Siemens SIMATIC PCS 7 Function Manual page 197

Example configuration
Startup of an OS server pair
The following applies, in general: An OS server pair consists of the OS server and its OS
partner server. The two PCs are configured with WinCC Redundancy in a redundant grouping.
When the OS server pair starts up, WinCC Redundancy first checks which of the two OS
servers is to be assigned the master identification. This depends on which OS server starts
up first.
● If one OS partner server is active already when the other comes online, the second OS
server receives the standby identification.
● If no other OS server is active when an OS server starts up, it is assigned the master
identification.
The internal WinCC tag @RM_MASTER is set to identify the master OS server. The internal
WinCC tag @RM_MASTER is reset to identify the standby OS server.
The "@RM_MASTER_NAME" tag contains the name of the OS server, for example, "Server
1". You can display this tag, for example, in an I/O field of a Graphics Designer picture. Other
applications or scripts can also evaluate these tags. The "@RM_MASTER" tag can also be
changed.
Fault-tolerant Process Control Systems (V8.1)
Function Manual, 11/2014, A5E34878832-AA
Failure, Switchover and Return of Fault-tolerant Components
6.4 OS server
197