Opc data Access Automation Specification

4.4.6

 

OPCGroup Events 

4.4.6.1

 

DataChange  

Description 

The DataChange event is fired when a value or the quality of a value for an item within the group 

has changed. Note the event will not fire faster than the update rate of the group.  Therefore, item 

values will be held by the server and buffered until the current time + update rate is greater than 

the time of the previous update (event fired). 

This is also affected by active states for both Group and Items.  Only items that are active, and 

whose group is active will be sent to the client in an event. 

Syntax 

DataChange (TransactionID As Long, NumItems As Long, ClientHandles() As Long, 

ItemValues() As Variant, Qualities() As Long, TimeStamps() As Date) 

 

Part Description 

TransactionID 

The client specified transaction ID.  A non-0 value for this indicates 

59 

 

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OPC Data Access Automation Specification2.02 

that this call has been generated as a result of an AsyncRefresh.  A 

value of 0 indicates that this call has been generated as a result of 

normal subscription processing. 

NumItems 

The number of items returned 

ClientHandles 

Array of client item handles for the items  

ItemValues 

Array of values. 

Qualities 

Array of Qualities for each item's value. 

TimeStamps 

Array of UTC TimeStamps for each item's value. If the device cannot 

provide a timestamp then the server will provide one. 

Remarks 

If the item values are changing faster than the update reat, only the most recent value for each item 

will be buffered and returned to the client in the event. 

Example 

Dim WithEvents AnOPCGroup As OPCGroup 

Private Sub AnOPCGroup_DataChange (TransactionID As Long, NumItems As Long, 

ClientHandles() As Long, ItemValues() As Variant, Qualities() As Long, TimeStamps() As Date) 

‘ write your client code here to process the data change values  

End Sub