Why Does My PXI Trigger Line Go to Logic High When My DAQ Task Stops?

Issue Details

I’m using LabVIEW and a PXI DAQ device to generate a rising edge trigger that I pass to the PXI backplane.  Other devices are continuously monitoring this PXI trigger line to take action each time they see a trigger signal from this PXI DAQ device.  However, when I stop my DAQ task, my PXI trigger line returns to logic high which creates an erroneous extra trigger event for my other devices.

I need to be able to start and stop my DAQ task without causing this unintended trigger.  How can I prevent this extra trigger from happening?

Solution

This is expected behavior for all PXI DAQ hardware controlled by NI-DAQmx.  The PXI Trigger lines are by default logic high.  When the DAQ device reserves a given trigger line, the trigger line goes low.  Then the DAQ device will control that trigger line until the task is stopped.  When the task is stopped, the DAQmx Task state will return whatever state it was in when the DAQmx Start Task.vi was called.

If the DAQmx task was not explicitly set to the Reserved or Committed state using the DAQmx Task Control.vi, then the task was in the Verified state.  Returning to this state will release the reservation of the PXI Trigger line which will return back to default logic high.  Any other devices still monitoring this trigger line for a rising edge trigger will see this trigger.

Alternatively, if the DAQmx task was explicitly set to the Reserved or Committed state using the DAQmx Task Control.vi, then the task will have reserved the PXI Trigger line.  Now when the task is started and later stopped, the DAQ task will return to respective Reserved or Committed state with the PXI Trigger Line still reserved and still in a low logic state.  A task configured in this way can be started and stopped without causing erroneous triggers. 

If maintaining the reservation on the PXI Trigger line(s) is necessary after stopping the DAQ task, use the DAQmx Control Task.vi as described above.