Network-Published Shared Variable

[msad66]

Hi

1) I would like to have better understanding about network-published shared variable and indicator which is bind to the shared variable. In this regard I create a vi consist of a shared variable and an indicator which is bind to the shared variable with access type read/write. The block diagram of the vi is as follows:

Since the SVE updates variables every 10ms or after the transmit buffer is filled to capacity (8KB) as mentioned in http://www.ni.com/white-paper/4679/en/, I expect “X Sh 2” has the new value after 15ms wait but the result is different:

However after 25ms wait this parameter is updated:

Why this take more time than I expected?

2) Also I want to see the difference between writing value to the shared variable and writing value to the indicator which is bind to the shared variable. In this regard, I changed the vi to the following form:

The result is as follows:

The result shows that “X Loc 1” and “X Loc 3” are updated but “X Loc 2” is not! Why?

Regards

[msad66]

Here is my come.

[crossrulz · Accepted solution]

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@msad66 wrote:

2) Also I want to see the difference between writing value to the shared variable and writing value to the indicator which is bind to the shared variable. In this regard, I changed the vi to the following form:

The result is as follows:

The result shows that “X Loc 1” and “X Loc 3” are updated but “X Loc 2” is not! Why?

Regards

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This one looks like a race condition.  You wrote to X.  After a short time, X self-updated itself with the shared variable which still had the old value, and then later self-updated with the shared variable again just this time the variable was actually updated.

Is the shared variable being hosted on this machine?  Is there a lot of network traffic which might be causing the SVE to be slow in updating?

[msad66]

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1) Is the shared variable being hosted on this machine?

2) Is there a lot of network traffic which might be causing the SVE to be slow in updating?

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Thanks for your quick replay.

1) Yes.

2) No. The attached code just have one shared variable.

According to your answer (If it is a race condition), using local variable of indicator which is bind to network-published shared variable is wrong since it causes race condition every time it is used. Do you agree?

[crossrulz]

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@msad66 wrote:

According to your answer (If it is a race condition), using local variable of indicator which is bind to network-published shared variable is wrong since it causes race condition every time it is used. Do you agree?

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That's what the evidence points to.  Regardless, I would use the shared variable directly anyways.  That way you know for sure you are getting the value that was last broadcasted.

[msad66]

Thanks.

Ok. I will do the same.

What about my first question?

Do you have any clue why it takes more than 10ms to update the value?

I think it takes exactly 20ms to do so.

Why?

[samil_07]

Hello guys,

I am currently working on a project that i need to use network shared variable. I am trying to send the incoming data stream to my PC using network-published shared variable to cRIO. When i debug the block diagram with probe i see that the data is being buffered network-published shared variable. I also checked that on NI distributed system manager. However,  I can't get any data on reader side. I created a simple while loop in cRIO and tried to get the stream data and observe it but i failed. I can upload the VIs if you'd like. Hence, Do you have any idea about what causes this problem? Thanks in advance.

[crossrulz]

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@samil_07 wrote:

I am currently working on a project that i need to use network shared variable.

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Right there is your first mistake.  Network Published Shared Variables are never a good choice (except for when trying to pass the CLED).  This is especially true since you are trying to stream data.  Replace the NPSV with a Network Stream or straight up TCP/IP.  In this case, I would make the cRIO the "listener" and have your PC open the connection to the cRIO.  The cRIO can then stream the data to the PC very easily.