Chapter 5. Ethernet Global Data78 PACSystems* RX7i & RX3i TCP/IP Ethernet Communications User Manual GFK-2224QObtaining Timestamps from the Ethernet Interface ClockIn this operating mode, the PLC CPU obtainsthe timestamp data from the time clock inthe Ethernet interface. The CPU only usesthis timestamp for Ethernet Global Dataexchanges. The timestamp from theEthernet interface does not affect the timeof the CPU's internal time clock.If time synchronization between the CPUand ETM is lost, as when the CHTIME StationManager command is used to change theETM time, the CPU uses its own clock for thetime stamp.CPU EthernetinterfaceCPUtimeclocktimestamp timeclockEGD withtimestampFigure 40: Obtaining Timestamps from the Ethernet InterfaceClockThe time clock in the Ethernet Interface is synchronized to either the clock in the CPU or an external SNTP serveron the network. Selection of the timestamp source for Ethernet Global Data is part of the basic configuration ofthe Ethernet Interface, as explained in Chapter 4.PLC's Time Clock: If this source is selected, the EthernetInterface’s built-in time clock is synchronized at power-up or at restart to the clock in the PLC CPU. Thetimestamp information produced by the PLC has aresolution of 100 microseconds. Because the timeclocks in the PLCs on the network are not synchronized,EGD timestamps produced by different PLCs cannot becompared accurately.CPU EthernetinterfacetimeclocktimestampCPU TimeCPUtimeclockFigure 41: Obtaining Timestamps from the PLCTime ClockSNTP Server's Time Clock: If this source is selected, the Ethernet Interface’s built-in clock is periodicallysynchronized to the clock on an SNTP server on the network. All Ethernet Interfaces configured to use SNTP willhave updated, synchronized timestamps. Therefore, accurate timing comparisons between exchanged datacan 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.
