Subclause 5.4.2 deals with Aggregate characteristics and behaviour that are common to all Aggregates.
To read Historical Aggregates, OPC clients shall specify three time parameters:
- startTime (Start)
- endTime (End)
- ProcessingInterval (Int) The OPC Server shall use these three parameters to generate a sequence of time intervals and then calculate an Aggregate for each interval. Subclause 18.104.22.168 specifies, given the three parameters, which time intervals are generated. Table 12 provides information on the intervals for each Start and End time combination. The range is defined to be |End - Start|.
All Aggregates return a timestamp of the start of the interval unless otherwise noted for the particular Aggregate.
Table 12 – History Aggregate interval information
|Start/End Time||Interval||Resulting intervals|
|Start = End||Int = Anything||No intervals. Returns a Bad_InvalidArgument StatusCode, regardless of whether there is data at the specified time or not.|
|Start < End||Int = 0 or Int ≥ Range||One interval, starting at Start and ending at End. Includes Start, excludes End, i.e., [Start, End).|
|Start < End||Int ≠ 0, Int < Range, Int divides Range evenly.||Range/Int intervals. Intervals are [Start, Start + Int), [Start + Int, Start + 2 x Int),…, [End - Int, End).|
|Start < End||Int ≠ 0, Int < Range, Int does not divide Range evenly.||(Range/Int( intervals. Intervals are [Start, Start + Int), [Start + Int, Start + 2 x Int),…, [Start + ( (Range/Int( - 1) x Int, Start + (Range/Int( x Int), [Start + (Range/Int( x Int, End).
In other words, the last interval contains the “rest” that remains in the range after taking away (Range/Int( intervals of size Int.
|Start > End||Int = 0 or Int ≥ Range||One interval, starting at Start and ending at End. Includes Start, excludes End, i.e.,[Start, End). a|
|Start > End||Int ≠ 0, Int < Range, Int divides Range evenly.||Range/Int intervals. Intervals are [Start, Start- Int), [Start– Int, Start – 2 x Int),…, [End + Int, End). a|
|Start > End||Int ≠ 0, Int < Range, Int does not divide Range evenly.||(Range/Int( intervals. Intervals are [Start, Start - Int), [Start –Int,Start - 2 x Int),…, [Start – ( (Range/Int( - 1) x Int , Start - (Range/Int( x Int), [Start - (Range/Int( x Int , End).
In other words, the last interval contains the “rest” that remains in the range after taking away (Range/Int( intervals of size Int starting at Start. a
|a In this case time is running backwards on the intervals.|
The calculation of all Aggregates when time flows backwards is the same as when time flows forwards with the exception that the ‘early time’ is excluded from the interval and the ‘late time’ is included. In most cases this means the value will be the same except the timestamps are shifted by one ProcessingInterval. E.g. when time flows forward the value at T = n is the same as the value at T = n + 1 when time flows backward.
Note that when determining Aggregates with MonitoredItem, the interval is simply the ProcessingInterval parameter as defined in the AggregateFilter structure. See OPC 10000-4 for more details.
Table 13 outlines the valid DataType for each Aggregate. Some Aggregates are intended for numeric data types – i.e. integers or real/floating point numbers. Dates, strings, arrays, etc. are not supported. Other Aggregates are intended for digital data types – i.e. Boolean or enumerations. In addition some Aggregates may return results with a different DataType than those used to calculate the Aggregate . Table 13 also outlines the data type returned for each Aggregate .
Table 13 – Standard History Aggregate Data Type information
|BrowseName||Valid Data Type||Result Data Type|
|Interpolative||Numeric||Raw Data Type|
|Data Averaging Aggregates|
|Data Variation Aggregates|
|Minimum||Numeric||Raw data type|
|Maximum||Numeric||Raw data type|
|MinimumActualTime||Numeric||Raw data type|
|MaximumActualTime||Numeric||Raw data type|
|Range||Numeric||Raw data type|
|Minimum2||Numeric||Raw data type|
|Maximum2||Numeric||Raw data type|
|MinimumActualTime2||Numeric||Raw data type|
|MaximumActualTime2||Numeric||Raw data type|
|Range2||Numeric||Raw data type|
|DurationInStateZero||Numeric or Boolean||Duration|
|DurationInStateNonZero||Numeric or Boolean||Duration|
|NumberOfTransitions||Numeric or Boolean||Integer|
|Start||All||Raw data type|
|End||All||Raw data type|
|Delta||Numeric||Raw data type|
|StartBound||All||Raw data type|
|EndBound||All||Raw data type|
|DeltaBounds||Numeric||Raw data type|
|Data Quality Aggregates|
The following issues may come up when calculating Aggregates that include time as part of the calculation.
All Aggregate calculations include the startTime but exclude the endTime. However, it is sometimes necessary to return an Interpolated End Bound as the value for an Interval with a timestamp that is in the Interval. Servers are expected to use the time immediately before endTime where the time resolution of the Server determines the exact value (do not confuse this with hardware or operating system time resolution). For example, if the endTime is 12:01:00, the time resolution is 1 second, then the EffectiveEndTime is 12:00:59. If the Server time resolution is 1 millisecond the EffectiveEndTime is 12:00:59.999. If time is flowing backwards, Servers are expected to use the time immediately after endTime where the time resolution of the Server determines the exact value.
If there is one data point in the Interval and it falls on the StartTime the time duration used in calculations is one unit of the time resolution of the Server.