Siemens SIMATIC PCS 7 Configuration Manual page 216

Configuring OS functions
10.1 Project settings
Settings in the "Time Synchronization" editor in the OS server project
Time synchronization of the plant bus is performed via the OS server and the redundant OS
partner server with the aid of the CP1613/CP1623/CP1628.
Note
If the ES does not have a CP1613/CP1623/CP1628 communication processor (as of PCS 7
V7.0 SP2), select the "Display symbolic name of the access points" option in the area
"Synchronization via Plant Bus (Master, Slave)". Select the communication processor
(CP1613/CP1623/1628) from the "Access point 1" drop-down list and select the "Master" check
box.
The OS servers act as cooperative time masters, in other words, the first OS server active on
the plant bus to not receive a time frame on the plant bus switches automatically to time
"master" mode. All other OS servers on the plant bus detect the time message frame from the
time master and automatically switch to time "slave" mode. A cooperative master does not
have to belong to a redundant server.
Select the following parameters in the OS server project:
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Compendium Part A - Configuration Guidelines (V8.2)
Configuration Manual, 08/2016, A5E38166869-AA