DAQ Assistant in subvi not updating output to DAQ board with each call...

Question

Hi All,
I am calling a simple subvi that creates a user-defined number of pulses with "Square Waveform.vi."  This square wave (with the given total number of pulses) is then used as an input to a DAQ Assistant controlling an analog output signal on a NI USB-6259 DAQ board.  I am using Labview 8.5 right now.
However, each time I call this subvi from my main program, the output I measure from the DAQ board is identical to whatever I set in the first call (i.e., if i created two pulses in the first call, I get two pulses on every call, regardless of the input I feed to the subvi).  The multiple calls to this subvi are made in sequential frames in a stacked sequence.  I believe stacked sequences are frowned upon by good labview people, right?  But putting that aside for the moment...
The "#-of-pulses" input I give to the subvi is updated in a subvi front panel number indicator and a graph of this waveform.  Just not in the real output I measure from the board.  Why is the hardware output being asserted (with the original input value) before this new number can reach the DAQ Assistant?
The sloppy fix to this is just to put that square wave creation code in my main program each time I need it.  This does work and fixes my problem.  However, I would like to use subvis to keep things clean.
I am not a good Labview programmer, but have used this software for a number of projects and am stumped by this.  Any ideas?
Thanks,
John

Answer 1

Hi John,
I am running your code over here and seeing
the same results.  I believe the problem is that the DAQ Assistant is
being called inside a loop (really a sequence structure, but
nonetheless more than once).  Sometimes it is difficult to troubleshoot
the DAQ Assistant in cases like this--it is trying to be "smart" and
seems to be avoiding re-configuring its parameters inside the loop. 
This is intended to improve loop speed for when customers are
performing continuous operations.  In this case, it is performing a
finite generation, and the number of samples generated appears to carry
over from one loop iteration to the other.
It sounds
like you have discovered one workaround for this already: putting a DAQ
Assistant in each frame of the main VI.  Two other options that come to mind are:
Use the lower-level DAQmx functions inside
the sub VI.  Here you will have explicit control over when the task is
created and cleared, and when parameters are set.  You can find
examples of how to use the DAQmx API in the Example Finder at:
Help >> Find Examples... >> Hardware Input and Output >> DAQmx
Write
a consistent amount of samples to the DAQ assistant by "zero-padding"
your signal.  For example, instead of writing [10, 1010], try writing
[1000, 1010].  In this case, it wouldn't need to reconfigure the number
of samples to generate.
One lesson to take away here is
that the DAQ Assistant is good for basic functionality, but for more
advanced control over the execution and configuration of your task you
should learn to use the lower-level DAQmx functions.  In this case it
sounds like the problem is actually a bug.  I'll file a bug report, since the DAQ
Assistant is not checking for waveform timing changes even though your
timing is set to Use
Waveform Timing.
Thank you for pointing out this odd
behavior--out of curiosity which version of DAQmx are you using? 
-John
John Passiak