Hello Everyone,
I have a question if we have a PLC redundancy system, will the application code for the passive plc (the one in standby mode) still be running? e.g. If there is a TCP server code in it will it still respond to any client request when it is in standby mode?
Or we can treat the plc in standby mode as if all the application code are “temporarily suspended” similar to when we use ST_tmp_suspend()?
Does it run its Init routine on bootup? Then it goes into suspended mode? When Redundancy switch over occurred does it run the Init routine, or it just behave as if we resume the task via ST_tmp_resume()?
Best,
Samuel
Hello,
when the PLC is in Standby the code is executed as the PLC would be in Active mode. Nevertheless the IOs or other parts are not transferred to the outputs or interfaces (to say it in simple words)
If the active one fails a seemless transition is then done as the standby PLCs code is at the same “position” and results as the active one
hope that helps
best regards
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If let’s take the TCP server for example, if the client access via the cluster IP I think the case is quite straight forward everything will as if it’s just connecting to 1 PLC. However, if the client is accessing the actual IP address of the primary and secondary PLC, in that case I assume only the active PLC will respond to the request?
If you connect to the direct IP of one of the two PLCs it will fail when the PLC fails or enters standby mode. As mentioned the standby PLC will not access the interface and therefor no longer communicate.
Therefor you should always connect to the cluster IP.
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