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05/09/2016
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Hi Team.
The OPC UA aggregates specification describes the Interval start and end consideration in general for forward time calculation as, include start bound and exclude end bound. The same is also true in the reverse time order.
Consider the below scenario;
Processing interval 5. Minimum aggregate.
TimeStamp Value Quality
0 10 G(good)
5 15 G
10 20 G
for the interval 5 to 10.
In forward direction the value of Minimum aggregate will be: 15 (start included, but not end)
In Time backward direction, the value of Minimum aggregate will be : 20 (start included, but not end).
So for the same interval, the value is different due to different flow of time.
Doesn’t this breaks the beauty of symmetry? Doesn’t the backward direction follow the symmetrically opposite behavior to forward direction, so to say if
start is included and end is excluded in forward direction, then
start is excluded and end in included in backward time direction.
What is your opinion? And why OPC UA chose not to follow the symmetry?
04/01/2014
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You are correct. OPC does not follow the concept of symmetry of time when doing aggregates in a forward and reverse direction. Your example is an excellent case of this.
The reason is exactly as you state, start time is included and end time is excluded. The reason for this is so that intervals that chain together do not include the start (or end) time twice. This simple rule is always followed so that the data gathering logic is always consistent.
Consistency is also the reason the reverse interval follows the same logic and the start time, in this case the later time, is included and the end time is not. This creates the lack of symmetry.
Rod Stein Manager of Technology Matrikon OPC http://www.matrikonopc.com
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