Problem using local variable in event loop

Question

I have a state machine from which I want to monitor various controls, including "Start" and "Stop" buttons.  Not every state needs to monitor the controls.  At present, most states run timed loops.  In the first state that reads the front panel, I have an Event structure (inside a While loop) that monitors the various controls' Change Value events.  For numeric controls, I update variables (in shift registers) as needed.  The "Start" button is used to end the While loop controlling the Event structure, allowing the State to exit to the next state.
My problem comes in subsequent states that employ this same idea.  Here, I put a Local Variable bound to the Start button and use the same code, but it frequently happens that when I enter this particular state, I cannot "turn on" the control -- I push the button, but it stays off.  Curiously, if it was On when I enter, I can turn it off, but then I'm stuck not being able to turn it on.
I mocked up a very simply routine that illustrates this.  There are two sequences (corresponding to the two states).  Both use an Event loop with a local variable bound to my Stop button (really this is an LED control with custom colors).  I've deliberately moved the "initialization" (the declaration of the control in the block diagram) out of the Event loops -- putting it inside the first loop modifies the behavior in another strange way.
Here's my thinking on how I would expect this to work:  The code outside Event Loop 1 should have little effect.  Assume the Stop button is initially Off.  You will "sit" in Event Loop 1 until you push the Stop button, changing its value to True; this value will be passed out of the Event case and cause the first While loop to exit.  You now enter the second sequence.  As I understand the Exit tunnel, it defaults to "False", so I'd expect to stay in the second Event loop until I turn the Stop button from On to Off, which will pass out a False, and keep me in the While for one more button push.  However, this doesn't happen -- I immediately exit, as though the "True" value of the Stop local variable is being seen and recognized by the Event loop (even though it hasn't changed, at least not in the context of this second loop).
An even more curious thing occurs if I start this routine with the Stop button turned on.  Now I start in my Event loop waiting for a change, but this time the change will be from On to Off, which won't cause an exit from the frame.  This will be reflected by having the While loop count increment.  We should now be in the state of the example above, i.e. in an Event loop waiting for the control to be pushed again, and turned On.  However, clicking the control has no effect -- I cannot get it to "turn on".
Where am I going astray in my thinking?  What is it about this method of doing things that violates the Labview paradigm?  As far as I can tell, what I'm doing is "legal", and I don't see the flaw in my reasoning, above (of course not -- otherwise I'd have fixed it myself!).  Note that because I'm using local variables inside Event loops (and I'm doing this because there are two places in my code where I want to do such testing), the Stop control is not latching (as required).  Is there something that gets triggered/set when one reads a latched control?  Do I need to do this "manually" using my local variable?
I'll try to attach the simple VI that illustrates this behavior.
Bob Schor
Attachments:
Simple Stop Conundrum.vi ‏14 KB

Answer 1

altenbach wrote:
Ravens Fan wrote:
NEVER have multiple event structures that share the same events. 
Actually, that's OK.  NOT OK is having multiple event structures in the same sequence structure.
See also: http://forums.ni.com/ni/board/message?board.id=170&message.id=278981#M278981
That's interesting.  I had always thought I read more messages discouraging such a thing rather than saying it was okay.  Your link lead me to another thread with this message. http://forums.ni.com/ni/board/message?board.id=170&message.id=245793#M245793.  Now that thread was mainly concentrating on registered user events which would be a different, but related animal. 
So if you have 2 event structures they each have their own event queue?  So if you have a common event, one structure pulls it off its event queue and it does not affect the other structure's event queue?  I guess the inherent problem with this particular VI was that the second event structure locked the front panel.  Since the code never got to that 2nd event structure because the  first loop never stopped because the change was from true to false.  After reading your post and the others, I did some experimentation and turned off the Lock front panel on the 2nd structure, and that prevented the lockup of the program.
Overall, the example VI still shows problems with the architecture and I think your answer should put the original poster on the right track.  I think as a rule I would probably never put the same event in multiple structures, I feel there are better ways to communicate the same event between different parts of a program,  but I learned something by reading your reply and about how the event structures work in the background.  Thanks.