4. Configuration
49
ATTENTION
If the PLC receives a time sync command from the control center, and this option is disabled, an
error answer will be returned to that command. But if this option is enabled then a success message
will be returned to the control center, even that the sync command be discarded for there be another
synchronism method active with higher priority.
This synchronism method should be used only as an auxiliary synchronism method, once the
precision of the clock sync process depends a lot on delays and traffic on the network, as well as the
processor load on the CPU, as this mechanism is treated by a low priority task.
ATTENTION
In redundants PLCs architectures, the IEC60870-5-104 Server driver is disabled on non active PLC.
This way, isn’t recommended the use of this synchronism method in redundant systems. Because the
non active PLC might take several seconds after the switchover until its clock is synchronized. To
redundant systems it is recommended the use of SNTP.
SNTP
For that, the CPU will behave as a SNTP client, which is, it will send requests of time
synchronization to a SNTP/NTP server which can be in the local net or in the internet. The SNTP
client works with a 1 ms resolution, 100 ms precision. The precision of the time sync through SNTP
depends on the protocol configurations (minimum error to clock update) and the features of the
Ethernet network where it is, if both client and server are in the same network (local) or in different
networks (remote). Typicallythe precision is in tens of milliseconds order.
The CPU sends the cyclic synchronization requests according to the time set in the SNTP
Synchronization Period field. In the first synchronization attempt, just after the service start up, the
request is for the first server set in the first server IP address. In case it does not respond, the requests
are directed to the second server set in the second server IP address providing a redundancy of SNTP
servers. In case the second server does not respond either, the same process of synchronization
attempt is performed again but only after the Period of Synchronization having been passed. In other
words, at every synchronization period the CPU tries to connect once in each server, it tries the
second server in case the first one does not respond. The waiting time for a response from the SNTP
server is defined by default in 5 s and it cannot be modified.
If, after a synchronization, the difference between the current time of the CPU and the one received
by the server is higher than the value set in the Minimum Error Before Clock Update parameter, the
CPU time is updated.
SNTP uses the time in the UTC (Universal Time Coordinated) format, so the Time zone parameter
needs to be set correctly so the time read by the SNTP will be properly converted to a local time.
The execution process of the SNTP client can be exemplified with the following steps:
1. Attempt of synchronization through the first server. In case the synchronization occurs
successfully, the CPU waits the time for a new synchronization (Synchronization Period) and
will synchronize again with this server, using it as a primary server. In case of failure (the server
does not respond in less than 5 s) step 2 is performed.
2. Attempt of synchronization through the second server. In case the synchronization occurs
successfully, the CPU waits the time for a new synchronization (Synchronization Period) and
will try to synchronize with this server using the primary server. In case of failure (the server
does not respond in less than 5 s) the time relative to the Synchronization Period is waited and
step 1is performed again.
As the waiting time for the response of the SNTP server is 5 s, the user must pay attention to lower
than 10 s values for the Synchronization Period. In case the primary server does not respond, the time
for the synchronization will be the minimum of 5 s (waiting for the primary server response and the
synchronization attempt with secondary server). In case neither the primary server nor the secondary
To Next Page
Loading...
General