Understand the run-time behavior of a redundant device
- Last UpdatedAug 26, 2025
- 2 minute read
The Communication Driver starts with the active device. The driver switches to the standby device when the active device fails to communicate. The value of the $SYS$Status determines the communication failure.
Note: The value of the $SYS$Status of the standby device must be TRUE in order to switch over to the standby device. Otherwise, there will not be any failover.
When $SYS$Status shows a FALSE value at both active and standby devices, the driver considers a complete communication failure and mark all the items subscribed to the redundancy device hierarchy with the current time and the appropriate OPC quality. The driver activates the slow-poll mechanism to retry the communication to both devices until either one of the Ping Items returns to a good quality and update its $SYS$Status item to TRUE.
When the driver switches to the standby device, the standby device becomes active and the originally active device becomes the standby.
When the active device becomes the standby device the Ping Item is not deleted from that the standby device. This ensures the standby is able to recover the communication again.
Note: The Ping Item must be a valid item from the controller that has not been rejected by the server for the failover to function properly.
This feature allows the Communication Driver to provide fail over support by providing one node which switches between two other nodes. The Redundant device is configured with a redundancy node which directs itself to one of the two nodes and switches to the other based on lack of communications to a common user-configured controller item. In this manner the Redundant Device Object can be used to direct client requests to the redundant node, which switches between device or communication pathway failure without intervention.
Note: Unsolicited message configuration is not supported in the Redundant Device Object (RDO) itself. You can still receive unsolicited messages directly from device groups defined in the regular server hierarchy.