"relative" real time

[Henrik_Volkers]

Maybe you should explain what you want to do, and not what you want to do in software, or how to solve a sub task.

A lot of people here are willing to help you, however it seems to me that you are stuck in the way you want to solve your main problem. Time to go back a few more steps.

[Ray.R]

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@Alain S wrote:

What is that VI doing for 1.666msec?

This is definetly not a task for a PC!

This is a task for a stand alone µC like this one for example. 

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I have to agree with Alain & Henry.

It does not make sense to rely on such timing to start and stop a VI.  In non-software terms, what is the end result of the system?  There may be other (better) solutions, including hardware solutions.