Siemens Simatic PCS 7 Function Manual page 208

Failure, switchover and reintegration of high availability components
7.4 OS server
The terminal bus as a whole and the communication between the AS and OS servers remains
unaffected.
Both OS servers are started and begin processing an activated WinCC project. If a disruption
in the network connection to the OS partner server occurs in this situation, WinCC Redundancy
reacts as follows:
● Both OS servers save the date and time of day of the failure.
● Both OS servers report the failure with a process control message in the message list.
● If the disrupted OS server is a master, the master/standby identification changes.
During the connection failure no online synchronization for alarm logging, operation messages
and user archives can be performed between the two OS servers. As soon as the connection
is restored, this is remedied by following actions:
● Both OS servers save the date and time of day of the restored connection.
● Both OS servers report the return with a process control message in the message list.
● Data from the alarm logging, tag logging and the user archives accumulated during the
connection failure are transmitted to the returning OS server.
● The @RM_MASTER and @RM_MASTER_NAME tags remain unchanged in both servers.
Disrupted network connection between the OS client the OS server
An OS server and the OS client connected to it are processing an activated WinCC project. A
redundant OS partner server has been configured for the OS server in WinCC Redundancy.
The OS server is defined as the preferred server for the OS client. A disrupted network
connection to the OS server may result from a cable break in the spur line from the network
to the OS server. The terminal bus as a whole remains unaffected.
If a connection failure occurs between the OS client and the OS servers, WinCC Redundancy
triggers the following reactions:
● The OS client is not switched over from the failed OS server to its redundant OS partner
server because the redundant OS partner server is also not available.
● When the connection is available again, the OS client automatically switches back to its
preferred server.
Disrupted network connection to the AS
If a fault occurs on the plant bus connection between the OS server and the AS, WinCC
Redundancy reacts as follows:
● The disruption of the plant bus connection is reported to the OS partner server.
● The OS partner server receives the message that the OS server has failed.
● The OS partner server saves the date and the time of day of the OS server failure.
● An OS client is automatically switched over from the failed OS server to its redundant OS
partner server. Condition: The "WinCC client switch in case of a process connection error"
option must be selected in the "Redundancy" dialog box for this.
When the process connection to the OS server is restored, the missing data in the archive of
the OS server is updated by the procedure described below. Condition: The "Synchronization
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High Availability Process Control Systems (V9.0)
Function Manual, 05/2017, A5E39221836-AA