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DAQMx Start Digital Edge (falling) or Digital Trigger is making PFI0 go low

My UUT requires PFI0 to enable and disable a diode.  I sample AI from a reference detector after PFI0 is enabled and disabled.  While using an oscilloscope to monitor PFI0, I see PFI0 go low when setting DAQMx Start Digital Edge (falling) and DAQMX Digital Trigger.

I use the same algorithm for the rising edge without any problems.  Why is this occurring?  Below is my vi.



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When you say that you are using the same algorithm for the rising edge, is the only difference in that VI the "falling" input into the DAQmx

Christine B
Technical Support Engineer
National Instruments
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Thanks for replying.


The falling input and the PFI0(DO) is being written to with a True value.

While doing more research I think I stumbled on the solution.  I hard wired the PFI0 to PFI2 and monitor the falling edge on PFI2.  This seems to work for my solution.  I assume that we cannot trigger on PFI0 since it is being used as a DO. 


I think this matter is resolved.


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Good if you've got something working, but you might have learned the wrong lesson.


It looks like you want to use a DO task to generate the trigger signal that the AI task is waiting for.  I've done this kind of thing a lot with desktop MIO boards and it works just fine without any need for physical wiring & screwdriver work.


The key is this:

1. Start the DO task and set an initial DO output state first.  Set it to the "pre-trigger" state -- i.e., set it False for a rising edge trigger or set it True for a falling edge trigger.


2. *Then* start the AI task that will be sensitive to this signal.


3. *Then* go back and toggle the DO state (I always toggle twice to make a full pulse).  This state change will trigger the AI task.


4. You're now free to stop the DO task if you like and the AI task can continue.


Voila, no need to hardwire PFI2 to PFI0 or anything like that.



-Kevin P

CAUTION! New LabVIEW adopters -- it's too late for me, but you *can* save yourself. The new subscription policy for LabVIEW puts NI's hand in your wallet for the rest of your working life. Are you sure you're *that* dedicated to LabVIEW?

(Summary of my reasons in this post, part of a voluminous thread of mostly complaints starting here).

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