Siemens SIMATIC PCS 7 Function Manual page 72

Configuring time synchronization
5.5 Configuring the time synchronization of the OS
receive a time signal on the plant bus, WinCC time synchronization automatically switches to
master mode. It will then send substitute time signals for the failed time master.
If a time jump of more than 5 seconds occurs, the computer defined as the time master
becomes a time slave. The message "Time synchronization has been permanently switched
to slave" is generated.
Note that also after a restart this computer is no longer defined as time master. Reconfigure
the computer as time master and load the configuration on the computer.
Configuring the OS client for time synchronization
To configure OS clients for time synchronization, proceed as follows for each OS client:
1. Open the OS client in the PCS 7 project on the engineering station.
2. Select the "Time synchronization" editor from the tree view of WinCC Explorer.
3. Select Open from the shortcut menu.
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CAUTION
The "Time Synchronization" dialog box opens.
Time synchronization (V8.0)
Function Manual, 03/2012, A5E02779849-02