Thanks for your reply and now I have a few ideas on how to implement this. Just now I have another problem regarding the crio issue. I use one analogue input of the digital module NI 9239 to acquire the PPS signal and when the rising edge of the PPS approaches, it will trigger some other measurements. And in my case, I want to illuminate the LED when the rising edge approaches. The attached file is my LabVIEW code on how to do this. But now I cannot get any response from the LED. I mean I can read the PPS values but the LED cannot be triggered to illuminate. Can you give me some help on that?