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From: Mike in Arkansas on 3 Feb 2006 14:16
A new Labview user, I've only been playing with Labview 8 for a couple
weeks and have a question about charts. I want to chart data over
time. The data comes in from an external instrument via RS232 and may
be one or more channels. The scan rate will be variable from 1 scan
per second upward. When I setup the chart I use the relative time
setting for the x axis. If I, say set it to 60 seconds and have a scan
rate of once per second all is okay. But if the scan rate is changed
to once every 2 seconds the 60 second display now represents 2 minutes.
In other words the 60 second x axis time represents 60 samples, not 60
seconds so when the scan rate is changed the time relative to scan rate
changes. Am I missing something? Is it possible to set up a chart
that actually accurately plots data relative to time? I want an
operator to be able to vary the scan rate and have the data accurately
plotted versus time. So that a chart with a fixed time interval of say
1 hour will plot data accurately time wise regardless of how often data
arrives. Obviously the distance between data points along the time axis
would change depending on the scan rate. Essentially, I wants to
emulate a mechanical strip chart recorder. Am I using the wrong type
of chart/graph? Does this even make sense? Thanks, Mike

From: Robst on 3 Feb 2006 15:10
Hi:

Waveform graph/charts are for displaying evenly sampled measurements.
In your case, the sample rate is variable so the best graph for your
application is a XY Graph. I've made a simple VI that uses a XY Graph
to plot data acquired (it this case simulated by the Random Function)
at a variable sampling rate. The samplig rate is simulated with the
Wait Function. Hope this helps you.

Regards.

Robst


VariableSampleRate.vi:
http://forums.ni.com/attachments/ni/170/165363/1/VariableSampleRate.vi
From: Mike in Arkansas on 3 Feb 2006 17:35
Robst, indeed it does help. As I suspected, I was barking up the wrong
tree. Many thanks. Mike

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