Hello;
The way to accomplish that is by using one of the 660x boards, and configure the counter to do "Edge Separation Method".
That method will increment the counter event count value every time a pulse comes into its source input. The counters only increments between the rising edge of the aux line and the rising edge of the gate. The current value of the counter is read when the counter is finished or an error occurs. You need to wire the out pulse to the AUX line, and the return pulse to the gate line of the counter.
Hope this helps.
Filipe A.
Applications Engineer
National Instruments