Switchover Behavior during Application Mismatch

Modifying the Application Variables

If a switchover occurs during application mismatch, the new Primary CPU executes its own, different application program with the data received from the other controller.

Depending on the modification, different behaviors occur:

Modification	Effect
Only code changed (same variables)	All the variables exchanged between the controllers are equal.
Variables added to the initial Primary CPU	Variables are not used by the new Primary CPU.
Variables deleted from the initial Primary CPU	New Primary CPU executes application program using the latest values for these variables.
Variables added to the initial Standby CPU	New Primary CPU executes application program using initial values for these variables.
Variables deleted from the initial Standby CPU	New Primary CPU does not use these variables

Modifying an SFC Section with Control Expert

Schneider Electric recommends not using the SFC programming language in a Hot Standby application.

NOTE: The SFC programming language is not available for 140 CPU 671 60S Hot Standby applications.

NOTE: Modification of existing SFC actions and transitions have no impact on the SFC execution. A Switchover does not reset the SFC to its initial step.

If SFC is used in a Hot Standby application, system reaction to online modifications depends on the setting of %SW60.3, the logic mismatch bit:

If a mismatch is not allowed, SFC modifications do not cause a problem. When the Primary CPU application changes, the Standby CPU goes to the RUN OFFLINE mode. the Primary CPU application must be transferred to the secondary CPU to move it back to the RUN STANDBY mode.

NOTE: A transfer carried out automatically by the application reduces the time that the Hot Standby function is not available to o a minimum.
If a mismatch is allowed, then:
- An SFC modification may cause a reallocation of the block containing the SFC data. This stops the exchange of this data with the Standby CPU.
- Also, after a Switchover, this SFC restarts at its initial step. This may have an impact on the operation of the Hot Standby application.
- To reduce these impacts, program the SFC in several sections. The modification of one SFC does not the rest of the SFC.

WARNING

UNINTENDED EQUIPMENT OPERATION

Ensure that the controllers contain the same application program during a Switchover.

Failure to follow these instructions can result in death, serious injury, or equipment damage.

If a Switchover occurs in the Run mode and there is an application mismatch between the two controllers, the Standby CPU assumes Primary CPU responsibilities and starts executing a different application program from the previous Primary CPU.

Remove any application mismatch by performing an Application Program Transfer as soon as possible after completing modifications.