To run redundant OPC Mirror clients, you must install OPC Mirror on both members of an Application Station pair with the Enable Redundancy for Assigned Modules option enabled. You can then configure OPC Mirror so that it participates in switchover decision-making for the Application Stations. The OPC Mirror clients inherit their active or standby redundancy roles from the core DeltaV redundancy subsystem.
A redundant OPC Mirror client can communicate with other OPC servers installed on the same Application Station or on another DeltaV or non-DeltaV workstation. A redundant OPC Mirror client works together with the redundant DeltaV OPC Data Access Server and Application Station where it is installed such that if a failure occurs with the active OPC Mirror, the entire Application Station where the active OPC Mirror is running will switch over to allow the standby OPC Mirror to assume the active role. The active OPC Mirror can monitor OPC servers to which it is connected, and transfer data between them; it does not transfer data from or monitor OPC servers connected separately to the standby OPC Mirror. Similarly, the standby OPC Mirror can monitor only OPC servers to which it is connected; however, the standby OPC Mirror does not transfer data between the OPC servers to which it is connected until it assume the active role.
You configure the active OPC Mirror with the data mapping between the appropriate OPC servers. After the active OPC Mirror is configured, the configuration information is copied to the standby DeltaV OPC Mirror, and OPC Mirror is ready for redundant operation. The active OPC Mirror monitors its configured OPC servers and transfers data as required. The standby OPC Mirror is in hot standby mode, meaning it can monitor its configured OPC servers, but does not transfer data. Hot standby allows the standby OPC Mirror to assume the active role very quickly after failure of the active OPC Mirror.
When configuring a redundant OPC Mirror client on a redundant DeltaV OPC Data Access Server, set the Hostname field to Local Machine in the New Pipe or Pipe Properties dialogs.
The primary differences between the active and standby OPC Mirror clients in a redundant configuration are the following:
The active OPC Mirror can transfer data between OPC servers; the standby OPC Mirror reads data from OPC servers to which it is connected, but does not write data.
You can start the OPC Mirror Configuration program to define pipes or set server monitor configuration options only on the Application Station where the active OPC Mirror is running.
There is a condition where both machines (the preferred active and preferred standby Application Station pair) are both in the Active state. This is called a dual active condition (sometimes called a twin active condition). While rare, this can occur when communication between the preferred active and the rest of the DeltaV network is disrupted and the pair is connected to the Plant LAN; or, the active is disconnected and reconnected while still in the active state.
This dual active condition occurs because when DeltaV detects that the active Application Station is missing from the network, it allows the standby Application Station to assume the active role. However, due to being disconnected, the status of the previously active Application Station is not updated and thus maintains its active status. When it is reconnected to (or communications resume on) the DeltaV Network, both machines of Application Station pair will be in the active state, resulting in a dual active scenario.
When this condition occurs, the expected behavior is for the preferred active Application Station to assume the active role and the preferred standby Application Station to assume the standby role.
If the option Unattended workstation reboot on failover or manual switchover is enabled, the preferred standby Application Station automatically reboots and resumes the standby role. Otherwise, the preferred standby Application Station does not reboot and is unavailable.