GFK-1527A Chapter 4 Normal Operation 4-25
4
Ethernet Global Data Production
When the two units of a CPU Redundancy system are synchronized, Ethernet Global Data
exchanges are produced onl y by the a ctive unit. This reduces the amount of t r a ffic on the Ethernet
network and simplifies the han dling of the exchange by the consum er . In part icular, the consumer
is able to consume the exchanges in th e same way as for exchanges from non-r edundant systems.
If the exchanges are to be produced b y both units, the units must be configur ed to have the same
producer ID. This way, the consumer does not need to know which unit is producing the
exchanges. The configuration of unique production exchanges for the two units is not
recommended since the exchan g es w ou l d only be produced when the unit was acti ve an d not when
it was backup.
If a unit stops bei n g the active unit , i t stops producin g Et h ernet Global Da t a exchanges so that the
other unit can start producing the EGD exchanges. The new active unit, if any, delays starting
production of E th ernet Global Da t a ex changes long en ou gh to let the other un it stop producin g.
This is necessary so that both units are not producing Ethernet Global Data exchanges at the same
time. That could become confusing to the consumer.
The following formula gives the maximum time after a unit becomes the active unit before it starts
producing a given Ethernet Globa l Data exchange. Note that in certain fa ilure conditions, it may
take up to 1 sweep + 2 failwait timeouts for the backup unit to detect the failure of the active unit
so that it can become the active unit.
Software Watchdog Timeout +
1 Network Production Period for the Exchange +
2 CPU Sweeps +
220 ms
If both communications links between the Redundancy Communications Modules and Bus
Transmitter Modules fail, both units are marked as Active Units and attempt to produce Ethernet
Global Data exchanges. If the application cannot tolerate this situation, then it must detect that
both units are a c t i ve and shut down on e of t he units with a ser vice request fun ction block or oth er
means. The program logic can detect this by sending a running counter fr om one unit to the other
via discrete I/O modules or other means and then checking if the counter still increments after both
links have been lost.
If outputs are disabled on the active unit, neither unit produces Ethernet Global Data.
SNTP Timestamping
Ethernet Global Data exchang es can be timestamped using either th e PLC CPU's local clock or
using a Simple Network Time Protocol (SNTP) clock from a user-provided server on the Ethernet
network. SNTP clock timestamping for a given Ethernet Global Data exchange is selected by
enabling timestamp synchronization in the configuration of the corresponding Ethernet module. If
timestamp synchronization is disabled for a given Ethernet module, then Ethernet Global Data
exchanges produced by that mod u l e are timestamped with the PLC CPU's l oca l cl ock.