ControlLogix MSG (Class 3) Utilization Swings Widely, Why?

Zero overload is good. If you haven't yet, be sure to put the connection size back to 4000 for this PLC type, and you should probably set concurrency to at least 2.

Yeah, it looks good without the AOIs. I know you want to reduce the Class 3 load on the PLC side... but there's only so much you can do. It's 50 requests/s just to service your 300ms tag group alone. You've gotta pay to play. Slow some groups down or accept it.

The fact that you have what sounds like a single parent UDT, or set of large parent UDTs, but 6 different tag group rates, isn't doing you any favors. All tags within a UDT should be the same rate for the best results.

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I have to admit that I would immediately deploy my alternate driver's Adapter Mode to move all the 300ms and many of the AOI values to Class1 I/O buffers.