Simple network time protocol (sntp), Timestamping egd exchanges, Configuring an ethernet interface for sntp – FANUC Robotics America GFK-1541B User Manual
Page 174: Normal sntp operation
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Simple Network Time Protocol (SNTP)
SNTP is a method of synchronizing the internal clocks in a network's Ethernet Interfaces to an
external source. In an SNTP system, a computer on the network (called an SNTP server), sends
out a periodic timing message to all of the SNTP-capable Ethernet Interfaces on the network,
which keep their internal clocks synchronized with this common SNTP timing message.
Timestamping EGD Exchanges
Each EGD exchange contains a timestamp that pinpoints when the associated data was transferred
from the producing PLC's CPU to its Ethernet Interface for transmission over the network. The
timestamp value is based on a built-in clock in the Ethernet Interface. It sends this value to the
PLC CPU, which uses it to timestamp its data exchanges. Its time clock is synchronized to one of
the following two sources:
PLC's Time Clock: If the Network Time Sync parameter of the Ethernet Interface is set to
None or DISABLED, the Ethernet Interface’s built-in time clock is synchronized once, at
power-up or restart, to the clock in the local PLC CPU. The disadvantage of this source is
that the time clocks in the various PLCs on the network are not synchronized, so timestamps
produced by different PLCs cannot be compared accurately.
SNTP Server's Time Clock: If the Ethernet Interface's Network Time Sync parameter is set
to SNTP or ENABLED, the Ethernet Interface’s built-in clock is periodically synchronized to
the clock on the SNTP server. This method has the advantage that all Ethernet Interfaces
configured to use SNTP will have updated, synchronized timestamps, because they will all be
synchronized to the SNTP server's clock. Therefore, accurate timing comparisons between
exchanged data can be made. For example, if alarm data were sent over the network from
several PLCs, it might be advantageous for troubleshooting purposes to know the order in
which the alarms were generated. Using SNTP, the data's synchronized timestamps could
help you determine this.
Notes
With SNTP operation, the time value passed from the Ethernet Interface to the
PLC CPU is used in the PLC only for timestamping EGD exchanges. It does
not affect the time of the CPU's internal time clock.
SNTP server dates before January 1, 1989 are not supported by the Ethernet
Interface.
Configuring an Ethernet Interface for SNTP
SNTP operation must be configured for each Ethernet Interface individually. For more
information for your particular Ethernet Interface, refer to the installation chapter.
Normal SNTP Operation
The Ethernet Interface will synchronize to a remote SNTP time server after receiving two
broadcast clock values within a 150-second period. The Station Manager can be used to view
server status information.
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