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71GFK-0827 Chapter 4 Operation
Genius Bus Controller Switching
Genius Bus Controllers will stop sending outputs to blocks when no data has been
received from the PLC CPU for a period equal to two times the configured watchdog
timeout.
If the CPU on the active unit becomes inoperative in an uncontrolled fashion (for
example, because of a power failure), the Genius Bus Controllers will detect this within
twice the watchdog setting, and stop sending outputs to the Genius blocks. After three
Genius I/O bus scans of not receiving data from the Genius Bus Controllers, the Genius
blocks will start driving data from Serial bus Address 30 (that is, the backup unit). If no
data is available from SBA 30, the blocks revert to default or hold last state (as
configured).
For example, if the system has a 200 ms watchdog timeout and 5 ms Genius bus scan
time, then if the active unit loses power, the Genius Bus Controllers on expansion racks
will wait 400 ms and then stop updating outputs on Genius blocks. 15 ms later, the
blocks will begin driving outputs based on data from the backup unit. Note that any
Genius Bus Controllers in the main rack would stop driving outputs immediately since
they would also lose power. Genius blocks on these busses would begin driving data
from the backup unit within 15 ms.
Note
For fastest switching, all Genius Bus Controllers in the Hot Standby CPU
Redundancy system should be in the main rack, or in a rack driven by
the main rack’s power supply. This will cause the Genius Bus Controller
to lose power at the same time that the CPU loses power and allow the
backup unit to gain full control of the I/O as soon as possible.