Timing for Data Acquisition Using Notifiers

Question

I'm trying to use notifiers to transfer data from an acquisition (master) loop to the slave loop. I want data to be transferred for analysis only when the VI is in a certain state - not in all states. That's why I'm preferring to use notifiers instead of queues (I want all data collected during the other states to be disregarded). I have attached a simplified version of what I'm trying.
The master loop generates a data point every second. The slave loop is in a "delay" state for 5 seconds and then in "acquire" state for 3 seconds. Given this architecture I would expect no data for 5 seconds and then 3 data points to be plotted during "acquire" state because only one data point is generated per second. But for some reason I get 5 or 6 data points during every acquire cycle. I haven't figured out why I get 5-6 data points instead of 3. It probably has to do with the timing functions I'm using.  
Thanks!
-Arnie 
Attachments:
Notifier Data Transfer Template.vi ‏62 KB

Answer 1

Here is an example of a race condition without the use of local or global variables.  What is happening is that the notifier is already queued up with a value even before the Elapsed Timer has even started.  So one extra value will be in the output array.  Also, depending on how the parallel loops perform (which goes first), there may be an extra value queued up before the Time Elapsed event could fire.  Typically, when I ran it, I got an array size of 6 at first, and then it went down to 5.  Clearly the architecture is not right for what you want to do.  Instead of depending on timers and parallel loop timing, you could receive all the values queued and just discard the ones you don't want.
- tbob
Inventor of the WORM Global