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1-2 Series 90™-70 Enhanced Hot Standby CPU Redundancy User's Guide
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May 2000 GFK-1527A
1
Enhanced Hot Standby CPU Redundancy
CPU Redundancy allows a critical application or process to continue operating if a failure occurs in
any single component. An Enhanced Hot Standby CPU Redundancy system consists of two CPUs
connected to one or more Genius I/O networks. One PLC is the Primary PLC and the other is the
Secondary PLC. The Primary PLC contains all externally redundant Genius Bus Controllers at
Serial Bus Address 31; the Secondary PLC contains all externally redundant Genius Bus
Controllers at Serial Bus Address 30.
Each PLC
must have a Redundancy CPU module (IC697CGR772 or IC697CGR935), a
Redundancy Communications module and a Bus Transmitter Module
. The Redundancy
Communications module provides the synchronizing link between the two units. The scanning
process of both CPUs is synchronized to minimize bumpless switching from one PLC to the other.
The CPU that currently controls the system is called the active unit, the other CPU is the backup
unit. Control automatically switches to the backup unit if certain system failures are detected in the
active unit. Control can also be switched manually by pressing a pushbutton on the Redundancy
Communications Module, or through the application program. When a manual switch of control
occurs, the CPUs switch roles; the active unit becomes the backup unit and the backup unit
becomes active.
The system runs synchronously with a transfer of all control data that defines machine status and
any internal data needed to keep the two CPUs operating in sync. The transfer of data from the
active unit to the standby unit occurs twice per sweep. These CPU to CPU transfers are checked
for data integrity.