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GFK-1527A 1-1
Introduction
This chapter introduces the method of CPU Redundancy for the Series 90-70 Programmable Logic
Controller, which is referred to as Enhanced Hot Standby CPU Redundancy. The contents of this
chapter describe:
Enhanced Hot Standby CPU Redundancy
Components of the Enhanced Hot Standby Redundancy System
Control Strategies
Basic Enhanced Hot Standby Operation
Basic CPU Redundancy Systems
Definition of Terms
Active Unit
The unit that is currently controlling the process.
Backup Unit
That unit that is synchronized with the active unit and able to take over the process.
CPU Redundancy
A system with two PLC CPU units cooperating to control the same process.
Critical Component
A component whose failure causes the PLC (either active or backup) where it resides to stop.
Hot Standby
A feature of Genius devices whereby the device prefers output data from the Bus Controller at Serial Bus
Address 31. When outputs from that Bus Controller are not available, the device takes output data from the
Bus Controller at Serial Bus Address 30. If outputs from neither Controller are available, the device places its
outputs in the designated default state.
Primary Unit
The unit in which the externally redundant Bus Controllers' Serial Bus Address is 31.
Redundancy
The use of multiple elements controlling the same process to provide alternate functional channels in case of
failure.
Secondary Unit
The unit in which the externally redundant Bus Controllers' Serial Bus Address is 30.
Synchronized
A unit is considered to be synchronized when it has received the latest status information from the Active unit
and is running the PLC program in parallel.
Dual Bus
The use of two Genius busses to control the same I/O devices. The busses are linked to the I/O devices by one
or more Bus Switching Modules (BSMs). A BSM will automatically switch to the other bus if the active bus
has a failure.
Local System
(LEDs on RCM) - The system where the RCM resides. LEDs indicate whether the local unit is ready to
become the active unit or is the active unit in a redundancy system.
Remote System
(LEDs on RCM) - The system to which the RCM is connected via the communications cable. LEDs indicate
whether the remote unit is ready to become the active unit or is the active unit in a redundancy system.
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Chapter