Shared Variables update behavior

[plessnick]

Hello,

I have a system deployed that use a network shared variable to transfer data from a cRIO to a RT PXI.  The variable is a 2D array of 8000*5 elements that is updated every second.  The variable isn't buffered (but it seems to be).

I have some issues right now with the system at one location and I'm suspecting network problem. I'm seeing the variable being update once per second, then no update for two or three second and then three updates in less than 500 ms... 

What would be the behavior of the shared variable if for some network reasons it wasn't able to complete its write operation before a new write is performed?  What is written in the shared variable and when (after all the data being sent or as soon as one part of the data is received)? 

Regards,

Patrick

[Peñata]

Hi plessnick,

My first inclination is that this may have something to do with you network connection. How are your cRIO and RT PXI system connected? Are they connected through a common router/switch? Do you have anything else on this network? What I am thinking is that your 2D array is being buffered somewhere between your cRIO and your RT PXI, perhaps at the router/switch. I am not too familiar with the intricacies of the Ethernet protocol, but there may be some internal buffering there.

Also, when you say that when your 2D array is being updated every one second, is that on the cRIO side or on the RT PXI side? By that, I mean is your cRIO publishing new values to the shared variable every one second or is your RT PXI reading from this variable every one second?

Thank you for choosing National Instruments.

Aaron Pena

National Instruments

Applications Engineer

http://www.ni.com/support

[plessnick]

Hello Aaron,

The cRIO is publishing a new 2D array every seconds and the PXI is polling the SV to see if it contains new data.   The SV are deployed on the PXI.  

I'm pretty sure it is a switch or something else in the network that is causing the problem.  I just want to know the internal behavior of the SV as the customer would challenge and tell that the problem is related with the SV and not the network.

Patrick Lessnick

[Peñata]

Hi plessnick,

I will look into the behavior of a Shared Variable in the case that you specified in your previous post. I will post again when I have more information.

Thank you for choosing National Instruments.

Aaron Pena

National Instruments

Applications Engineer

http://www.ni.com/support