Why I am facing an issue where writing to the PLC tags happens quickly, but reading the tag values has a very big delay

[nandhakumar.baskaran]

I am facing an issue where writing to the PLC OPC tags happens quickly, but reading the tag values has a significant delay.

This is my Tag group settings

this is showing overload, I am having this much tags 189459.
What could be the reason behind this?

[Samuel_Sueur]

Wow, your device just can’t keep up with what you are asking.

It shows that you are sending 2003 requests every second.

I think your system needs a massive rework, are you sure you need this many tags sampled at 1second ??

Reduce the number of tags, especially if you don’t use them, increase your sample rate, I’m sure you have tags coming from this PLC that do not need to be refreshed every second…

Have a look at this topic as well for some information :

The device connection overload is excessive

In the project, I need to connect the new Siemens 1500plc to read and write data. In this process, I encountered some problems. I only added 372 points, but overload reached 161% and the writing delay reached 3129ms, which was the result after I increased the polling rate to 1200ms. Normally 372 points wouldn't be such a high overload, would it? What are the factors that affect overload? What should I do next? [f2a97ae52ed6c1967e1367d286e5763]

I’ll the experts guide you on the proper steps to take, but expect massive rework on your side.

[nminchin]

I’m guessing this is an AB plc? Logix driver?

If so, are you reading AOI parameters? (2000 requests suggests so) These will kill performance. Read UDT members or AOI locals making sure each item within is set to have at least Read ext. Access

You hopefully didn't just drag over all of the tags from the opc browser into your project, but it’s certainly been done before… definitely don’t do that if you did.

Ignition & Logix Best Practice Tips for better communications

I recently decided it was time to actually test the theories with scan rates for different structures in the PLC, and found the results below. What I found (obviously) backed Phil's recommendations to only read UDT tags and to ensure that all members of them are set to at least read access. However what I found was negligible difference in response rate at 100ms when reading the UDT tags from within different structures (e.g. individual UDT instances at the root/controller level, addressing UDT…

Also, set the cip connection size to 4000 for capable hardware.

Ignition & Logix Best Practice Tips for better communications

I always forget about this... Holy moly [image] [image] Edit: Updated results with tests using different CIP Connection sizes: Test Ext Access CIP Connection Size Tag Count Sample Rate (ms) Actual Sample Rate (ms) Request Count Overload UDTs All Read/Write 500 6787 10 45.48 14 355% UDTs All Read/Write 4000 6787 10 10.06 2 1% AOI Local Tags (UDT) All Read/Write 500 6787 10 50.86 14 409% AOI Local Tags (UDT) All Read/Write 4000 6787 10 10.08 2 1% AOI Parameter Tags All Rea…

[tgarrison]

Also, if you are using an AB, and it's not v20? or later, make sure you're using the legacy driver.

[code_skin]

I believe the exact versions are

• Logix Driver >= 20.12
• Legacy Driver <= 20.11