WF Graph ignores x-scale increment and changes value?

[MIMaracuya]

I've got my vi here that takes points from an oscilloscope and plots them. I choose the length of the record and the x-increment value is based on the record length and the points retrieved from the oscope ( I input -1 to get the maximum ). However, the x-increment value inexplicably changes when it is plotted.

When probing the vi to find the problem I see that within the fetch waveform.vi the values are calculated correctly and once they are sent to the waveforms they suddenly change to new values.

Does anyone have any idea as to why these value change and why my plots do not match my indicated record value?

Thanks!

The VI is attached for reference.

[smercurio_fc]

This appears to be a duplicate of your other post: http://forums.ni.com/t5/LabVIEW/How-to-fix-waveform-points-plots-read/m-p/2176958

[MIMaracuya]

Sorry, I wasn't able to find it and had figured out seemingly where the problem was occuring- I wasn't sure how to update it.

[MIMaracuya]

Do I delete this one and update the last one?

[smercurio_fc]

Since this thread contains more recent information it's best to continue the discussion here.

FYI: Clicking on your username will show all posts you've made.

[MIMaracuya]

I've been over weekend to see if I could fix this but to no avail, has anyone else got a bit of insight to share?

[altenbach]

You need to provide more details:

• You have 4 graphs, which one is wrong?
• You did not attach the "fetch wavform" subVI. What does it do? What are the two outputs values for dt?
• OK, so dt changes. Changes to what, exactly? What do you want and what do you actually get? Is it random or always wrong in the same way?
• Are the wire value correct? What are the "new values" in the waveform? Do you use units?

Can you reduce your VI to a single graph, containing diagram constants containing the data from fetch waveform?

[MIMaracuya]

Sorry, I'll try to be a little more clear.

Of the four graphs plotted the one with the incorrect axis i the one labeled Channel 1 and 2, which would be the top left-most one. I've  attached the fetch waveform sub-vi. It gathers the y values (voltage in this case) and x values from the oscilloscope and brings the data into Labview. The 3 values the vi are the y-values, the x-incremented or my delta_t (which is calculated from the record length I specify and the number of pts gathered i.e. time/ # of pts = delta_t), and the intial x-value with x being my the time axis. Now when I run the program I should be able to calculate the delta_t based on how long the record time is and how many points are gathered. However, once I run the program the Ch1 and 2 plot x values change: the record length that is plotted is much longer than I have specified. I probed the wires going right into the Ch1 +2 plot and that indicated that the delta_t value had changed to a longer time (it also varies according to the record length you indicate). However, if I probe the value of delta_t within the Fetch waveform vi, the correct values are displayed.

[MIMaracuya]

Also I think I should mention, upon further inspection it looks like within the Ch 1+2 graph it looks like only Channel 1's values are affected by this- and by extension so is the top right plot, Ch 1 Auto-correlation (which makes sense) Channel 2 seems to be plotting okay.

[MIMaracuya]

I've also made a chart of how the values change:

Requested time record           # of points            expected delta_t                actual delta_t            new time record

1e-5                                       10000                       1e-9                                  5e-9                        5e-5

1e-4                                       50000                       2e-9                                  50e-9                    2.5e-3

2e-3                                    1000000                       2e-9                                 1e-6                           1s

1e-3                                      499980                       2e-9                                 500e-9                     .25s

2e-2                                    1000000                       2e-8                                 10e-6                        10s

1e-2                                    1000000                       1e-8                                 5e-6                            5s