Siemens SIMATIC S7-400 Operating Instructions Manual page 51

The CP supports the following methods of time-of-day synchronization:
● SIMATIC mode
The SIMATIC mode cannot be configured.
If you leave the time-of-day synchronization via NTP disabled, the SIMATIC mode is
enabled automatically on the CP. In this case, the CP can only adopt the time of day from
the station.
Note that with the CP as OPC UA server when using the SIMATIC mode the time is not
displayed as UTC but as the local time of the station.
● NTP
If the option is enabled and the Security functions disabled, the CP synchronizes its time
of day via NTP.
When NTP is enabled, the CP as OPC UA server displays the time of day in the UTC
format.
You configure the synchronization method, the local time zone of the station, the
synchronization interval and the addresses of the NTP servers.
● NTP (secure)
If the security functions are enabled, the time of day can be synchronized via
NTP (secure).
The secure method NTP (secure) uses authentication with symmetrical keys according to
the hash algorithms MD5 or SHA-1.
NAT configuration: Unsynchronized NTP server
If the "Accept time of day from non-synchronized NTP servers" option is enabled, the CP
also accepts the time from NTP servers that are not synchronized with a stratum 16.
If the option is disabled, the response is as follows:
If the CP receives a time of day frame from an unsynchronized NTP server with stratum 16,
the time of day is not set according to the frame. In this case, none of the NTP servers is
displayed as "NTP master" in the diagnostics; but rather only as being "reachable".
Setting the SYA bit when synchronizing via NTP
If the time is forwarded to the CPU, with each time frame the CP sets bit 0 of the time status
word (SZL-ID 132, index 8). The status bit indicates on the CPU whether the time is
synchronized with the external time master.
If you evaluate the time status using SFC 51 "RDSYSST", configure a synchronization cycle
shorter than 3 minutes. This avoids the status bit being reset cyclically by the CPU after 3
minutes if NTP servers are unreachable or not synchronized.
CP 443-1 OPC UA
Operating Instructions, 01/2017, C79000-G8976-C427-02
Configuration and operation
4.7 Configuration of the CP in STEP 7 V5
51