Append Waveform, keep time stamp

[RavensFan · Accepted solution]

Use a case structure.  When i=0 but the waveform directly into the shift register.  On all other iteraation append the value from the shift register with the new waveform then put it in the shift register.

[COIEngineer]

I'm reading from a USB. Here is my attempt to initialize with a t0. As you can see, the t0 for the First run and that of the appended waveform are different. Essentially, I want a true timestamp from each of my samples. The ULx read data maxes at approximately 500 Hz, but can vary fairly significantly. I will need to append several samples to allow realtime calculations. If I drop a few samples because of a lag in read time, that's ok, but I'd like to be able to adjust for that by looking at the timestamp rather than a sample count.

If there is a better way to achieve that, I am certainly open to suggestions.

[JÞB]

use Raven Fans advice in that case

[COIEngineer]

The case structure looks like it does it, but I'm not sure I believe the results (so slightly different topic). I moved the sampling frequency up to 10 kHz, much faster than the one-sample read that is possible (500Hz). Even if I output to a different indicator for each of the loops, the timestamp is only incremented by 0.0001. Does this mean the data are being back-logged, or is this a false timestamp?

Thank you for your help. I'll go ahead and mark it as solved since the main question for the thread is answered.

[crossrulz]

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@COIEngineer wrote:

The case structure looks like it does it, but I'm not sure I believe the results (so slightly different topic). I moved the sampling frequency up to 10 kHz, much faster than the one-sample read that is possible (500Hz). Even if I output to a different indicator for each of the loops, the timestamp is only incremented by 0.0001. Does this mean the data are being back-logged, or is this a false timestamp?

Thank you for your help. I'll go ahead and mark it as solved since the main question for the thread is answered.

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The timestamp coming out of the driver is changing by 0.1ms?  That means you are only getting one sample at a time.  You will overrun whatever buffer that instrument has quite quickly.  You need to be able to read a lot of samples all as once.

[Dennis_Knutson]

Are you really talking about the timestamp or dt? A sample rate of 10 kHz is a dt of .0001.

[COIEngineer]

I did that sampling rate only as a short-term test. From your response, it sounds like the timestamp comes from the device (not Labview). If that's the case, I think the problem is easily solved by, as you said, reading several samples at once. I only need 500 Hz, so it shouldn't be a problem.

Thank you!

[COIEngineer]

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@Dennis_Knutson wrote:
Are you really talking about the timestamp or dt? A sample rate of 10 kHz is a dt of .0001.

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I set the sample clock on the device very high, and put the read data on a loop (which as about 500 Hz timing), reading out a waveform.

[Dennis_Knutson]

The timestamp comes from the driver (pc time) - not the device. The dt comes from the device.

[COIEngineer]

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@Dennis_Knutson wrote:
The timestamp comes from the driver (pc time) - not the device. The dt comes from the device.

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This is what I'm looking at, The results at the bottom are a new indicator from the case structure at the bottom of the diagram:

From each loop: