GFK-1527A Chapter 1 Introduction 1-9
1
Basic Enhanced Hot Standby Operation
In an Enhanced Hot Standby CPU Redundant system, Genius I/O Blocks are normally configured
for Hot Standby operation. Genius I/O Blocks can also be configured for the less frequently used
Duplex operation, but only with the GDB Control Strategy. When configured for Hot Standby
operation, the blocks must choose between outputs from the Genius Bus Controller at serial bus
address 31 and the Genius Bus Controller at serial bus address 30. If outputs from both Genius Bus
Controllers are available, then the blocks will prefer the outputs from bus address 31. If there are
no outputs from bus address 31 for three consecutive Genius I/O bus scans, the blocks will use the
outputs from bus a ddress 30. If outp u ts are not availabl e from either bus ad dress 31 or 30, the
outputs go to their configured default (OFF or hold last state).
For Hot Standby CPU Redundant systems, the Genius Bus Controllers in the Primary Unit are
normally configured at serial bus address 31 and the Genius Bus Controllers in the Secondary Unit
are normally configured at serial bus address 30.
It is possible to configure Genius I/O networks in which there is not a redundant bus controller in
the synchronized PLC. It is not necessary for the serial bus addresses to be 31 in the Primary unit
and 30 in the secondary for such networks.
In an Enhanced Hot Standby CPU Redundancy system, only the active unit may control the
redundant Genius outputs. This is accomplished differently in the two control strategies:

Output Control with GHS

In the GHS control strategy, the PLC CPU allows only the active unit to control the outputs. When
the Primary Unit is active (GBCs at bus address 31), the PLC CPU allows both units to send
outputs to the blocks. The result is a bumpless switchover if the Primary Unit fails while it is the
active unit.
If the Secondary Unit is active, the PLC CPU automatically disables outputs from the redundant
GBCs in the Primary Unit. That means the Genius I/O blocks will only receive outputs from the
Secondary Unit (bus controllers at serial bus address 30).

Output Control with GDB

In the GDB control strategy, both the Primary and Secondary Units send outputs regardless of
which one is active. The user is resp on sible for ensuring that all redun d ant outputs are transferred
from the active unit to the backup unit. Because the same output values will then be present in both
units, the blocks will receive the same outputs (regardles s o f wh ether the Primary or the Secondar y
Unit is active). There is no output glitch (data interruption) on switchover since both units are
always sending outputs.