size and is what most scopes are missing.
As mentioned, always check the properties tab and a good tip is to use a trigger


HTH
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