QuickOPC User's Guide and Reference
Mapping Operations
There are several things you can do with OPC Data: you can read from them, write to them, or subscribe to their changes. Not all mappings should be involved in all operations, and you therefore need to specify the operations that are of interest to you.
For DAClientItemMapping (OPC Data Access items), doing this means properly setting the Operations argument of the DAItem attribute. You can set it to one of the following values, or their combination:
If you want a combination, you can use a logical OR of the individual values, or use a predefined symbolic name such as ReadAndWrite, ReadAndSubscribe, etc. By default, the Operations argument is set to DAItemMappingOperations.All, meaning that the mapping will be involved in all operations.
There is just one mapping operation for DAClientPropertyMapping (OPC Data Access properties) – the “Get” operation that obtains the value of the OPC property. For OPC-DA property mappings, it is therefore neither necessary nor possible to specify the operation(s) that certain mapping should be involved in.
For UAClientDataMapping (in OPC-UA), you specify the operations by properly setting the Operations argument of the UAData attribute. You can set it to one of the following values, or their combination:
The operations do not get actually executed until you write and run a code that does it. See Invoking the Operations for details.