Control Logix Device Load Factor

[Varsha_Gottipati1]

We’ve been tracking the load factor of our ControlLogix PLC, which typically averages 0.55—a high but manageable level. However, we’re seeing occasional spikes to 1.1 that persist for several hours before returning to normal without intervention. These unexpected overloads are alarming, as their cause remains unclear. We’ve identified a daily spike at 6:00 PM that aligns with IT server backups, but other prolonged spikes lack an obvious trigger. We need to understand the factors causing these spikes and determine if they indicate a failing PLC.

Kindly Advise.

[pturmel]

Do you have Studio 5000 users coming and going? (Same PLC resource.)

Consider monitoring your Class3 Message loading through the PLC's built-in web page (or task monitor tool if the web interface isn't available).

[robertm]

Unlikely a failing PLC. How are the device drivers configured? What CIP size and number of connections? What’s the network architecture between Ignition and the PLC? What model and firmware PLC? How many tags? Are they set to poll or leased? If polled, what are the poll rates? Are you accessing AOI tags?

Have you applied these best practices:

Ignition & Logix Best Practice Tips for better communications

I misunderstood. I guess it depends a lot on how much and how often you’re reading from the PIDE data blocks. The main reason reading directly from an instruction like that is generally not a good idea is because it blocks the Ignition driver from efficiently reading the data. Each element of each PIDE will be its’ own read operation. While normal UDTs with all of their members exposed with read/write access can be read with a single read operation. If you’re reading, say 90% of a block all the…

[Varsha_Gottipati1]

I am the only studio 5000 user and I see the Overload factor increase slightly when I go online with the PLC.

As for the Class 3 Message, what parameter should I monitor on the web page or Task monitor? No idea what it is.. Thanks.

[michael.flagler]

Still a lot of questions that need answered:

• Controller Model
• Controller Firmware
• Ignition Driver (Allen Bradley, AB Logix, Automation Professionals Client Driver?)
• Driver Settings (Concurrent Connections - Default is 2 and CIP onnection Size - Default is 500)
  • Typically I'll increase the CIP Connection Size to 4000 bytes but this won't work on some older models.
• Tag Count
• Scan Groups (Polled/Subscribed, Direct/Leased, Rates)
• What kind of tags are you communicating with (Mostly atomic, UDTs, AOIs)?

On the PLC web page (if you can get to it), there will be a Class 3 Comms gauge on one of the pages that shows what percentage of load the processor itself is seeing. Once this starts maxing out (90%+) you'll see comms performance issues with Ignition also.

[Varsha_Gottipati1]

We got it figured out. I did find a device under the Class 3 Message Connections around the same time our Load factor increased. So, that device is a laptop of an ex employee who installed Ignition, created projects accessing tags from the control Logix.

[robertm]

Even so, a 50% load could be indicative of a poorly configured system. I have near 80k tags with 0% load on my drivwr statistics.

[nminchin]

There seems to be a mixture of terminology here... There's load factor which is what the logix driver used to use where anything under 100% meant the plc was keeping up with demand. And then there's the newer Overload, where anything above 0% means it's not keeping up with demand. 0.55 or 1.1% is perfectly fine whatever the measure

[robertm]

I read it as 55% and 110%. But yes, a mixture of terminology between post 1 and post 4.