topic Re: reference to a type in LabVIEW

reference to a type

bigdave — Mon, 07 Nov 2005 17:52:22 GMT

Hi,

I have created a subvi which I now use in my main VI, however, I now want to use the values from the subvi WITHOUT having to have a control in my main VI. I can't seem to find a tool to convert a reference to a type.

Anyhelp would be much appreciated.

Thanks,

Paul.

Re: reference to a type

ThomasD — Mon, 07 Nov 2005 18:01:45 GMT

Hi Paul,

can you not take a constant? You have to wire the SubVI to start it. If you like to transfer values to a started SubVI, you can take local or global variables for example.

Regards ThomasD

Re: reference to a type

bigdave — Mon, 07 Nov 2005 18:32:14 GMT

Hi,

Thanks for your prompt reply.

When I try and connect my subvi to a variable I receive the following error

"A wire must be connected to one and only one data source, such as a control's terminal or a function output. This wire is connected to two data sinks but zero sources. You must add a source, perhaps by changing an indicator to a control."

I need to convert the reference type outputted by my subvi to a type (i.e. string, integer, etc) than I can use it without the need for a reference from a control or indicator.

Thanks again,

Paul.

Re: reference to a type

Dennis_Knutson — Mon, 07 Nov 2005 19:00:47 GMT

If, in your subVI, you are using a value property to write to a reference, at the input to the value property, right click and select Create Indicator. If, in your subVI you are using a value property to read from a reference, at the output of the property node, right click and select Create Control. Delete the wire from the value property and wire your control to where the property output was. In either case, you should wire the control/indicator to the connector pane in order to pass/get the value from a main.

Re: reference to a type

bigdave — Mon, 07 Nov 2005 20:18:55 GMT

Hi, thanks for your reply.

When I connect the indicator or control in the subvi to the connector plane, the value from the mainvi does not get passed down in run time, hence why I used references. Is it possible to dereference (as such) the reference?

for example

SUBVI ->passes value to main -> change reference to a type (boolean, integer) -> to a case statement in main vi WITHOUT a control or indicator

Thanks again,

Paul.

Message Edited by bigdave on 11-07-2005 02:21 PM

Re: reference to a type

Dennis_Knutson — Mon, 07 Nov 2005 20:24:59 GMT

If you wire something from the main VI to the subVI, it will get passed to the subVI. If you expect to pass a new value from main to a subVI while the subVI is running (i.e. it's got a while loop running), that is not how a subVI or a function call in any other programming language works. To pass a new value once a subVI has started and before it returns to main, you have to use a reference or a global variable.

Re: reference to a type

bigdave — Mon, 07 Nov 2005 21:02:54 GMT

Hi Dennis, thanks again for your reply,

This is exactly what I have done, use references. In the main vi to access this reference you must then create a control or indicator, then right click and create a reference, then link this reference to the subvi.

I don’t want to have to create an indicator or a control… I just want to use the value outputted by the subvi.

Thanks,

Paul.

Message Edited by bigdave on 11-07-2005 03:03 PM

Re: reference to a type

bigdave — Tue, 08 Nov 2005 15:34:44 GMT

Hi again,

Just to let you all know, I found the "hide" function on the indicators and controls, so I create my controls, then a reference so that I can get the values in/out of my subvi, then hide the controls and connect up the references. Very poor way of doing what I want, but it works.

Re: reference to a type

Ben — Tue, 08 Nov 2005 15:43:46 GMT

Hi Bigdave,

It sounds you are approaching this in a manner that is not consistant with how LV is intended to operate.

Please post a zip of your top level and sub-VI's so we can look and advise.

We are very good if we have pictures to work with!

Trying to get you started out on the right foot,

Ben

Re: reference to a type

bigdave — Tue, 08 Nov 2005 18:13:03 GMT

Hi Ben,

I have included a simple VI of what I would like to achieve. Any questions just ask.

Please keep in mind that this is a simple model of what I would like to do.

Thanks again,

Paul.

Message Edited by bigdave on 11-08-2005 12:13 PM

Re: reference to a type

AndrewAlford — Tue, 08 Nov 2005 18:22:29 GMT

The change the reference passed from the main vi to the subvi is a control (this is correct). Change the ref passed back to the main vi from the sub vi to an indicator. This should fix your problem.

Re: reference to a type

bigdave — Tue, 08 Nov 2005 19:46:56 GMT

Could you take a look at the image attached?

Thanks.

Re: reference to a type

AndrewAlford — Tue, 08 Nov 2005 20:20:06 GMT

Sorry, my bad. I would just pass the value back and forget the references.

Instead of passing the Reference back to the main vi, create an indicator in the sub vi for the value you want to pass. Connect that to a terminal the output of the subvi and that will pass the value to the main.

See attached pics.

Re: reference to a type

bigdave — Tue, 08 Nov 2005 20:26:52 GMT

Unfortunately I have to use references because I need the value to update in real time.

At this point I don't think labview has the facility that I want. To get around this I have used a control and hidden it.

Re: reference to a type

AndrewAlford — Tue, 08 Nov 2005 20:57:38 GMT

Don't give up just yet. Here is how you do it with references. You need to create an indicator for the reference you want to pass back to the mainvi.

See attchments.

Re: reference to a type

Ben — Wed, 09 Nov 2005 01:56:21 GMT

Hi BigDave,

"At this point I don't think labview has the facility that I want. " You are dissin my sweatheart (LV) She can do most anything.

I am just a volunteer here so I can only get serious about answering Q's at night or on week-ends.

I re-wrote you code to do what I THINK you want.

The top level and sub-VI's are illustrated here.

I pass a reference to the control, the indicator and a stop boolean to the sub-VI.

After that the top level just waits for the sub-VI to complete.

The sub-VI reads from the control, does some math and updates the indicator.

If the stop button is true it stops and returns to the top level VI.

The code is attached.

************************************

Andrew,

You can make you images show up in the posting by posting as you have and then...

Click on you link for your attached jpg.

Copy the URL

Go back to your post.

Edit it.

Go to the place in the text where the image should appear then click on the "Inset image" button.

Paste the jpg URL in the dialog box.

Re-post your edited answer.

Ben

Message Edited by Ben on 11-08-2005 08:01 PM

Re: reference to a type

shoneill — Wed, 09 Nov 2005 07:20:12 GMT

Big dave,

What you want to do with your picture below is really unclear to me. If I have understood correctly what you want to do, it won't work. I'm also not sure that what you want to do will work in any other language either.

By using a property node, you need a reference. This you have clearly understood. However, ask yourself what the reference is TO. It must be a reference to an object, and the data contained in the reference will in some way be associated with this particular object. A reference is NOT a class descriptor. It is a reference to an actual object.

If you want to simply generate a reference to an object, then the object must exist first, otherwise you have a problem. Other languages may allow such invalid references, but LV does not. This object is created on the front panel, and can then be hidden. This is the way LV works, and there's nothing wrong with it.

You MUST have a non-constant object (either control or indicator on the front panel) in order to be able to create VALID references.

Simply create the control you require on the FP, hide it and use this as the source for the reference.

Hope this helps

Shane.

Re: reference to a type

bigdave — Wed, 09 Nov 2005 14:31:27 GMT

Hi Shane,

A reference is NOT a class descriptor. It is a reference to an actual object.

If you want to simply generate a reference to an object, then the object must exist first, otherwise you have a problem. Other languages may allow such invalid references, but LV does not. This object is created on the front panel, and can then be hidden. This is the way LV works, and there's nothing wrong with it.

I know a reference is not a class descriptor, the problem I have is that the only way to dereference this reference is to use a control/indicator then hide that control/indicator. One should be able to use the "Type Cast" object to simply dereference and type cast the reference from the subvi WITHOUT the need for hiding an indicator/control. Hiding a control/indicator just so I can get the value out from the subvi is ugly in my opinion.

Simply create the control you require on the FP, hide it and use this as the source for the reference.

You have answered my question in your above quote,

Thanks for the help guys,

Paul.

Message Edited by bigdave on 11-09-2005 08:34 AM

Re: reference to a type

shoneill — Wed, 09 Nov 2005 14:46:34 GMT

Hi Dave,

There are ways of "re-casting" references, but they all require current valid references of a new sort in some way compatible with the old type. I suppose the crux of the problem is that LV doesn't accept these references as being seperate from the objects as you have already noticed early on. The only way to actually get AT the references is to have an object........

Catch 22

Hope this helps

Shane.