Siemens SIMATIC PCS 7 Function Manual page 189

Permanent operability
If OS Server 1 fails, OS Client 1 is connected to redundant OS Server 2. The identity of the
redundant partner server of OS Server 1 comes from the downloaded server data on the OS
client.. OS Client 1 is not available during the failover to redundant OS Server 2. However, if
redundant OS Server 2 is specified as the preferred server for OS Client 2, you can continue
to operate the plant during the failover from the failed OS Server 1 to redundant OS Server 2.
Once OS Server 1 becomes available again, OS Client 1 is connected to the returning OS
Server 1 because it is the configured preferred server.
Permanent operability is restored after the failover is complete. OS Client 1 is not available for
the duration of the failover to OS Server 1. OS Client 2 remains operable.
The status of the "@RM_Master" redundancy tag does not apply to the OS client with preferred
server configuration. The @RM_SERVER_NAME tag indicates the OS server to which this
OS client is currently connected.
Note
Information on updating operator stations with redundant OS servers in runtime can be found
in "guidelines on updating a redundant OS in runtime (Page 191)".
Reaction of an OS client without a preferred server
If no "preferred server " is configured for the OS client in the "Configure Server Data" dialog
box, the OS client connects to the OS server of a redundancy configuration for which the
"@RM_Master" redundancy tag is set.
If the active OS server fails, its redundant OS partner server becomes the master server. You
can recognize which of the two redundant OS servers is currently acting as the master server
by the status of "@RM_Master" redundancy tag. You can trigger a manual switchover by
setting or resetting this tag. All OS clients then connect to the "new" master server.
Failover criteria of the OS client
The following faults trigger an OS client failover. It is not relevant here whether or not a
preferred server has been configured.
● The network connection to the redundant OS server is disrupted.
● The redundant OS server fails, e.g., due to power loss.
● The WinCC project of the redundant OS server is deactivated.
● A disruption of the network connection between OS server and AS, when the option "WinCC
Fault-tolerant Process Control Systems (V8.0)
Function Manual, 03/2012, A5E02779471-02
client switch in case of a process connection error" is selected in the "Redundancy" dialog
box.
Failure, Switchover and Return of Fault-tolerant Components
6.7 OS clients
189