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Differences in Operation for CPU 780
The following features operate differently with the CPU 780 than they do with other
Series
HRUN/DISABLEDmode
HConfiguration of Fault Actions
HSTOP to RUN mode transition
HBackground Window Time (default is different)
RUN Disabled Mode
RUN/DISABLEDmode causes all physical outputs to go to their default state in that PLC. Inputs are still scanned and logic is solved. A CPU in RUN/DISABLED mode may be the active unit.
There are several guidelines that you should be aware of regarding the use of the
RUN/DISABLEDmode.
1.If a unit is in RUN/DISABLED mode, its LOC_RDY %S reference and the remote unit's REM_RDY %S reference will not be set and the corresponding LEDs on the RCMs will not be ON. This indicates that the unit (with LOC_RDY reference off) is not available to drive outputs.
2.You cannot command a role switch from an active unit that is in RUN/ENABLED mode to a unit that is in RUN/DISABLED mode. The RCM role switch pushbuttons and SVCREQ #26 will be ignored if a role switch is attempted in this situation.
3.If the units are transitioned when the primary unit is active with outputs disabled and the secondary unit is backup with outputs enabled, the primary unit will continue to solve logic and transfer outputs to the backup, and the backup unit will drive the transferred outputs.
4.If units are transitioned in any manner where the secondary unit is active with outputs disabled and the primary unit is backup with outputs enabled, then the units will automatically role switch so that the primary unit becomes active in RUN/ENABLEDmode.
5.If a unit is in RUN/ENABLED and the other unit is in RUN/DISABLED, the unit in RUN/ENABLED will not use its synchronized fault action table. Instead, it will use the
Note
If the backup unit is in RUN/DISABLED mode, then the backup unit will continue to NOT drive outputs upon failure of the active unit and therefore is not a true backup.
Chapter 4 Operation | 65 |