Shift register updates

[JakubWaw]

Hi,
question might be ridiculously obvious, but I am new to LabView software.
Why isn't the state of the Boolean 3 LED indicator updated every iteration but only on VI termination?

[dsb@NI · Accepted solution]

Structured Data Flow.
 - Node runs when all inputs available

 - Node returns outputs when it completes execution

Looking at the While Loop as a node, the output of the right shift register is not returned until Boolean 4 stops the loop. Then, Boolean 3 updates!

Check out this tutorial (especially the Dataflow Programming Section):
https://learn.ni.com/courses/labview-tutorial

[altenbach · Accepted solution]

It all follows the principles of dataflow, one of the core concepts of LabVIEW and the first thing you should learn.

• Nodes only execute if all inputs have received data
• Nodes only output data once they have completed.

In your case, the "Boolean 3" can only update once the while loop stops, before that, the output value is not defined.

A powerful side effect of all this is the fact that anything that does not depend on earlier code can run in parallel.

LabVIEW is a full features programming language, not an electrical circuit simulator or similar.

A few quick comments on your "program".

• Give all your terminals reasonable names for code readability, help debugging, and avoid confusion.
  • "ms wait" instead of "Numeric"
  • "Inside LED" instead of "Boolean 2"
  • "After LED" isntead of "Boolean 3"
  • "Stop" instead of "Boolean 4"
• Boolean 4 should be a latch action button instead of a switch, else it could be left on TRUE at the next run. Familiarize yourself with the various mechanical action settings.
• Your shift register should typically be initialized (unless this is an action engine subVI), else it will start with whatever value it had from the previous run, i.e. not defined.
• Waits typically don't change at runtime, so a diagram constant for the wait is more reasonable.

[JakubWaw]

Thank you for all your comments on my code, but that was just an example, to illustrate my question - no factory will ever use it as a crucial component. 

So i guess your answer extends to all nodes, that would mean timed loops as well. Am I right?

[altenbach]

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@JakubWaw wrote:

So i guess your answer extends to all nodes, that would mean timed loops as well. Am I right?

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It applies to absolutely everything that has inputs and/or outputs! (structures, subVIs, terminal, primitives, etc.)

For an old very simple example, watch this video.