Having trouble making my VI work as a Sub VI

Question

I am having trouble getting the terminals to pass any data to what they are connected to because the controls they connect to are in a while loop. My frustration level is high since I would have already had this done if I wrote it in C.

First, let me say this might get a little long so if you don't want to read it, then don't. Here goes. I have watched a couple of tutorials, read a lot, and even tried a few things out in code. I get why this can't be done directly in a while loop. Having said that, it seems that I have no choice but to use while loop(s) in my VI.

My VI is loosely based on Queued Message Handler in the Templates section of creating a new VI. I have 2 things that must take place. One - I have created a TCP client where I constantly send messages to get status from the equipment I am communicating with. This is a timed event and must be handled in a while loop so I can maintain the connection to the server. I am not doing the Open, Send, Close, Reopen, Send, Close, etc. type of message handling. Too inefficient. This is the lower half of the example template.

Second - On occasion the user will press a button on the front panel which creates a message that is sent to the equipment to make it do something. And this, it would seem, needs to be in a while loop also, hence my problem. Some/most of the controls exist with the event structure. This is the top half of the example template.

I actually have this working as a front panel, but every thing is in just one while loop and I cannot get the terminals to work. Here is where my confusion comes in, if I am passing something to the while loop, I only get its value once and if it changes, you don't get that change, and if you are passing the data out of the while loop, you only get it when the loop ends. These two things are really baffling me. How can pass data that changes while using a while loop, because I have to, but the while loop breaks using the terminals. Seems circular. The TCP communications cannot stop, and I cannot find an example of how to do this using my friend Google. Am I the only person on this planet that needs to do this? Doubt it.

Not going to show my code, as this in not a code problem. It is an understanding how LabView does things vs. how you would just write the code in C using some library. And also just being unfamiliar with all the things you can do in LabView, not to mention how things are different. I don't know what I don't know, but I can learn.

I want to be able to give the VI I have created to any user and let them use it to control my equipment. If they just want to run it as a front panel, or if they want to use it as a Sub VI that is OK too. I just need to be able to make the terminals actually pass data when used that way.

Thanks, I did order a book on LabView today, but I won't get it soon. I really need to put this problem to bed.

Answer 1

Cannot help that much without seeing the code. But I can try to give you a little bit of an idea of what is going on.

Dataflow is an important concept to understand in LabVIEW. elements (VIs, loops, etc.) will not start until all of their inputs (ie. terminals) have been received or set by something called before, and then they only take their inputs once. If your terminal is outside the loop, then the loop can only read it's starting value. (See "Infinite Loops" on this page). A simple way of solving this would be to put the terminal inside of the loop rather than outside, so it is then read on every iteration of the loop.

As for passing values outside of the loop, there are a number of methods for this. Again, because of dataflow, you will not usually be able to access the value of something inside the loop until the loop finishes executing. However, there are a number of ways to read those values in a different loop. Local or global variables would be the simplest way, but they are not recommended by NI. The proper way of handling this is using something on the synchronization pallet. More info on the options can be found here.

Seeing as you are basing something on the Queued Message Handler, a queue might be a good way to start. LabVIEW has built in examples of code to show you how to use these functions.

Loop synchronization and asynchronous programming are fundamental concepts for writing LabVIEW code. If these are not concepts you are familiar with, I would say that you will gain a lot from showing others your actual code and having people help you with the issues. If you are concerned about sharing something proprietary, try making a simple example and posting that code instead to understand the concepts better.

Answer 2

Event structure to react to evr8and functional global to pass data out.

Suggest pasting block diagram.