6 Data Access specific usage of Services ToC Previous Next

6.2 PercentDeadband ToC Previous Next

The DataChangeFilter in OPC 10000-4 defines the conditions under which a data change notification shall be reported. This filter contains a deadbandValue which can be of type AbsoluteDeadband or PercentDeadband. OPC 10000-4 already specifies the behaviour of the AbsoluteDeadband. This sub-clause specifies the behaviour of the PercentDeadband type.

DeadbandType = PercentDeadband

For this type of deadband the deadbandValue is defined as the percentage of the EURange. That is, it applies only to AnalogItems with an EURange Property that defines the typical value range for the item. This range shall be multiplied with the deadbandValue and then compared to the actual value change to determine the need for a data change notification. The following pseudo code shows how the deadband is calculated:

DataChange if (absolute value of (last cached value - current value) >              (deadbandValue/100.0) * ((high–low) of EURange)))

The range of the deadbandValue is from 0.0 to 100.0 Percent. Specifying a deadbandValue outside of this range will be rejected and reported with the StatusCode Bad_DeadbandFilterInvalid (see Table 28).

If the Value of the MonitoredItem is an array, then the deadband calculation logic shall be applied to each element of the array. If an element that requires a DataChange is found, then no further deadband checking is necessary and the entire array shall be returned.

Previous Next