LabVIEW
Not Understanding Data Value References
Solved!
If you understand dataflow for normal variables, then you understand dataflow for DVR's.
DVR stands for "Data Value Reference". It is a variable that is simply a reference to a piece of data.
When you give a VI this reference, it can look up and see what that reference is pointing to. You still have to pass around the reference, but the difference is that the reference can be supplied just one time, whereas normal variables it is the *value* that is supplied each time.
I'll use an internet example. Let's say you want to send a picture to some friends. You open an email, and attach the picture, and send it to them. Once the picture has been sent, you can never change it or modify it- it's gone. This is "by-value" data, like when you branch a wire.
Let's contrast that with sending them a link TO a picture. Instead of attaching the image to your email, you send them www.example.com/FunnyCatPicture.jpg. Now, when they click the link, they see the picture. You didn't send them the picture, you sent them a reference to a picture. What's the difference? Well, for one, you don't make copies of the picture when you send it this way- there's just the one copy. Two, whoever owns example.com can now change what that picture is, and the same link now points to a NEW picture- without having to send a new email!
With this use-case, let's say you want to host a new picture of your cat every day- the website owner can send out a link to www.example.com/CatPictureOfTheDay.jpg *one time*, and every day he can change what picture is hosted at that address. The reference never changes, but the data does. That's what a DVR does for you. It's an advanced topic, to be sure!
My point here is this. With your setup, you will need to first create the DVR somewhere, then share THAT SPECIFIC reference to all of your subVI's that need to access that data somewhere else. You cannot just open a brand new VI and say "Get my DVR and look up the data to which it points" any more than you can click on a link if no one sends you a link. You will need to share THAT link.
This is why a branching object will work if you assign the DVR *before* the branch but not *after* the branch.
Last, in your situation I don't think a DVR is the right way to go. An Action Engine is likely the better choice for you for now, as it's much simpler to get your head around, but I wanted to try to give you some more understanding about why some of your examples work, but some don't. Hope that helps, and good luck with your project.
(PS- your method of reducing the problem to simple test VI's is good. Keep doing that! Keep each test simple until you understand it, and build from small blocks. Trying to go for the whole program all at once is a bad way to do things.)
Skip the class for now, just make an Action Engine with Init, Write, Read and Close.
Init creates the DVR and stores it in a shift register.
Write uses the internal DVR to write.
Read uses the internal DVR to read.
Close Closes the DVR.
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