System Model > Variable Tags > Link tags to an External Data Source | System Model

Link Tags to an External Data Source

Linking is an I/O device specific operation. When you add an I/O device to Citect SCADA, you can choose to link it to an external data source (see Link an I/O Device 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, Citect SCADA 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. Citect SCADA can update whenever one of the external tags is changed.

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 compile your project in Citect SCADA, the change is copied from the external database to Citect SCADA's variable tags database.

If a tag 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 Citect SCADA. Also, some will not be in a format that Citect SCADA can read. Each I/O device has an associated format file in Citect SCADA. It is this file that determines what information is copied to Citect SCADA's variable tags database and how this information is to be converted. In most cases, you will not have to edit this file.

Linked Unity Tags

Note the following for Citect SCADA tags linked to Unity tags:

  1. The addition of a new tag on either side, will result in the addition of a new tag on the other side.
  2. The deletion of an existing tag on either side, will result in the deletion of the corresponding tag on the other side.
  3. The modification of an existing tag on either side, will result in the modification of the corresponding tag on the other side.
  4. The delete operation in either Unity or Citect will take precedence over other operations (such as modify).
  5. If there is some contention between the Unity database and the Citect database for the same tag, that is modifications are done from both databases at the same time, the Unity database item will take precedence over the Citect database item.
  6. The linked I/O Device live update synchronization will be triggered in the following cases:
    1. Unity configuration update.
    2. Citect configuration update.
    3. Manual refresh.

See Unity Support Matrixes for information about levels of support between Unity and Citect SCADA.

See Also

Refresh the Tags for a Linked I/O Device
Breaking the Link to the External Data Source

Published June 2018