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Using a Quantum 984 HSBY System
106
840 USE 106 00 January 2003
Synchronizing Time-of-Day Clocks
Clock
Synchronization
In a Hot Standby system, the Primary and Standby controllers have their own time-
of-day clocks. They are not synchronized. At switchover, the time of day changes by
the difference between the two clocks. This could cause problems if you are
controlling a time-critical application.
To solve this problem, program the Standby controller to reset its clock from the
state table provided by the Primary controller. If you are controlling your system via
configuration extension screens, put the logic for time synchronization first.
Otherwise, put the logic for time synchronization in segment 1, but do not put it in
network 1.
Since both controllers run the same program, you must read CHS status register bits
12...16 to be sure that only the standby clock is resetting. If bits 12...16 are 01011,
you know three things:
which controller is the Standby
that the remaining controller is the Primary
that both controllers are running the same logic
If these conditions are true, then the logic should clear bit 2 and set bit 1 of the time-
of-day control register. The clock in the Standby controller will be reset from the state
table of the Primary controller at the end of a scan and bit 1 will be cleared.
Note: Be sure that the registers for synchronizing the time-of-day clocks are
included in the state RAM transfer area.