Shall I use global or local variables for my program?

Question

Hello,
1) I am using 2 while loops in parallele. One 50 ms timed and the other one 1000 ms. The 50 ms one gets data from a sensor. I want to display that data every 1000ms in the 1000 ms loop. I have used global variables to do that. Should I better use local variables?
2) This question deals with the VI I have attached
a)  Shall I initialise the "  Quitter programme " global variable in the main VI aswell ? Or is what I have done enough?
b)
I want the sequence to continue ( finishes the last sequence and goes back to the first one without any delay if possible ) until the user presses Quit. I am concerned about the " wait until " icon of the exterior main while loop ( set to be 250 ms at the moment ). This time, 250 ms, is the time it takes between the end of the last sequence and the beginning of the first sequence? Is that right? Once u start the first sequence, the while loop doesn’t do anything until the end of the last sequence ( my 3 sequences can take several minutes ) ?
Hope I am clear… lol
Don’t hesitate to ask any questions if not clear enough
thanks a lot,
David
Attachments:
Example1.zip ‏28 KB

Answer 1

Bonjour Marc.  You will definitely have a race condition because you are writing to the global in a parallel loop.  You could click the stop button in the quitter program, then before it is read in the main loop, the lower loop writes a false to it, and your click is lost.  Why do you want a separate quitter program?  You may have a good reason but it isn't clear here.  I would not use globals or locals for this, just put your stop button on the main vi.  You already have one, so why do you want another one in another vi?  If you must do it this way, eliminate both lower loops.  They are not necessary.  The global default is set to false, so when you run the vi, it is false.  The button mechanical action is set to latch when released, so after clicking it will turn true until the main vi reads it, then it will automatically turn false.  So you don't need the lower loops to keep writing false to it.  If you just eliminate both lower loops, it will work fine.  The only purpose I could see for the quitter vi is to have one button cause several vi's to stop.  Instead of doing it this way, put one stop button in your main vi, create a reference (right click - create - reference), and wire this reference into all subvi's that the main calls.  The subvi's must be changed to accept the reference and use it.  This would be a much better method to close all vi's with one button.
You could also get rid of the sequence structure (it hides code and makes it more difficult to read).  Just put all your code side by side.  The error in/out wire will ensure that the execution order will be fixed, just like in your first sequence with the open file and write file functions.  Just wire the error out of the write vi to the second write vi, and then the error out of the second write vi to the close vi.  No sequence structure needed, and all your code is viewable on one screen.
- tbob
Inventor of the WORM Global