Real-Time Measurement and Control

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Slow DAQmx Read

DAQmx Read is being "slow" for me and I don't see why.


I've put a DAQmx Read into a timed loop where the frequency of the loop and the sample rate of the DAQmx Input task are equal at 5kHz. When I check the duration of the DAQmx Read VI, I find that it is consistently close to the period of the timed loop. I'd like to perform some processing and control off inputs but I can't do that if the Read VI's take up 3/4+ of the loop period.


I've looked everywhere and I haven't noticed anything that would cause this behavior. If it's "a feature, not a bug" to not retrieve data as quickly as possible, I haven't found a way to tell it not do that. My DAQmx Write VI's had durations of about 5 µs, which is about where I had expected the DAQmx Reads to clock in at.


Here's an example of the behavior I'm referring to:



The above example was timed with this setup:



The input task is configured to use a PXIe 6612 (Slot4) and the timing stuff (pulse train generator and timed loop source) are on a PXIe 6356 (Slot2). The whole thing is running on PharLap ETS 13.1 with a PXIe 8135 as the controller.


- A. Brooks


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I've gotten the DAQmx Read duration back to where I had expected it to be, but I don't understand why the code would be operationally different.


I changed:





After the change, the DAQmx Read duration is about where I expected it to be between 2µs and 4µs. I don't understand why the change would have a significant impact though. If someone could explain for documentation purposes, I'd greatly appreciate it.


-A. Brooks



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