I was frustrated by the practice as follows,
1, My application retrieves a certain tag, say WndSpeed, from PI on a regular basis, i.e., 1~2 seconds;
2, Upon my application's receiving of WndSpeed, the real time value of this tag is wrote back to PI right away by the name of WndSpeedWriteBack.
When I check the historical values of WndSpeed and WindSpeedWriteBack in the ProcessBook Trend, they don't match!
After a few hours struggling all check-out approaches that I know, I suddenly found out that my ProcessBook is showing me historical data for WndSpeed around 3 minutes later. For instance, when the timeline stands at 19/3/8 19:30:00, the reading of WndSpeedWriteBack (the blue curve) is exactly the one I recored by hand, while the measure of WndSpeed (the red curve) is the one at 19:33:00 or so. It's about 3 minutes later than it should be.
Accordingly, when I track those two tags in real time, the WndSpeedWriteBack always display itself in real time, while the WndSpeed just hide itself by drawing a straight line. Approximately, the line lasts for 3 minutes.
In another word, the value at each red arrow should be equal.
I guess there might be something wrong with my time clock, based on which my PI records or trends data.
Could you please drop me a hint, or any weblink that related?
Thanks in advance.