Multiple sntp servers, Loss or absence of sntp timing signals – FANUC Robotics America GFK-1541B User Manual
Page 175
ries 90™ PLC User's Manual
–
May 2002
GFK-1541B
5
Multiple SNTP Servers
To guard against loss of SNTP timing messages, multiple SNTP time servers can be tracked on a
network. An Ethernet Interface can maintain timing information from up to four total SNTP time
servers at a time. Each server assigns a stratum number that determines its priority. The message
from the server with the lowest stratum number is used by the Ethernet Interface until
communication with that server is lost. Then the server with the next lowest stratum number
becomes the server of choice and the Ethernet Interface synchronizes to it if it receives two of its
timing messages within a 150-second period. A server is considered "lost" if more than 150
seconds elapse between timing messages.
The Station Manager command, CHSNTP, can be used to override the Ethernet Interface’s
automatic server selection strategy and identify specific server(s) to be used. For more
information on using the Station Manager, refer to GFK-1186, TCP/IP Ethernet Communications
for the Series 90 PLC Station Manager Manual
.
Loss or Absence of SNTP Timing Signals
If an Ethernet Interface is configured for SNTP, but does not receive two timing messages from
an SNTP network time server within a 150-second period, the following will happen:
A fault entry will be placed in the PLC Fault Table.
A fault entry will be placed in the Ethernet Interface's exception log. This log can be read by
using the GE Fanuc Station Manager. Details can be found in GFK-1186, TCP/IP Ethernet
Communications for the Series 90 PLC Station Manager Manual
.
The Status word within a consumed exchange will indicate new data with a value of 3,
instead of the normal 1 value, indicating that SNTP is selected, but the Ethernet Interface is
not synchronized to an SNTP server. This Status word value can be obtained from the PLC
register configured for the particular exchange.
Note
The SNTP error condition is considered the least important of all possible error
codes. Therefore, if another error condition exists, its status code will appear in
the Status word instead of the SNTP error code.
Upon loss or absence of synchronization, the Ethernet Interface’s built-in clock will operate as
follows:
If the Ethernet Interface, after its last power-up/restart cycle, has never received an SNTP
server's timing message, it will continue to use the PLC CPU's local clock value that it
received at power-up/restart for its time base.
If the Ethernet Interface has, at some point, been synchronized to an SNTP server but lost its
signal, it will use the most recently received SNTP time message as its time base.
The Ethernet Interface will continue supplying time values to the PLC CPU for timestamping,
while it "listens" for SNTP timing messages from the network. If SNTP messages are received
later, the Ethernet Interface will then synchronize to them.
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