Multifunction DAQ
Issue with DAQmx Write and timed loop: Cycle Time depending on size of playback file
Dear all,
I am having an issue with the attached VI regarding the cycle time of the timed loop. Purpose of the VI is to "playback" csv files (values in 1 column, sampling rate 500Hz) and to create an analogue voltage signal between -9,9V and 9,9V from the csv files. Acc. to the constant sampling rate of the csv files the "dt" of the 1kHz timed loop is "2" which is supposed to yield 500Hz loop frequency.
The issue is that the cycle time of the timed loop seems to depend on the size of the playback file:
- With relatively short files (60000 rows => 120 secs) the loop runs faster than "real time" (120 file seconds played in 71 real seconds => 1,685 times too fast).
- With relatively large files (1574000 rows => 3148 secs) the loop runs slower than "real time" (3148 file seconds played in 5036 real seconds => 0,625 times too slow).
- A file with 1009000 rows => 2018 secs almost runs in "real time".
There seems to be a linear correlation between what I call "playback factor" PF (e.g. 0,625 or 1,685) and Filelenght FL which is like:
PF = -0,00035*FL + 1,73
The positive thing about that is: Since small files run faster than real time it doesn't look like a hardware limitation (e.g. caused by desktop DAQ via USB) which is described in other posts here. Playback with a sampling rate of 500Hz seems to be achievable with the hardware used.
When I remove only the "DAQmx Write" from the schematic the playback always is correct (500Hz, neither too fast nor to slow) and independent from filelength.
What can be the reason for that undesired feedback of filelength to cycle time? Any idea is highly appreciated.
Thanks in advance.
Christoph
NI-cDAQ-9172 with NI9263 (1 AO used)
LabView 2017
You should be trying to use a hardware sample clock when you want precise output timing. Here's a minimum working example (MWE) to show how you can replay column 0 of the data in your file at 500 Hz.
-Kevin P
Dear Kevin, thank you very much, your VI works almost perfectly. I had to add a "-1" to the "DAQmx Wait until done" because I received an error 200560 for each of my playback files regardless of their lenght.
The only drawback with the VI is that without the loop (timed loop or any other) I seem to have no chance to observe my playback data online e.g. using a waveform chart. The files are of 1 minute to 45 minutes playback duration (@500Hz sampling rate). Do you see any possibility to add such a feature as I used it in my original VI? I would really like to be able to monitor the signal like in an oscilloscope.
