DAQMx has no output when the pulse width is less than 0.1sec

Question

Hi,
    I am developing a application to generate square wave(voltage) and out put from a DAQMx card (PCI-6723   - 32 point analog in/out).  I attache a simplified code (labview8.5) I wish somebody can help me to figure out what is the problem
    When I set pulse width longer than 0.1 sec. everything works fine. If I set the pulse width less than 0.1 sec, the output from DAQ eithet gets nothing or incorrect(I used a Osclliscope to monitor the  DAQ output), although my labview monitor showed correct output.   I wonder if it is a sampling issue of the DAQ card.
     Thanks
    Guang
Attachments:
test21.vi ‏69 KB

Answer 1

I think one or two things could be causing the problem.
1.  Your DAQmx write is set for 1 sample.  I don't know how that dynamic signal type works with such a setting.  Perhaps it is just putting out the first sample of the signal that is generated.
2.  The math for the frequency of the square wave does not make sense.  You are adding a pulse delay with the pulse period, then inverting it to get the frequency.  So a 1 second delay with 1 second pulse is a 1/2 Hz wave.  A 4 second delay with a 2 second pulse would be a 1/6 Hz wave.  That doesn't make sense to me.
I would recommend building your own waveform or array and not using the express VI.  If you have a 1000 Hz frequency established let's say, build a 1000 element array.  For a .1 second pulse, set replace 100 of those elements with a 100 element array of the voltage value you want.  You can index it wherever you want in the array to account for whatever delay you want.  If you want a 1 msec pulse, then just replace a single element of that array wherever you want.