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Using a Quantum IEC Hot Standby System
840 USE 106 00 January 2003 145
Synchronizing Time of Day Clocks
Primary and
Secondary
controller time-
of-day clocks
In a Hot Standby system, although the Primary and Secondary controllers have their
own time-of-day clocks, they are not implicitly 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.
Assign the time-of-day clock eight 4x registers in the Specials dialog of the
configurator. Be sure that none of these 4x registers resides in the nontransfer area,
all of them need to be transferred to the Standby controller after each scan. Then
use somewhere in the IEC logic the ‘SET_TOD’ EFB, which resides in the system
library under the HSBY group.
Elementary
Function Block
(EFB) to set the
PLC’s time-of-
day clock
While the full IEC Hot Standby system is running, meaning the Standby controller is
also online, your application logic should trigger (rising edge of the S_PULSE input)
the EFB. This would then not only set the time-of-day clock in the Primary, but the
one in the Standby as well, at the same time. The trigger on the clocks might again
run at slightly different speeds, this time-set process should be repeated
periodically, for example within a period of 1 minute.