7 Time Synchronization
7.3 External Time Synchronization via Ethernet NTP
131
SICAM I/O Unit, 7XV5673, Device Manual
E50417-H1040-C484-A3, Edition 05.2012
7.3
External Time Synchronization via Ethernet NTP
General
To synchronize the time via an external source, SICAM I/O Unit is equipped with an SNTP client (SNTP = Sim-
ple Network Time Protocol) that can be connected to 2 NTP servers (NTP = Network Time Protocol), the pri-
mary and the secondary (redundant) NTP server.
The chapter 6.3.3.5 describes how to set the parameters of the two servers.
NTP is used for external time synchronization via Ethernet. The SNTP client sends a time request to the NTP
server once a minute. The time synchronization error is ±5 ms referred to UTC time of the NTP server.
The time stamp of the NTP server has a 64-bit format. Counting is accomplished in seconds and fractions of
seconds.
Time Synchronization Procedure
The device was set to external time synchronization (
Ethernet NTP
) during the parameterization. After
switching on or resetting the device, the FAIL bit is first set to "1" (=invalid) and the device sends a time request
to the NTP server. After receiving the time information from the NTP server via Ethernet, the FAIL bit is set to
"0" (=valid) and the internal timer RTC (Real-Time Clock) is updated. The SNTP client repeats the time request
to the NTP server cyclically once every minute.
If the primary NTP server fails (for example, no response to a request twice or one of the criteria at "Redundant
NTP server" satisfied) and if the secondary NTP server is operational (always polled in parallel), the device
switches to the secondary NTP server. The FAIL bit remains = 0. In this case, the operational indication
"Primary NTP Server Error" is displayed, see chapter 12.
If the secondary NTP server is also invalid, the FAIL bit will be set to 1 after the programmable timer
Error in-
dication after
(see Figure 6-31) has expired, and the "Clock Error" indication is output.
Redundant NTP Servers
The time synchronization supports a primary and a secondary NTP server. Different IP addresses are set for
the two NTP servers, see chapter 6.3.3.5.
SICAM I/O Unit cyclically polls both NTP servers once every minute, but during normal operation it is synchro-
nized by the primary NTP server. The device automatically switches to the secondary NTP server if one of the
following criteria are met:
•
No response from the primary NTP server to two successive requests
•
The "Alarm" indication is set in the time information of the primary NTP server.
•
The primary NTP server responds with zero.
•
The message runtime in the network is > 5 ms.
•
The stratum of the primary NTP server is 0 (unknown) or > 3.
Switching to the secondary NTP server is prevented if:
•
The secondary server does not provide better time information (see criteria that initiate the switch from
primary to secondary NTP server; "Secondary NTP Server Error" indication was already output) or
•
The secondary server has recently been available for less than 10 minutes.
In these cases, SICAM I/O Unit is not synchronized anymore. The device uses the internal clock (on millisec-
onds time basis) and the last valid drift. After the programmable delay time, the device reports "Clock Error",
see chapter 12.
NOTE
The time format is described in detail in the RFC 5905 (Request for Comments 5905 for NTP).
MAN_SICAM_IO-Unit_7XV5673_US.book Page 131 Thursday, May 10, 2012 10:59 AM
