Chart history explanation?

[Caseyw]

When you wire a waveform data type into a waveform chart, the chart history length dialog shows "number of waveforms in chart history buffer." The minimum value is 10.

If you were to generate 20 enormous waveforms (with the chart history set to 10), and then plot them after they were generated, every data point in every plot would show. The value of 10 pertains to the number of waveforms per channel that can be displayed. So if you generated a waveform of 100 data points inside of a loop, and plotted the waveform during each iteration of the loop, then only the last 10 iterations would show on the waveform chart. Therefore, you would see 1000 data points.

This is exactly what Ben mentioned earlier.

I hope this helps.

Casey Weltzin

National Instruments

[Ben]

Watch your memory usage!

That data has to be somewhere.

Ben

PS NIOSH

Did you by any chance attend any of my presentations?

Message Edited by Ben on 06-09-2006 04:10 PM

[GReaper]

I do notice that now, how the text in the set chart history dialog says "# of waveforms". 

Just some additional observations on chart history length:

    - sample scenario:
       sample rate = 10 hz, continuous buffered daq, grabbing 1 buffer of 10 samples every second
          - chart history length = 10 gives me a window of 10 seconds
          - Now the funky mojo
             - The above holds when I have the x axis on autoscale.  It automatically goes to the size
                implied by the history length
             -  If I turn x autoscale OFF and set the x axis minimum to a much earlier value, the chart is happy enough to just fill that all in and not enforce the implied window size.  Why isn't that data being dropped off?

Ah well.  TGIF.

[Caseyw]

Assuming that you have a waveform data type wired into your waveform chart, and data is being passed to the chart during each iteration of your collection loop (once per second), then you should only be able to access the last ten iterations (ten seconds) worth of data.

I would try to turn on execution highlighting, and make sure that this is indeed what is happening. It might also help to probe each of the first ten incoming waveforms to make sure that the data is as expected. If you are viewing more than ten seconds worth of data, I would appreciate it if you could please post an example or some steps to reproduce the issue so that I can investigate further.

Casey Weltzin

National Instruments

[GReaper]

I'll refer you to the diagram screen shot I posted earlier in this thread as to how the daq loop is running. 

To do what I explained above, stream data to a chart (using waveform data) and set the chart history length to give you, say 10 seconds.  With autoscale ON for the x-axis you will get a 10 second window.  If you turn autoscale OFF and manually edit the x scale minimum value you can create a window longer than 10 seconds and it seems to plot fine.

I note it more as a curiosity than an issue becuase I'm not retrieving any data from the chart history.

Cheers,

[Caseyw]

I am curious about your experience as well. When I try to reproduce your issue, autoscaling does act as you explained. I can view ten waveforms of data and the waveform chart automatically resizes to fit all ten waveforms. When I turn off the x autoscaling and decrease the x scale minimum value I see only blank space. So the waveform chart acts as predicted and only stores ten waveforms at a time.

I have also tried to reproduce the issue running the VI several times with different t0 values each time. Still, it appears to me as if each time the VI runs the waveform chart is displaying only information from the last 10 waveforms passed to it.

Let me know if you have any more information that might enable me to reproduce what you are observing.

Casey Weltzin, National Instruments

[GReaper]

Ok, I worked up a quick VI.  Doing what I said above, works on this VI on my machine.  Hopefully you can replicate it.

Cheers,

[Caseyw]

Thank you for the example code, I was able to reproduce your issue exactly. Here is what I think is happening:

The waveform chart can only store 10 waveforms in it's data buffer, but the chart front panel display itself does not refresh when the data buffer begins to erase waveforms and store new ones. As you said earlier, the waveform chart will show more than ten waveforms with your example. However, if you make the x scrollbar visible and attempt to scroll (forcing the chart to refresh), you will only be able to see the ten waveforms stored in memory.

I hope this helps.

Casey Weltzin, National Instruments

[eximo]

See the attached picuture (it's chart history length is set to 1)

If one of the knights would finish this with their synopsis of the problem or issue.  I set my chart history length to one, and can see no difference from when its set to any other number.  I'm addressing this issue because I was setting my chart length to 1M  and it was banging out 700MB of memory when it populated the chart with the data.

Does 2011 have an issue with waveform charts?