stop trigger for edge counting

Hi

I need to program a function to exactly count the edges of a quadrature signal between the rising edges of the start and stop signal.

I'm using the NI USB-6361 and i not very familiar with their counter functions.

So, this is my test code...

The start trigger is working perfectly but stop counting by polling digital input is not working very reliable, because the polling rate is much to slow. 

Is there any trigger like the "AamStart" or a way to stop the counting task at the rising edge of the stop signal?

Thanks