Tag Expansion for OPC Clients

Tag expansion is the method by which a group of items on the OPC server are automatically associated with similarly named class members in your project.

Note: This feature is not supported in projects that are configured to run on the Embedded target platform. For more information, see About target platforms, product types, and target systems.

An OPC server acts as a container for OPC groups, and in turn, each OPC group acts as a container for OPC items. The OPC group is roughly equivalent to a class tag in your project, and the OPC items are roughly equivalent to class members.

If you create a class tag in your project that has class members with the same names as the OPC items, it is possible to automatically associate those class members with the OPC items. You need to configure only a single row in the OPC client worksheet that associates the class tag with the OPC group, and then tag expansion will manage the associations between the individual class members and OPC items during project run time.

You can disable tag expansion for a single OPC client worksheet without affecting any other worksheets, and you may choose to do so if you want more control over how your project tags are associated with OPC items. To disable tag expansion, clear the Enable Tag Expansion option in the worksheet's advanced settings.

The following illustrations provide an example of how tag expansion works. (This is only an example; you do not need to use these exact names nor follow these exact steps.)

First, on the OPC server, you can create a group of three items.

Then, in your project, you can create a class with similarly named members.

The class serves only as a sort of template for project tags, so you need to create a new project tag and then set its type to the appropriate class, thereby making it a class tag.

Note: Multi-dimensional or "nested" classes are not supported.

Finally, when you create your OPC client worksheet, you can configure a single row of the worksheet to associate the class tag with the OPC group.

You do not need to configure any additional rows, because the one-to-one associations between the class members and the OPC items will be managed automatically during project run time, thanks to tag expansion.

Taking this a step further, you can create an array of classes in your project by increasing the Array property of the class tag. In this example, increasing the Array property to 3 creates an array of four classes in positions 0 through 3.

It is technically not possible to create an array of OPC groups on the server, but it is possible to create a series of OPC groups that are sequentially numbered like array elements, using the standard syntax for array indices ([n]).

Tag expansion will detect the numbering of the OPC groups, so that when you create your OPC client worksheet, you can configure a single row of the worksheet to associate the array of classes with the series of OPC groups.

Again, you do not need to configure any additional rows, because all twelve associations (that is, four classes/groups, each with three members/items) will be managed automatically during project run time, thanks to tag expansion.

Note: Make sure the size of the array in your project is equal the number of OPC groups on the server. If it is not equal, you might see unexpected behavior during project run time as tag expansion tries to make invalid associations between class members and OPC items.

Note: The Check node type option, in the advanced settings for OPC UA connections, can interfere with tag expansion. Therefore, if you are using tag expansion in an OPC UA Client worksheet, you might need to clear the option for the connection used by that worksheet.