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:

The OPC Server shall use these three parameters to generate a sequence of time intervals and then calculate an Aggregate for each interval. Subclause 5.4.2.2 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 IntRange

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 IntRange

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

Interpolation Aggregate

Interpolative

Numeric

Raw Data Type

Data Averaging Aggregates

Average

Numeric

Double

TimeAverage

Numeric

Double

TimeAverage2

Numeric

Double

Total

Numeric

Double

Total2

Numeric

Double

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

Counting Aggregates

AnnotationCount

All

Integer

Count

All

Integer

DurationInStateZero

Numeric or Boolean

Duration

DurationInStateNonZero

Numeric or Boolean

Duration

NumberOfTransitions

Numeric or Boolean

Integer

Time Aggregates

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

DurationGood

All

Duration

DurationBad

All

Duration

PercentGood

All

Double

PercentBad

All

Double

WorstQuality

All

StatusCode

WorstQuality2

All

StatusCode

Statistical Aggregates

StandardDeviationSample

Numeric

Double

VarianceSample

Numeric

Double

StandardDeviationPopulation

Numeric

Double

VariancePopulation

Numeric

Double

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.