Siemens SIMATIC PCS 7 Function Manual page 19

Time synchronization using strata
The following figure shows an example of time synchronization using strata:
The above example shows time synchronization with the following structure:
● Stratum 0
● Stratum 1
● Stratum 2
● Stratum 3
● Stratum 4
Meaning of the strata for a PCS 7 plant
The higher the stratum level, the higher the potential time difference in relation to the original
time signal source. As time forwarding in a network also takes time, the time of lower strata
time masters is more precise than the time of higher strata time masters.
Time synchronization (V8.0)
Function Manual, 03/2012, A5E02779849-02
The time source is an atomic clock. This atomic clock synchronizes the GPS satellites or
the DCF 77 senders, for example.
The central plant clock (SICLOCK) receives the time via a radio signal (e.g. GPS decoder
or DCF 77 receiver). SICLOCK transfers the time to domain controller 1 in stratum 2.
Domain controller 1 is the time slave of the central plant clock. Domain controller 1 transfers
the time to several domain controllers in stratum 3. Domain controller 1 is the time master
of the domain controllers in stratum 3.
The domain controllers in stratum 3 synchronize the time of their own process units in
stratum 4. The domain controllers in stratum 3 are the time masters of their own process
units.
The process unit components are time slaves.
2.6 Time levels for a PCS 7 plant (stratum)
Fundamentals
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