4-24 Series 90™-70 Enhanced Hot Standby CPU Redundancy User's Guide – May 2000 GFK-1527A
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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 synchr onized system can consume Ethern et Global Data.
Consumption by individual units requires separate Ethernet Global Data configurations for the two
units and therefore separat e folders. If an exchange should be consum ed by both units in a
redundant system, the exchan ge must be multicast and the exchang e mu st be configured to be
consumed in each of the two units.
A single folder may be used for Eth ern et Global Data configuration if th ere are no exchanges
consumed or prod u ced only by one of th e 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 indep endently on the two CPUs
even when the same exchange is con s u m ed in both units. The Ethernet modules obta in 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 swe ep.
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 exchan g e pr oduced 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.