DataChange event on OPC Servers


Thread Starter

Urban J.

I have a question that probably someone has an answer on.
Do the attributes in the DataChange event always containing all items in the group or just the items that have been changed.
And can different OPC Servers work different regarding the datachange event

Urban J.
Hello Urban,

In a DataChange callback from an OPC server, only the active items that have changed within the group's update period should be present in the
message (note that a change in value or quality, but not timestamp constitutes a changed item as far as DataChange callbacks are concerned). This is explicit in the OPC Data Access specification, and as such, all servers should behave in this manner. If they do not, then they are non-compliant.

I should point out that the above is true for subscription callbacks. A DataChange callback can also be invoked in response to client's Refresh request, in which case the current values for active items in the group should be present, irrespective of whether or not they have changed.

Best regards,
Gord Ellis

Gord Ellis, P. Eng.
Northern Dynamic Inc.
J103-155 Frobisher Drive
Waterloo, Ontario
Canada N2V 2E1
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