You should be able to do this with a continuous counter pulse generation task using DAQmx on a cDAQ. cRIO shouldn't be necessary.
You'd use one of the chassis counters and route its output to a DO pin. Note that you may need to write both freq *and* duty cycle together to make the duty cycle change "take hold." See this thread for more info.
-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).
