The following example shows a step-by-step trend process. Figure 1 shows this same process graphically.
The following trending example assumes:
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field value is a linear, increasing value
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trend interval is 60 minutes (3,600 seconds)
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Temporary Status Item Expiration Time is 60 seconds
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initial field value is 71.8
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Present Value is not updated by any other means
Trend process example:
- Trending begins when the network engine receives a trend request for the trend point’s Point Value (PV). Trending reads the initial PV at 9:00 A.M., which is 0.
- Polling begins and the Temporary Status Item Expiration Timer starts.
- Polling acquires the point’s current value and stores it as the update value of 71.8. At this point, the trend has a value of 0 with a timestamp of 9:00 A.M., even though the actual field value is 71.8.
- Sixty seconds later at 9:01 A.M., the Temporary Status Item Expiration Timer expires and polling stops (PV is now 72.0).
- Fifty-nine minutes later at 10:00 A.M., the hourly trend request restarts the trending process and reads the last stored PV (72.0).
- Polling begins and the Temporary Expiration Timer starts.
- Polling acquires the point’s current value and stores it as the update value of 74.1. At this point, the trend has a value of 72.0 with a timestamp of 10:00 A.M., even though the actual field value is 74.1.
- Sixty seconds later at 10:01 A.M., the Temporary Status Item Expiration Time expires and polling stops (PV is now 74.3).
- Fifty-nine minutes later at 11:00 A.M., the hourly trend request restarts the trending process and reads the last stored PV (74.3).
- Polling begins and the Temporary Status Item Expiration Timer starts.
- Polling acquires the point’s current value and stores it as the update value of 76.5. At this point, the trend has a value of 74.3 with a timestamp of 11:00 A.M., even though the actual field value is 76.5.
- Sixty seconds later at 11:01 A.M., the Temporary Status Item Expiration Timer expires and polling stops (PV is now 76.7).
The process continues for the duration the trend settings dictate.