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LabView RT DAQmx Signal From Task

I can create a RT timed loop timing source using the Digital Change Detection flavor of the DAQmx Create Timing  This works well, but I cannot use the same port line to detect the polarity of the input when i wire the line into both the rising edge and falling edge inputs.  To detect the polarity, I have to use a DI task on another coupled line input.


I would like to use the "Signal from task" flavor, but it doesn't seem to function with LabView RT (2015) and my PXI-6281 cards.  Does anyone know how to get it to work or why it doesn't work?


There was a discussion about this years ago here but no indication of why "Signal from task" doesn't work on M-Series boards which support change detection like my 6281s. 





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Hi xl600,


Could you tell us a bit more about how the "Signal From Task" instance of the VI doesn't work? Does it hang, throw an error, etc? Also, if you could post some snippets, that would be helpful. We might be able to replicate it, or better see what it is you're doing that way.

William R.
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Best would be to attach the entire VI that you are trying to run, attaching it as a .VI (not a picture, please).


Bob Schor

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I'll try to come up with an example.  The system is too complex to post the vi directly so I'll have to try it in a standalone vi.

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Here's a demonstration.


  • PXI-6281 "ANALOG"
  • PXIe-1082 chassis
  • PXIe-8135RT controller
  • LabView RT 15.0.0
  • DAQmx 15.0.0


To test, I externally tie PFI7 (ACQ_TRIG_OUT) to port0 line 6 (Sense) and port0 line 7 (Detect).  Then drive PFI7 via MAX in single sample on demand mode.


The first VI uses two tasks

  1. A normal DI task to read the sense channel to determine polarity
  2. A timing source of the Digital Change Detection variety to drive a timed loop

The second VI attempts to use a single Signal from Task timing source on either the sense or detect channel (Detect channel is shown wired).  According to NI, Signal from Task auto-starts the task which is why I don't include the Start Task VI.


  • When I drive PFI7 with the first VI running, change detection works great.
  • When I drive PFI7 with the second VI, edges are never seen and the time loop never wakes up.

Thanks for taking a look!






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