Hello,
Here some examples:
Duty Cycle Measurement:
http://zone.ni.com/devzone/cda/epd/p/id/2592
Triggered counter measurements:
http://zone.ni.com/devzone/cda/epd/p/id/2410
With a M series card (PCI-6229) you can use the arm start trigger to arm the counter.
http://digital.ni.com/public.nsf/allkb/A00D601140D0F8EC86256F8D007C53F9?OpenDocument
Best regards
Ken