Linking is an I/O Device specific operation. When you add an I/O Device record in CitectSCADA (using the Express Communications Wizard), you can choose to link it to an external data source. The external data source is where the tag data was saved when the actual I/O Device was programmed. If you link to the external data source, CitectSCADA automatically creates variable tag records for every tag in the I/O Device.
These variable tag records are dynamically linked to the tags in the external data source. CitectSCADA is updated whenever one of the external tags is changed. For example, you might program your I/O Devices, configure your project, then add some new tags or edit existing tags. In this situation, CitectSCADA is automatically updated with your changes.
This update occurs when you attempt to read the changed tags in CitectSCADA (for example you compile your project, display the tag using the Variable Tags dialog box, modify or paste the tag, or perform a manual refresh, etc.). For example, if you change the address of a tag using a third party I/O Device programming package, the external data source is changed. Then, when you display the variable tag record or compile your project in CitectSCADA, the change is copied from the external database to CitectSCADA's variable tags database.
You can tell if a tag is linked because the status line at the bottom of the Variable Tags form will read Linked: Yes. If it is linked and you change a field, your change will not be overwritten when the link is next refreshed. Generally, however, any field which takes its value from the external data source is disabled.
Note: Some properties defined for the external tags will not be relevant to CitectSCADA. Also, some will not be in a format that CitectSCADA can read. Each I/O Device has an associated format file in CitectSCADA. It is this file that determines what information is copied to CitectSCADA's variable tags database and how this information is to be converted. In most cases, you will not have to edit this file.
Note the following for Citect tags linked to Unity tags:
See Linked Tags for information about levels of support between Unity and CitectSCADA.