Good Quality, Stale Value: Diagnosing an OPC Link That Is Lying to You

Five boxes in a row from the controller through the driver, the server cache and the subscription to the client, with four numbered red break points marked underneath

Seal_Temp has read 182.4 °C since about 04:00, the quality field next to it says Good, and the trend behind it is a flat line eleven hours long. The bagger has run four product changes in that time and the seal jaws are visibly hot. Nothing on the SCADA is in alarm, nothing in the … Read more

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