Control references vs. global variables

I guess my question centers on the appropriate application of control references.
I am writing a complex program that has a state machine at the top level, and, for one frame of the state machine, a while loop containing a couple subVIs, each of which contains many subVIs. The lower-level subVIs are affected by buttons on the top-level VI. Information from the lower-level VIs must be shown on the top-level front panel within the loop. My question is: what is the best way to do this?
(I have a previous version of the program that I wrote that keeps just about everything on the top level so that I can update the front panel, but this is not at all modular.
In addition, I considered breaking things up into more than on
e front panel, but this doesn't seem to be a good option for this section of this program.)
Last week I read about control references, and decided that this could be the way to deal with this issue. I have not worked with them before, so I want to know: Are control references an appropriate solution for this issue? Is it good programming practice to use them, especially to update data on charts and graphs? How do they compare programming-wise and efficiency-wise to global variables?
Global variables are another solution, I think, but I would have way too many variables to update. I think the advantage with global variables is that one could just insert a global variable in a subVI and not have to wire it up through the different subVI levels, which one would have to do (I think) with control references.
Can you offer any advice?
Thanks!

Paul,
The actual implementation of the solution is up to you. This is indeed a complex question.
I would offer the following advice:
On globals:
Try to avoid them. I try to restrict the use of globals to unidirectional. If you have ever dealt with a race condition, you will know why. Mistakes are often made with globals, and they are quite apt to become the biggest source of bugs in a program, especially in one as large and/or complex as your appears to be. If you must use globals, try using a shift register, or LabVIEW 2 style global instead. Basically, you use an unitialized shift register with two (or more) cases: Read and Write. This prevents race conditions, but only if the VI is non reentrant. However, implementation is still difficult.
Control References:
I also have read and heard about control references. I have had much use for them, for controlling the appearance of controls and indicators (and decoratiopns too, I guess...). However, I would caution you on using them to pass data around. This violates dataflow in a big way, as you are passing around pointers, instead of data. I believe this is difficult to implement, and even more difficult to keep straight. I personally recommend using them only for attributes of FP components, not for passing data.
So, what do I believe is the most elegant solution? I think a data server is your best bet. This is close to a LabVIEW 2 style global, but a little more active. Simply write all of your data into a subVI in any manner, a queue, a FIFO, LIFO, etc. Then, retrieve the data as desired. Buffering, queuing, etc are all handled by the data storage and retrieval subVI. It becomes sort of a small active database. The two sides (storage and retrieval) are independent of each other, and the code is all handled in the function. A basic version is a LIFO, Last In First Out, wherein the data is overwritten constantly. The data is then read by the reading VI, the front panel VI in your case, at any time, and the data is always the latest. You can add complexity as you need, such as making it a FIFO. This is done by simply writing the data to an array, then reading back the oldest value (by reversing the array and reading index 0.)
I believe this is an elegant solution to your problem. It does maintain dataflow, except that time is taken out of the equation, depending on how you look at it. It simplifies your program because you only need to do three things. Write the data handler, then drop in the function into the VI that writes, and the one that reads. It is also flexible in handling the data.
Good luck, and let us all know how you do.

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Hi,
I've created a vi to read an instrument and to initiate its calibration with labview. This VI stores data in a global variable containing measurments and status of the instrument.
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I would make an array of values and then you just pass in which value to update. I would actually take it a step further and make an Action Engine that can update the data and return the data.
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Actually you could create the same sort of facility in LV. All you have to do is create a LV2-style global that behaves as follows:
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To read a value, you supply a value name. The VI looks up this name in the value name array to return an index into the value array. Output the value you find.
Frankly though, it sounds like what you really need is not hundreds of globals, but a good way to manage configuration data.
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Certified LabVIEW Architect
LabVIEW Champion
"... after all, He's not a tame lion..."
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I am quite new to labview. I have to update an existing program and I have a problem whith a tabcontrol. In attachement, I give you a very simple solution showing my problem.
I have defined a simple tabControl in a typedef ctl. I had it to a VI and in block diagram of this VI, I want to fill a global variable with this control and this global variable is set as "Include Data Type".
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Attachments:
test2.zip ‏11 KB

The problem is that the tab is an enum, and you can't wire two strictly typed references of two different enums together, because they are considered different data types. What you can do is right click the reference control in the global variable and select Show Control, which will show the data type of the reference. You can then replace the control inside the reference with the typedef, which will cause it to update correctly.
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Go to Solution.

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Get_Ref_to_MCL_in_Global.zip ‏158 KB

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I have created a project that consists of several VIs. Only the main VI has a front panel and the others perform functions. The function VIs are dependent on controls on the main VI's front panel. I have several ways of passing the value of the controls. One is to use a global variable and just place it on the dependent VIs. Another option is to strictly connect the terminal from the control to a VI connector block and pass the value directly. My last option is to create a reference of the control and reference it inside the dependent VIs, but this would also require connections to be made to the VI block.
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5thGen wrote:
I have created a project that consists of several VIs. Only the main VI has a front panel and the others perform functions. The function VIs are dependent on controls on the main VI's front panel. I have several ways of passing the value of the controls.
1) One is to use a global variable and just place it on the dependent VIs.
2) Another option is to strictly connect the terminal from the control to a VI connector block and pass the value directly.
3) My last option is to create a reference of the control and reference it inside the dependent VIs, but this would also require connections to be made to the VI block.
What are the advantages/disadvantages of these options?
-Stephen
1) Globals are evil and introduce race conditions.
2) The sub-VI only get the value when it was called and updates that occur while the sub-VI is runing are not sensed by the sub-VI
3) This uses property node "value" or "value signaling" both of which run the user interface thread which is single-threaded and you incur a thread swap hit to performance. You also have a potential for race conditions.
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It should work the way you did it. But their are some circumstances wich might prevent it:
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Uh - I am almost getting crazy with that
Kevin, that solution is the right one for programming an application. In that case everything will work fine. But during testing, it is more convenient to call all VIs from their front panels one after another. That's how I'm doing it for everything that needs to be exchanged between such VIs: store them either in a global or in a functional global variable. In that way the user can "play around" with the VIs without wiring them together in a main VI. Therefore, after some thinking I liked Davids idea with that daemon VI (I solved that by adding a boolean in the initialization VI to choose if that daemon is necessary - during testing phase - or not - when using the VIs in a main application). BUT:
David: It does not work!!! I have exactly done what you proposed and this daemon VI is really running, having a True/False frame with a constant of False wired to the selector and holding both the functional global and the dynamically to call VI in the True frame. In that way, after initialization is finished, both VIs are still running but idle. Anyway, when I read back the value of the reference or the DAQmx task, it is invalid again!
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Simplification of scenario:
class myObject definition.
public section.
    methods:
      set_global_table
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      m_pointer_to_table " not sure how to type this"
endclass.
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method set_global_table.
    m_pointer_to_table = i_table. " this needs to "
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    refresh m_pointer_to_table. "this should change "
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endmethod.
endclass.
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create object go_myobject.
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call method go_myobject->set_global_table
                exporting i_table = gt_itable.
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endif.
The code here doesn't work and I've tried messing with field-symbols, etc. all to no avail. Thank you for any help you could provide!
Brett

Just typed in this editor - so no syntax-check but you will get the idea:
Pass the internal table and get the reference of it
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Hope this helps
Christian
>
> class myObject definition.
> *
>   public section.
> *
>     methods:
>       set_global_table
> importing reference(i_table) type standard
> standard table,
> *
>       change_global_table.
> *
>   private section.
> *
>     data:
> m_pointer_to_table type ref to data
> *
> endclass.
> *
> class myObject implementation.
> *
>   method set_global_table.
> *
     GET REFERENCE of i_table into m_pointer_to_table.
>   endmethod.
> *
> *
>   method change_global_table.
> *
field-symbols: <lit_table> type any table.
    assign m_pointer_to_table to <lit_table>.
* manipulate <lit_table>
>   endmethod.
> *
> endclass.
> *
> *
> data: gt_itable   type standard table of t_widget,
>       go_myobject type ref to myobject.
> *
> * Main code
> *
>   create object go_myobject.
> *
> *
> *
> * phantom code fills gt_itable
> *
> *
>   call method go_myobject->set_global_table
>                 exporting i_table = gt_itable.
> *
>   call method go_myobject->change_global_table.
> *
> *
>   if gt_itable is initial.
> *
>      write 'this should output'.
> *
>   endif.
>
>
> The code here doesn't work and I've tried messing
> with field-symbols, etc. all to no avail. Thank you
> for any help you could provide!
>
> Brett

Table Reference Global Variable

Hi,
Inorder to update a table in One VI from another VI, i need a Global Variable Reference of that Particular Table. How shall i create it.
Anybody please comment on this.
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Attachments:
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Global variable reference problem

Hai all,
Am facing a problem in assigning a control as a global one.
Actually i have assigned two OK buttons g1 and g2 as global ones. In 1.vi i have created control c1 (Switch when pressed) and in 2.vi i used g1. when i run both VI's, the g1 get updated with the change in values i gave in c1 in 1.vi (NO PROBLEMS).
But in 1.vi i created another control c2 (Latch when released) and in 2.vi i used g2. when i run both the vi's, the g2 didn't get updated with the change in values i gave in c2 in 2.vi
Anybody please suggest a immediate reply.
Regards,
Mathan

Hai,
Thanks for the timely help. Actually am using parallel port read and write loop.vi from the example finders in Labview 8.5 in my application.
In my project, USB device test using USBCV utlity, am in need of two controls (buttons). One will be a switch which give a high/low value when pressed/released. Other one will be a latch which will be used for remote wakeup of the device (a pulse). i have done this in labview.
But the test needs to be automated. That is when the USBCV utlity asks to connect/disconnect a device, automatically the switch control should be handled accordingly. when the utility asks for a remote wakeup, the latch control will get pressed automatically.
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