LabVIEW Idea Exchange

I propose a property node to tell if a control was wired when This VI was called as a subVI.

Sometimes it is practical to be able to distinguish if the value in a control was wired to it (data arrived via dataflow), or if the control was unwired and the value hence the control's default value. Even comparison with the known default value of the control isn't enough, as the program logic may depend on the value source.

Consider an FG that stores two data elements:

You'll have to have two Set functions for this, one for Data 1 ("Set data 1"), and one for Data 2 ("Set data 2"). If you have only one Set function, you add the constraint that both data sets have to be present (wired) every time you set any of the two data sets, or else the unwired set will get overwritten in the FG with the default value of the unwired control. If the FG could decide programmatically if the value was wired to the control, or if it was merely the default value of the unwired control, it could decide whether to overwrite its stored copy or not. The data values could be the same in the two cases, but the actions very different. This is just a simple example, but I have many much more complex cases, that would get much simpler if I could tell where the control value came from. In the above example you could even dispose of the Function input alltogether, if I could tell if any data inputs were wired.

A polymorphic VI will remedy some of these issues, but not all, and in other cases a polymorphic VI is a drastic solution to a simple problem. It's also not an issue in a Top-level VI, as such run only once (only subVIs get called again and again, while depending on prior runs).

This post is intended to present a solution to a problem that I was struggling with using LV2 global variables in my applications.

I don't claim that this is a unique or elegant solution or even the first of its kind, but I'd be curious to hear comments and suggestions about it.

Since it might be a long post, I will split it into 3 parts.

Part 1 (below) will discuss the intended functionalities.

Part 2 will present my current implementation.

Finally, I will try to summarize all this into Part 3, opening the discussion.

So you may want to wait until that part is published before posting your comments (if any).

Part 1: What do I mean by Generalized Functional Global (or GFG)?

The LV2 global variable (or functional global, FG in short) is a well known structure (see for instance the beginning of this thread). It is a subVI which can store data locally in between calls. Usually it comes with a "Write" (or "Set") action and a "Read" (or "Get") action and a few inputs (and the same number of outputs). The inputs are the variables you want to update and the outputs, their corresponding values.

The advantage of this design over the standard LV global is that there is only one copy of your variables in memory. If you are only using scalars or string, etc., using LV globals might not necessarily be a big problem, but if you start storing arrays of large data structure, it could result in a significant performance penalty.

Note that there are other ways to pass data from VIs to VIs (queues and notifiers). Here, I am concerned with long term storage available from many parts of an architecture (e.g. subVIs or dynamically launched Vis).

To begin with, there are two major limitations with any design based on FG (at least that I am not happy with):

1) First, as emphasized above, due to the limited connectivity of a VI's connector pane, a FG cannot store a large number of variables. You can use clusters, but that is not always very practical.

2) Second, if you try to cramp as many variables in a single FG, you will probably run into the issue that you don't necessarily want to reset all variables at once, but maybe just one or two. That calls for a more sophisticated set of actions (Write Variable 1, Write Variable 2, etc, and possibly Write All, Clear All, etc).

In practice, if you use a lot of those FG, you will encounter a third problem:

3) In which FG did I store this @$%&! variable?

Some of my applications contain many of these FGs and I figured that this had become impractical to handle when I started duplicating them, having forgotten that I was handling variable "X" in an existing FG.

The obvious solution (apart from NOT using FGs) is to have a single FG that is designed such as to handle an unlimited number of variables of ANY type that can be modified at ANY TIME from ANYWHERE.

I first looked at the WORM (Write Once Read Many) design of tbob (you’ve got to go to the end of the thread to download the final version of the WORM VI). It is a very clever and compact solution to store different types of variables within a single variant.

Two of the limitations I saw in this design are that you need to know:

1) the NAME of the variable you want to access.

2) the TYPE of the variable that you are WRITING or READING.

Let me clarify those two points.

It seems obvious that you HAVE TO know the name of the variable you are interested in. Well, that’s maybe true when you are designing your application. But after a month or more doing other things, it is not obvious anymore whether you’ve stored the number of components as “# Components” or “Component #” in that other part of the program that…where did I save it, BTW? You get my point…

The second point is apparently taken care of by tbob’s solution of outputting the variable (as a variant) as well as its type. The problem is that this “type” provides a very limited description. For instance, what do you do with a “cluster” type?

Finally, since I want to be able to modify any variable at any time, the “Write Once” feature is not cutting it for me. This is could be easily modified, but considering the previous comments, I decided to change the architecture some more.

I will describe my solution in the next part.

Often times, I have a set of code in a for loop that takes a while to complete, so I put in a progress bar on the UI.  The problem is the progress bar defaults to 0-100 range, and my for loop normally does not have 100 iterations. 

So.... I can either rescale the progress bar to the number of elements, or I can use the "N" and "i" to calculate the percent complete and use that.

Since I like to keep my progress bars as 0-100 (since that works nicely to also show a percent complete) and % complete is useful elsewhere, I generally always end up calculating % complete of my for loops.

Well, why can't NI calculate this for me automatically???  Something like expanding the "i" iteration counter in the loop also have a "%" box right next to it.

Example shown below should probably wire as a double instead of an int, but you get the idea!

In many of my programs, it is frequent that I have to issue a command, then wait for a period of time before reading a response. Usually, if there is a timing delay in code, it is usual to end up with something like this:

To do this more neatly, I created a timing subVI with errors wired in and out, and a number wired to it, to instruct the program to stay in that subVI for that period of time. It takes up much less space.

By doing this, I can easily control timing and execution. However, it's be nice to rework the 'milliseconds to wait' diagram so we could wire errors through it, as this would make the block diagrams so much neater...