Logging timing discrepancies


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I have a question regarding logging intervals. The logging is set up to record a reading every 15 minutes. Most of the time, the readings are in fact recorded at exactly 15 minute intervals. However, this is sometimes off by as much as 25 seconds. Please let me know what could be causing this.

We are polling (6) Devices via ModbusRTU every 10 seconds. I am getting some Modbus Timeout errors that I have been unable to resolve, but I do not think the two issues are related since I am getting good data back every 10 seconds.

I have attached an excel file of the logged data.




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When you do a snapshot logging, its looking at whatever the latest data points are every interval specified. That latest data point may be older because the logging set and data acquisition are out of sync. A timeout is the most likely cause of this. I would have expected the interval to be smaller than 15 minutes after one that is longer, but I think the logging set is only triggered to read with a new data point, not at a specific interval, meaning it will sit there until a data point comes in, then look and see if the interval has passed. It doesn't wake up on its own every interval and log whatever it sees at that instant. This means the interval will never be less than what you specify, but can be more if data is delayed for any reason.

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