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4-24 Series 90™-70 Enhanced Hot Standby CPU Redundancy User's Guide
–
May 2000 GFK-1527A
4
Ethernet Global Data in a Redundancy CPU
Ethernet Global Data is enhanced to provide optimal use with Redundancy CPUs. Configuration
of Ethernet Global Data requires the use of Control Programming software, release 2.1 or later.
Ethernet Global Data Consumption
Either or both of the PLC units in a synchronized system can consume Ethernet Global Data.
Consumption by individual units requires separate Ethernet Global Data configurations for the two
units and therefore separate folders. If an exchange should be consumed by both units in a
redundant system, the exchange must be multicast and the exchange must be configured to be
consumed in each of the two units.
A single folder may be used for Ethernet Global Data configuration if there are no exchanges
consumed or produced only by one of the two units.
Consumption of configured Ethernet Global Data exchanges occurs in RUN mode regardless of the
Active/Backup state of the CPU and regardless of whether or not the units are synchronized.
The consumption of the Ethernet Global Data exchanges occurs independently on the two CPUs
even when the same exchange is consumed in both units. The Ethernet modules obtain a copy of
multicast exchanges at the same time, but polling of the exchange in the two CPUs may be phased
by one or more sweeps. This can result in the two units seeing different values for the same
exchange in a given sweep.
For example, an exchange might be consumed by the CPUs at a rate of 500ms. If the CPUs had a
sweep time of 100ms, the same exchange might be seen 400ms later in one CPU than in the other.
It may or may not be from the same exchange produced by the host.
Example
The diagram below shows an example with a sweep time of 100ms and an exchange that is
produced every 300ms and consumed every 500ms.
CPU Sweeps
Exchange Production from Host
Consumption by
CPU
A
Consumption by CPU B
XX
X
XX
X
X
X
X
If data from the exchanges must be seen identically on the two units, the reference data for the
exchanges can be transferred from the active unit to the backup unit during the input data transfer.
That transfer occurs shortly after the Ethernet Global Data consumption portion of the CPU sweep.
Exchange variables transferred must be placed into %I or %AI memory to participate in the input
data transfer.