GE PACSystems RX7i, User Manual

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Chapter 5. Ethernet Global Data
78 PACSystems* RX7i & RX3i TCP/IP Ethernet Communications User Manual GFK-2224Q
Obtaining Timestamps from the Ethernet Interface Clock
In this operating mode, the PLC CPU obtains
the timestamp data from the time clock in
the Ethernet interface. The CPU only uses
this timestamp for Ethernet Global Data
exchanges. The timestamp from the
Ethernet interface does not affect the time
of the CPU's internal time clock.
If time synchronization between the CPU
and ETM is lost, as when the CHTIME Station
Manager command is used to change the
ETM time, the CPU uses its own clock for the
time stamp.
CPU Ethernet
interface
CPU
time
clock
timestamp
time
clock
EGD with
timestamp
Figure 40: Obtaining Timestamps from the Ethernet Interface
Clock
The time clock in the Ethernet Interface is synchronized to either the clock in the CPU or an external SNTP server
on the network. Selection of the timestamp source for Ethernet Global Data is part of the basic configuration of
the Ethernet Interface, as explained in Chapter 4.
PLC's Time Clock : If this source is selected, the Ethernet
Interface’s built-in time clock is synchronized at power-
up or at restart to the clock in the PLC CPU. The
timestamp information produced by the PLC has a
resolution of 100 microseconds. Because the time
clocks in the PLCs on the network are not synchronized,
EGD timestamps produced by different PLCs cannot be
compared accurately.
CPU Ethernet
interface
time
clock
timestamp
CPU Time
CPU
time
clock
Figure 41: Obtaining Timestamps from the PLC
Time Clock
SNTP Server's Time Clock: If this source is selected, the Ethernet Interface’s built-in clock is periodically
synchronized to the clock on an SNTP server on the network. All Ethernet Interfaces configured to use SNTP will
have updated, synchronized timestamps. Therefore, accurate timing comparisons between exchanged data
can be made. If SNTP is used to perform network time synchronization, the timestamp information typically has
±10 millisecond accuracy between PLCs on the same network.