Clearing daqmx read value

Hello,
When doing a single sample counter read using DAQmx in a while loop, is there any way to reset the data output on the DAQmx read task? For instance, say the value is 200, is there any way I can keep the task open and running and reset it to read zero?
Thanks,
David Joseph
Message Edited by KY Wildcat on 11-16-2007 10:57 AM

Hi KY
There isn't a property or method in DAQmx for setting or resetting the count value when a task is running. You need to Stop and Start the task again. Otherwise, you can
create your counter in software. If you wire the counter output to an indicator, you can then create a local variable and create the logic that will
programmatically reset its value, e.g. when it exceeds 200.
http://digital.ni.com/public.nsf/allkb/65E3DBC715998C3286256E900075B7F8
I hope that helped,
KostasB
NI Applications Engineer

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Message Edited by gkvishnu on 10-12-2005 08:10 AM

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[SRQ 211371]
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Attachments:
Log Monitor Block.JPG ‏292 KB

Yeah, as you and Erik said, just specifying (Sample Rate / 5) as the '# to read' could do the trick. Then you can ditch the 'Wait (msec multiple)' function. I don't think I'd recommend the -1 = infinite timeout though. I'm pretty leery of stuff that can lead to an infinite wait or an infinite loop. Even a 1 sec timeout should easily be way more than enough. Note however that this method depends on all your processing code executing in under 200 msec on average. Otherwise, your reads will fall behind and you'll eventually get a DAQ buffer overflow error. My earlier suggestion to first query for # available samples and then read the MAX of (# available, SampleRate/5) will prevent cumulative fall-behind effects.
There's too many unknowns to speculate with any confidence on exactly why the software timing method didn't work. I can point out some additional things that bear greater scrutiny though.
1. You've got some sort of function dealing with file streaming that takes a path input and produces a path output. This probably means that every loop iteration you're using the path to open a file, write data, and close the file again. This actually may consume more than 200 msec, at least some of the time. Because this runs in parallel with the msec wait function, something like the following could be happening:
A. Iteration 1 proceeds as expected. The DAQ read collects 20 samples, the file write consumes only 100 msec, and the wait msec function ends after 160 msec in order to end on a 200 msec multiple. The wait function took longest, so your whole loop ends on a 200 msec multiple.
B. On iteration 2, the wait msec function ends after 200 msec on the next 200 msec multiple. The DAQ read collects another 20 samples (because it's been pretty much exactly 200 msec since the previous loop iteration started) right away. However, Windows was busy messing with the file cache so this time your file write consumes 375 msec. The file function took longest so your whole loop ends 175 msec into the next 200 msec multiple.
C. On iteration 3, the wait msec function ends after 25 msec on the next 200 msec multiple. The DAQ read collects 37 samples that have come in since the prior call 375 msec ago. The file function consumes only 50 msec this time, ending the loop 25 msec into the next 200 msec multiple.
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Message Edited by Dejun on 08-30-2007 05:16 PM

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Hello,
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Try and do exactly what the driver does. This
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http://digital.ni.com/public.nsf/allkb/5D42CCB17A70A06686256DBA007C5EEA
this link says that
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Thanks,

thanks,
im using LV 8.6, daqmx driver 8.9.5.
yes the program u posted get the currect time. but see attached program thats what i have in my program exactly
if you took out the time to using the get waveform components using the waveform from daqmx read. i still have the timestamp as the application start time. plz see attached code 8.2v and FP
the waveform time x axis is not current, it is starting from the application start time.
PS: samples to read = 10,000 and sample rate = 1000
plz help..
Attachments:
daqmx read problemp.png ‏8 KB
daqmx read problem.vi ‏36 KB

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Attachments:
example code.png ‏44 KB

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Clearing daqmx read value

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