Configuring Variables for Virtual Units

The Plant SCADA I/O server does not write to any tags if a unit is offline. Virtual units are useful to implement virtual tags that write to other units on the same port. They provide a way to write to some tags on units regardless of whether the unit is online or offline.

For example, consider the following project units and tags:

Unit 42

Tag1 address: ForceIntegrityPoll

Virtual unit (65534)

Tag2 address: UNIT42. ForceIntegrityPoll

If unit 42 is offline, it is not possible to write to Tag1. If the virtual unit stays online, then it is possible to write to the virtual tag Tag2. This causes the driver to do an integrity poll to unit 42 even if unit 42 is offline.

The virtual unit can only access the physical unit information on the I/O Server local to the virtual unit. To monitor or control the primary and standby units via virtual tags, configure a virtual unit on the primary unit I/O Server, a virtual unit on the standby I/O Server and then define two virtual tags on these virtual units.

For example, to monitor the time to the next poll, define a tag with address "Unit<x>.TimeToNextPoll" for one virtual unit and a tag with address "Unit<x>.TimeToNextPoll" for another virtual unit. At runtime only one of these tags shows data. The tag configured on a virtual unit is located on the same I/O Server as the active physical unit. The other tag sets to 0.

If a switchover happens, the physical unit on this I/O Server becomes active, and the tag on the second virtual unit starts showing data. Use UnitActiveState.Primary and UnitActiveState.Standby virtual tags to find out the active redundant unit and the virtual tag that is currently showing the data.

For the usurp virtual tags, configure the following only on one virtual unit, as the driver replicates this information between the redundant units.

• RequestUnitActive.Primary

• RequestUnitActive.Standby

• UnitActiveState.Primary

• UnitActiveState.Standby

• UnitPortState.Primary

• UnitPortState.Standby.

Virtual Unit Data Types

• Virtual Units - Polling Regime Data Types

• Virtual Units - Usurp Data Types

• Virtual Units - Control Data Types

• Virtual Units - Monitoring Data Types