Features of network redundancy

A PLC, which is connected on two physical networks, is considered to be two logical devices having two different network addresses. The two devices are termed as Primary and Secondary. Only one network at a time is used to access the PLC.

CIMPLICITY network redundancy for Allen-Bradley Communications works in the following manner:

On start-up, the Allen-Bradley Communications software uses the primary device to access the PLC.

When the communication software determines that the primary device has failed, it switches communications to the secondary device. When this switchover occurs, REDUND_DEVICE_DOWN and the communication software generates DEVICE_FAILOVER alarms.

If subsequently , the primary device comes up, the communication software clears the REDUND_DEVICE_DOWN alarm. However device communications continues to use secondary device for communication with the PLC.

If secondary device fails while the communication software is using it, the communication software will switch back to primary device after generating proper alarms.

As long as the communication software can talk to the PLC over a network, the device will always remain on-line (Alive).

The device will be declared dead only when both the networks are down.

The Resource ID for all secondary devices is $SYSTEM. Because the Resource ID is used when REDUND_DEVICE_DOWN and DEVICE_FAILOVER alarms are generated and cleared, it is recommended that you not enter $SYSTEM in the Resource field when you configure redundant devices.

With this scheme, there is a single alarm listed for all REDUND_DEVICE_DOWN alarms on the secondary network. You can use the View Stack option in the Alarm Viewer to look at individual alarms.

All DEVICE_FAILOVER alarms are generated using Resource ID you configure for the device.

More information

Network/Cable redundancy support.