Why knob need to use property node to change its value

[sunflower2772]

Refer boiler vi in CLD exam sample question. 

In the vi the knob vlaue is changed with a property node, it is not wired directly to a constant. The comment in the vi is something like "writing using property node because of the latch action of the booleans in the cluster"...

Huh? How do the booleans influence the knob even though they are in a same cluster? What principle is this called? I need to google this up,  I didn't read it in.

my Labview books. :womansad:

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[RavensFan]

That comment doesn't make any sense.

There are two main reasons I can think of why you want to use a Value property node.

1.  You want to control the order of execution by using the error wire.

2.  You want to use a property node on multiple controls by feeding it references to different controls.

Neither of these appear to apply in the screenshot you show.

However, looking deeper, it looks like you can set the value of a specific item in the cluster by way of the property node.  Check out the Link to section of its shortcut menu.  I don't think you can use a local variable to set a given element of the cluster.  Now you could change the value of the entire cluster.  Read the cluster, bundle the new value for that one element, write to a local variable of the cluster.  But you won't be allowed to do that because of the latched booleans that are a part of the cluster.  Hover over the context help of that property node and read the description there as well.

[smercurio_fc]

That comment actually makes sense. If you try to use a Bundle By Name on that cluster you'll get an error because the cluster contains a button with a latching action. In fact, you'll get a broken run arrow. Thus, the only way to set the value of that knob is through a property node.

[Omar_II]

I think the note explained it.

First, you can not write to a Boolean Control using a local who's mechanical action is one of the latching types (bottom row).

That applies rather the Boolean control is a single Boolean control or the Boolean control is contained inside a cluster.

When you write to a cluster using a local you are writing to every item in that cluster even if you only interned to over write only one of the items in that cluster.

Try it for your self.

Create a cluster with a Boolean and numeric on an empty front panel.

Create a local of that cluster on the block diagram.

Then try to write to the cluster using a local. If the Boolean is one of the latching type (bottom row) you will get a broken arrow.

But if the Boolean is one of the switched type (top row) you can write to the whole cluster. No broken arrow.