Siemens SIMATIC CPU 410 System Manual page 152

Time synchronization and time stamping
CPU 410 as time master
If you configure the CPU 410 as the time master, you must specify a synchronization
interval. You can select any interval between 1 s and 24 h.
Select a synchronization interval of 10 s if the CPU 410 is the time master on the S7-400
backplane bus.
The time master does not send time frames until you have set the time. You can set the time
as a time slave (NTP client/slave) with Step 7 or an interface.
Definition of time stamping
Time stamping refers to the assignment of an event to its acquisition time. The more precise
this assignment is, the more precisely the acquisition time corresponds to the event.
For time stamping, the IO controller/DP master sends its time of day to the local IO
subsystem/DP line.
The IO device/DP slave receives this time of day and uses the time information for time
stamping.
This time stamping is referred to as "high-precision time stamping" in the context of SIMATIC
PCS 7.
Example:
In the ET 200SP HA, signal changes can be acquired and time stamped with a precision of 1
ms.
Relationship between time synchronization and time stamping
To examine the chronological relationship between time stamped events from different S7
stations, the S7 stations must be time synchronized.
The synchronism among the individual systems is dependent on the selected time
synchronization method, the topology and the utilized interface in the S7 station.
Precision
The precision of the time stamping is the maximum difference of the time stamps that result
from signals that were recorded simultaneously by digital input modules.
The precision depends on the hardware used and the configuration of the plant.
Resolution
The resolution is the smallest possible time difference between two different time stamps.
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CPU 410 Process Automation/CPU 410 SMART
System Manual, 05/2017, A5E31622160-AC