What architecture is best for accurate data logging

Question

Hello,
I'm desiging some LabVIEW code for a standard DAQ application that is required to plot about 100 variables onto the screen on several different graphs and numeric indicators, as well as perform some simple feedback control and log data into a file once a second.
I've done this before, and used a simple state machine architecture, where one state takes care of my logging, and I have a timer vi in there that countsdown 1 second and then writes to file.  However, this method makes me miss a second every once in a while.
I started looking into the producer/consumer architecture as a possible remedy for this.  Because I hear it's good for running two things at different times, so I"ll have my quicker loop handling data acquistion, plots and feedback control, and my slower logging loop, executing once a second.  But I don't see how to implement this
questions:
1. is a simple producer consumer the right topology for my application?
2. when I create my queue do I create it a 100 element array (my data for logging) and then enqueue that in my producer loop from my data acquistion, then pass that to the logging VI.... this seems wrong to me, cause I'm going to be enqueing alot of 100 element arrays... and will be de-queing them slowly at once a second..
3. How do I trigger my consumer loop to execute every second, should I set it up as a timed while loop? or should something from the producer loop tell it to?
I'm sure this is a pretty standard thing to do, I'm jus tnot sure how to implment the correct architecture.
much thanks! 

Answer 1

Ok, let's try this.  I've put together an example that should do what you need.  I put notes in the block diagram, but essentially it runs data in a while loop at whatever execution rate you specify, then sends the data to another graph (or in your case, a log) every one second.  Basically, I've used a 100ms execution rate for the while loop, then every 10th time (you can change this if you want), it sends a boolean 'true' to a case structure within the while loop that contains the enqueue element.  The graphs that I included show that it does indeed add a new point to the second graph once a second while the first one is adding a point every 100ms.
The actual wiring of this Vi could be cleaner for sure, but it was a quick and dirty example I put together.  Hopefully this will help you accomplish what you're trying to do.
Regards,
Austin S.
National Instruments
Academic Field Engineer
Attachments:
Enqueue array 2.vi ‏28 KB