Can an output data generated within a loop in a sub-vi be transfered to the main program for each iteration of the loop ?

[Yoyo87]

Hi LV users,

I have a very basic question, which I did not succeed to asnwer using basic considerations.

I made a sub-vi which performs a current-voltage sweep using a Keithley source-measure unit and a FOR loop. 

This sub-vi outputs 2 arrays containing my data (basically a voltage array and the corresponding current array). I defined also an output terminal to output a cluster of these 2 arrays in order to plot them on a XY-graph. More specifically, I indexed this cluster to be updated after each iteration of the FOR loop, in my sub-vi (the indicator is placed outside the loop, of course).

My problem is that I would like my main program to display the XY-graph in real time, with an update after every iteration of the FOR loop which is in my sub-vi.

I have a problem as the output terminals of my sub-vi are onlu available to the main program as soon as the sub-vi has finished its own execution (which is what we expect from sub-vi to do).

How can I use a sub-vi (because it is very convenient) and get in the main program during its execution data that are generated from the sub-vi loops ?

Thanks in advance for your help,

Yoyo87

[dan_u]

You could use a queue to transfer data.

[Yoyo87]

Thanks for your reply.

How should be implemented the queue ? In the sub-vi ?

I don't see how this will solve the problem if it is in a sub-vi...

Should I forget using a sub-vi ? (which I wouldn't).

Yoyo

[dan_u · Accepted solution]

Enqueue elements in the SubVI and dequeue them in your main VI where you want to display the data.

It's similar to producer/consumer architecture. There are examples for that in LabVIEW. The examples work with 2 parallel loops, in your case one loop (the producer) will be in the SubVI.

[Zekasa]

Hi,

can use a global variable, write to it  inside the loop in the subvi. On the main vi add another loop in which you read the global variable and update the xy graph.

[Yoyo87]

OK for the producer/comsomer structure. But I expect the same problem as the sub-vi will have to finish its execution completely before giving back the execution to the main program. So, during the execution of the sub-vi, the main program is in stand-by, isn't it ?

Or does the queues worl differently ?

could you give me simple example where a number is randomly generated in a FOR loop in a sub-VI, and the main program is used to display the numbers for each iteration of the loop ?

Thanks for your kind help.

Yoyo

[dan_u]

Just try it... When you enqueue elements in a loop of the SubVI you'll get them in the main VI while the SubVI is running. Just pass the queue ref to the SubVI when you call it. Pretty simple.

[Ray.R]

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@Zekasa wrote:

Hi,

can use a global variable, write to it  inside the loop in the subvi. On the main vi add another loop in which you read the global variable and update the xy graph.

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NO!! NO!!  NO!!!

DO NOT USE GLOBAL VARIABLES!!

It is not a good idea.  The queue is an appropriate method.  Using Globals is NOT!!

And yes...  I was screaming..

Zekasa,  being a CLD, you should know better!

[Ray.R]

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@Yoyo87 wrote:

OK for the producer/comsomer structure. But I expect the same problem as the sub-vi will have to finish its execution completely before giving back the execution to the main program. So, during the execution of the sub-vi, the main program is in stand-by, isn't it ?

Or does the queues worl differently ?

could you give me simple example where a number is randomly generated in a FOR loop in a sub-VI, and the main program is used to display the numbers for each iteration of the loop ?

Thanks for your kind help.

Yoyo

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Put your sub-vi in parallel with the loop from your mainVI.  Not within the loop.

[Yoyo87]

I'm not going to use global variables, that's for sure as it won't solve my problem.

To reply to Ray.R, my loop is in the sub-vi, not the main program.

I'll make some test using the queues, and I'll see if I can manage it to work.

I'll keep you informed of my progresses....

Thanks guys.

Yoyo