Hello,
You basically have this VI set up correctly to trigger your digitizer off of your counter's falling edge. The only problem is how you are starting your tasks. When I change the pulse high time to 1 second, I would expect the waveform graph to update 1 second after I start the VI (when the falling edge occurs). However, I still see the graph update immediately.
What is happening here is that the route between the Counter output and PXI_Trig0 is being connected when DAQmx Start Task is called. When this route is connected some glitching occurs on the RTSI line which causes the digitizer (which has already started and is awaiting a trigger) to false trigger. I verified this by removing the DAQmx Start Task and replacing it with a DAQmx Control Task set to commit. When I commit this Task, the digitizer triggers, even though the counter never generates a pulse.
To work around this problem, I inserted a DAQmx Control Task set to commit before the sequence structure and the niScope Initiate Acquisition. This way, the route is connected before the digitizer task is started, and the digitizer misses the glitches which caused the false trigger before.
See the attached VI which demonstrates this fix. I increased the pulse high time to 1 second so you could observe the time between the start of the VI and the falling edge of the pulse. I also put the DAQmx Control.vi in a case structure so you can observe the difference in behavior with and without that function.
I hope this helps. Please let me know if you have any further questions.
Regards,
Sean Close
Product Support Engineer: High Speed Digitizers
