GE PACSystems RX7i, User Manual

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Chapter 5. Ethernet Global Data
GFK-2224Q January 2017 77
Timing Example 2
More than one sample can be produced per producer period expiration and stale samples are produced to the
network.
Producer Period = 2/3 Time of CPU Sweep
Producer PLC Output Scan
Ethernet Global Data
Production Timer Expires
Sample on Network
Stale Data is Produced
Figure 39: EGB Timing Example #2
5.5
Time-Stamping of Ethernet Global Data Exchanges
The CPU adds a timestamp to each Ethernet Global Data Message it produces. The timestamp indicates when
the data was transferred from the producing PLC's CPU to its Ethernet interface for transmission over the
network.
The timestamp is an 8-byte value representing the time elapsed since midnight, January 1, 1970. The first four
bytes contain a signed integer representing seconds and the next four bytes contain a signed integer
representing nanoseconds. This value can be examined to determine whether a packet received from the
network has a new data sample or if it is the same data received previously.
In its default operating mode for SNTP synchronization, the PLC CPU obtains the timestamp data from the time
clock in the Ethernet interface, which can be synchronized to either the clock in the CPU or an external SNTP
server on the network. For details on SNTP operation, see page 86.
Alternatively, the timestamp data can be obtained from the CPU TOD clock when the CPU TOD clock is
synchronized with an SNTP server. Synchronizing the CPU TOD clock to an SNTP server allows you to set a
consistent PLC time across multiple systems. This operating mode must be configured by an Advanced User
Parameter and enabled from the application logic. For additional information, see “Obtaining Timestamps from
the CPU TOD Clock” on page 79.