Hi raider,
For applications such as this, you would typically use event counting. However, this data acquisition task type requires that your hardware have a counter, which unfortunately, the 6515 does not.
The only suggestion I might make is modifying the Event Response.vi example program included with LabVIEW. This VI uses Digital Change Detection, which the 6515 supports, to control the iteration and execution of a timed loop. You could perhaps modify the example so that with each execution of the loop a numeric indicator was incremented. This might be done by taking a numeric constant of 0, wiring it to the loop and replacing the tunnel with a shift register to pass values from one iteration to the next, and then placing the increment function in the loop.
This example program can be found by navigating to the Help Menu>> Find Examples>> expanding the Hardware Input and Output folder>>expanding the DAQmx folder>> expanding the Control Folder>> and then looking in the Event Response file. When you click on an example VI, a brief description will appear in the Information box to the right of the Example Finder.
Hope this suggestion helps,
Mallori M.