Hello,

I've spent a few hours troubleshooting the OPC UA server accesses and i thought the feedback could be useful to anyone using this kind of communication.

My application is composed of a S7-1515 CPU hosting the OPC UA server and a National Instruments cRIO 9045 as an OPC client.

After configuring the OPC server through TIA-Portal in the hardware configuration and selected some DB to share i was ready to go.

The use of the "UA Sample Client" available on OPC UA foundation website is recommended. It allows you to connect to your server with certificates, inspect data nodes, server architecture and read/write variables.

On the Labview side i was using the OPC UA toolkit 2018 with the MultipleRead and MultipleWrite functions.

My first test was to read and write 70 REAL variables individually. Each loop was calling for a specific nodeID in the server. The labview function is probably spamming the server with as much requests as variables you want to process resulting in a non efficient refresh rate.

refresh rate when reading 70 variables : 400 ms

refresh rate when writing 70 variables : 1.4 s

Those measures were acquired with a program running on the S7-1500 CPU. They are obviously dependend of your CPU type and program cycle time.

I dropped to 800ms in writing with the CPU in stop mode.

A second test i made was to read and write the same variables in a single array of REAL and here are the times I measured :

refresh rate when reading an array of 70 variables : 100 ms

refresh rate when writing an array of 70 variables : 100 ms

100 ms seems to be the minimum refresh rate possible for the server which is all right.

I've then repeated this operation with an array of 1000 variables and obtained the same results of 100ms for both reading and writing.

The SIEMENS documentation called "OPC UA .NET client for the SIMATIC S7-1500 OPC UA Server" advises you to use array in case of accessing many tags on the server. I'd say you should use arrays in any case.

Greetings

Hi Yaouankiz,

thanks for your effort. Did you already know the measurements provided by Siemens?

Check out that page, there's a category for OPC UA:
https://support.industry.siemens.com/cs/ww/en/view/25209605

By the way: You can definitely read or write data much faster than 100ms response time. It looks like you either use subscriptions or you've locked your minimum cycle time to 100ms.

It also looks like you were reading those 70 REAL values one after another (sure that takes a long time), but It's possible to read them in one request.

Hi Owei,

Thanks for the link I was not aware of this page. I'll definitely take a deep look at it.

I couldn't find any setting in TIA Portal (V15) with my CPU to set OPC refresh rate so i guessed it was locked by default to 100 ms. I was not using subscriptions.

This could also be limited by the the internal code of the Labview functions. I do agree with you, "MultipleRead" and "MultipleWrite" functions must be calling the nodes one by one.

So, you are saying I could possibly read several values in a single request without using Arrays ?

According to the  OPC client i used, each variable declared in a DB is associated to a specific nodeID. So, unless I declare arrays in my DB (1 array = 1 nodeID), I can't access more than one variable at a time.

Regards

Well, this depends on the OPC UA implementation of your client. But OPC UA in general is able to provide multiple read/write operations within one request even though every node has it's own node ID. I'm quite sure these 100ms are caused by the implementation of your client. For example reading just one value should be done in less then 10ms if the server or the network are not overloaded.

I also advice you to use the free demo client from unified automation. It's a nice to handle tool for such things. It has a build in performance analysis function.
https://www.unified-automation.com/products/development-tools/uaexpert.html