Shared Variable Polling Rate

[Stocki]

Hi all,

I use the NI-DSM on the windows machine running an LV application that shares variables on the network.

In the NI-DSM you can setup the update rate and I can see that the rate changes as expected.

BUT, on my IPad I still have only a rate of 200ms thus 5 Values per second.

Bastian

[MikeeB]

Hi

You cannot change the rate of update for the shared variable on Data Dashboard. You can alter it if use Poll Web Service but it will still only go to 300ms. Please bear in mind, as a remote device, that you typically want to limit update rate. And, as of version 2 you can send arrays of values and supply multiple values at once.

Mike

[Kira_T]

Hey Bastian,

What version of Data Dashboard are you using?  Depending on the version that you are using, this document is a good resource to tell you the different update rates for the different types of connections.  Please let us know if you have any questions about it!

Thanks,

KiraT

Kira Theuer | Product Support Engineer | LabVIEW R&D | National Instruments

[Stocki]

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@User002_MikeB wrote:

Hi

You cannot change the rate of update for the shared variable on Data Dashboard. You can alter it if use Poll Web Service but it will still only go to 300ms. Please bear in mind, as a remote device, that you typically want to limit update rate. And, as of version 2 you can send arrays of values and supply multiple values at once.

Mike

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As a remote device you typically want to limit update rate? Why?

Here is my idea. The company I am working for is producing position sensor integrated circuits. I build LV executables to access the ICs RAM, and reading the sensor data via an serial interface. This should be done at least at a data rate of 25 values per second. So if you want to show someone how our sensor works it should be faster than the eye can recognize value changes (smooth updates).

Don't get me wrong. The Dash Board App is great. But I want more.  😉

I like the idea to have just one PC running, measuring the data via an USB device and sending the data via WLAN to several IPADs. Thumb up!

Bastian

[AdamKemp]

The processing power on a tablet is still very limited compared to a desktop. New values from the server have to be pushed into the controls in the UI thread, which is the same thread that handles user input. If you do that too often then you wouldn't be able to interact with the app because it would constantly be busy updating the display.

Technically the shared variables in Data Dashboard receive every update pushed from the server, but we have to throttle how many of those updates are sent to the UI thread to be displayed and how often we do that in order to keep the app responsive.

You should also keep in mind that shared variables are not an appropriate communication method for streaming data. They are lossy. The protocol itself does not guarantee that you get every value. You are essentially just getting snapshots of the data. As Mark mentioned, it would be better to send whole waveforms at a sower rate (but still possibly multiple times a second).

[Stocki]

Hi Kira,

yeah I found that document earlier and that is why I asked. I use the latest version of the data dashboard 2.1.1 and the document tells me when using Network Shared Variables the update rate depends on Network Shared Variable publishing rate, network capabilities, and processing power of the device.

I tried to setup the rate via the NI-DSM and I saw the rate increasing on the machine I run the NI-DSM, but not on the IPAD.

Bastian

[Stocki]

Hi Adam,

OK understandable. But it could be configurable somehow. Since in my case I don't care if there is data loss or not it could retrieve data faster. Its just for demonstrating sensor movement and the human eye cannot recognize thousands of data value changes per second it would be cool to increase speed from 5 to lets say 25 values per second.

Please take this into consideration because for me this app is a great advantage.

And some more Indicators would be great! 😉

Bastian

[AdamKemp]

The throttling isn't about controlling data loss. It's about maintaining responsiveness. We basically have a timer in the UI thread that fires at a fixed rate and pulls any pending updates from variables and then pushes those updates into the controls. We tuned that timer to fire at a rate that allows the UI to remain responsive. For the record, we currently fire that timer 20 times a second.