Export (push)

Exporting histories pushes data from a source station to a target station or to an external file.

Exported histories from remote stations may create histories in the Supervisor that have an unreachable source. If the source station that contains the history is unreachable, and you modify any history configuration properties using the histories Property Sheet under the Supervisor’s NiagaraNetwork, the next export from the remote station may overwrite the changed properties.

If you added metadata to a history extension, an export of a history from a remote source station does not overwrite or drop metadata values with the history in a Supervisor station unless the metadata property has the same name in both stations.

If you are exporting histories, it is better to add system tags at the remote station so that they are exported up to the Supervisor station.

Exporting data to an external file and importing it back in is also usually done by a Supervisor station. Enhancing the data and importing it back may improve reporting, which is almost always the function of a Supervisor.

Import (pull)

Importing pulls data from the source station or from a file (such as a CSV file) to a target station. Usually a Supervisor station imports histories from one or more remote stations for the purpose of archiving the data. This direction of data flow works best because you have more control over the timing of a data transfer when importing from the Supervisor than is possible when exporting from a remote station.

An import descriptor in the Supervisor station configures properties for each remote source history. Changing the Full Policy on the history Property Sheet view in the remote station, changes the same property on that history’s import descriptor under the Config Overrides property in the Supervisor station.

Figure 12.   Full Policy property in a import descriptor and remote history component

On the left is the import descriptor in the Supervisor station for the remote history. On the right is the history file’s Property Sheet in the remote station.

Local histories behave in the same way since local they and their history import descriptors should always be available to the Supervisor station.