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Unwanted delay when using DAQ in Labview

I think you're seeing a *latency* effect as your device's on-board FIFO gets filled up and the new waveform has to work its way through the FIFO before being generated as a real-world signal.

 

Dunno what device you have, but I know some common MIO boards have an 8k FIFO.  At your 1k sample rate, that'd give you 8 seconds of latency.  There's also a software task buffer that can add a little more latency.

 

To reduce latency, there's an advanced DAQmx Channel property you can set once you switch over to using the standard DAQmx API instead of the DAQ Assistants.

 

low latency.png

 

The above setting makes the board keep its FIFO nearly empty, leading to min latency but highest risk of buffer underflow errors.  (The default is for the board to keep its FIFO either full or half-full, leading to more latency but lower underflow risk.)  I'm not sure how suitable this setting alone would be if you're using a USB device, I've only used it with desktop devices.

 

 

-Kevin P

 

Message 11 of 18
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Thank you for the suggestion, but I am completely new at using Labview, and I am very unsure of how I should implement the thing you are suggesting. Could you maybe show it, or maybe even implement in my circuit.

If of course you have the time and generosity to do so 🙂

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Message 12 of 18
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A few other things to think about here:

 

Calculate one phase of your signal into a double array. Make the AO loop iterate through the array over and over to generate that signal. When something changes recalculate the phase and build a new array. This way you do not need to continuously generate the signal information. You only generate the double array if something changes.

 

If you are iterating through the double array and you want to change something it will also give you a way to stop immediately and go to the new signal.

 

Try to get rid of the wizards. They are a good starting point but are not efficient. Make sure that you put a wait in your loops so you do not bog down the processor.

Tim
GHSP
Message 13 of 18
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I really (REALLY) appreciate your replies guys, but I have no idea of how to implement these things you tell me. Can you please show me how I should do it 🙂
Thank you

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Message 14 of 18
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Highlighted

This should get you started.Example.png

Tim
GHSP
Message 15 of 18
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I will try sir. Thank you very much 🙂

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Message 16 of 18
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EDIT: Oops, too slow.  Looks like Tim's posting had a similar starting point but he took it quite a bit farther.  Study it well.

 

----------------------------------------------------------------

Just as a starting point...

 

I right-clicked on the DAQ Assistant for your analog output task and selected "Generate NI DAQmx code".   Then I did some very minimal cleanup on the auto-generated code.   The result is attached.

 

I *highly* recommend the following 2 exercises:

1.  In your code, open the DAQ Assistant for analog output.  Start cross-referencing all the settings and properties there to the DAQmx code I posted.  Do you find some things that aren't accounted for in one place or the other?  Dig into the detailed help for the DAQmx functions and then come here with further questions.

2.  On your DAQ Assistant, do the same right-click and "Generate...".   Ignore the advice (like I did) about modifying the DAQmx Timing setup.   Now carefully compare that raw result to what I posted, notice what cleaning up I did, try to understand it, then ask questions as needed.

 

 

-Kevin P

Message 17 of 18
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@BirgerBrosa wrote:

Thank you for the suggestion, but I am completely new at using Labview, and I am very unsure of how I should implement the thing you are suggesting. Could you maybe show it, or maybe even implement in my circuit.

If of course you have the time and generosity to do so 🙂


By the way, it is not a "circuit".  A circuit implies wires and actual electricity.  In LabVIEW, we call the visual representation of the program code a block diagram.

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Message 18 of 18
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