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on a M580 Hotstandby system, why the application cannot be transferred from Primary to Standby?

The M580 redundant system is made in a way that data related to tasks are exchanged between the Primary controller to the Standby at every cycle.

Synchronized situation :
At each cycle:
-          The Primary CPU “packs” the data in preparation to the transfer
-          The Primary CPU then starts to execute the application code of the task, and in parallel the data is being transferred to the Standby
-          Once the data are received, the Standby unpacks the data, and start executing its local code (depending on the configuration)
All of this should happen within the configured task (Mast of Fast) period in Unity Pro.

In this situation, simultaneous spare time exist in both CPUs at the end of the cycle, and an application transfer (low priority) can be managed by the system.

Not synchronized situation :
Depending on the tasks (mast of fast) execution duration, on the data transfer duration, and on the associated chosen periods, it is possible that the duration of the data transfer exceeds the task period, even if the primary tasks durations are lower than the period settings.
in this case, the primary CPU will not receive the data transfer acknowledge from the standby CPU, => the status of the standby task will be “not synchronized”
In this situation, no simultaneous spare time exist in both CPU at the end of the tasks, and no application transfer can be managed by the system.
In order not to jeopardize the system, the Standby CPU will remain Standby, however, the indication that the data was not fully received during this cycle will be available in the system bit: ECPU_HSBY_1.MAST_SYNCHRONIZED (or ECPU_HSBY_1.FAST_SYNCHRONIZED)
According to your process, having the data not synchronized between Primary and Standby for several cycles might have an impact on your application. For instance, the system might not have enough time to transfer the application from Primary to Standby.
in order to analyze if there could be an impact, do the following:
-          Get the system in a stable state (Run Primary and Run Standby)
-          Connect to the Primary CPU with Unity Pro
-          Observe the Device DDT bit: MAST_SYNCHRONIZED and / or FAST_SYNCHRONIZED (if FAST used)
-          If MAST_SYNCHRONIZED and / or FAST_SYNCHRONIZED do not always show value = 1, then adjust the configured period online until this cycle is stable and MAST/FAST_SYNCHRONIZED are set to 1


another advice is to use these system bits in the logic of the application or in the scada in order to trigger warnings
Feel free to contact your local Schneider Electric representative for additional information.
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