DNPR Redundancy

In other Plant SCADA drivers, the I/O Server controls the redundancy switchover based on the redundant I/O Devices state (online/offline) and configured I/O Device priorities. The DNPR driver redundancy support is different. It performs the redundancy switchover (makes either the primary or the standby I/O device active and usurps the connection) by itself, or you can do it manually by using the virtual unit control tags.

In normal driver redundancy, if both primary and standby units (the representation of the I/O device in the driver software) are connected to the device, communication happens via the primary unit. If the connection to the primary unit is lost, the I/O Server switches communication to the standby unit. If the connection to the primary unit is restored, the I/O Server switches communication back to the primary unit.

In DNPR driver, the priority for unit use is not based on the primary unit when a driver is started. The unit that is in use continues in service. If both drivers become active (that is, peer connection lost) then the priority for unit use is based on the primary unit when the peer connection is reestablished.

In the redundant configuration, the primary and standby I/O Device pair is configured on different I/O Servers. The DNPR drivers on these servers communicate to each other to synchronize the redundant unit states, the tag and the event/trend cache data.

The driver only communicates with a device if the unit is in the active state. The unit is active only on one peer driver at one time, except for the case when the communication between the redundant I/O Servers is lost and both units go into the active state.

When a unit is active, the driver reports the current online/offline state to Plant SCADA. If the unit is not active, it appears as offline to Plant SCADA. The driver does not communicate to the device if the unit is not active. It does not send any DNP3 frames to the device, or process any frames received from the device.

The following diagram illustrates a typical redundancy configuration showing a pair of redundant I/O servers and a network of DNP3 devices:

SCENARIO ONE:

Normal operation, with devices communicating via the primary server.

SCENARIO TWO:

Redundancy switchover due to an interruption to the communication channel between the primary I/O server and device two. Even after the communication channel to the primary I/O Server is restored, communication to device two continues via the standby I/O Server.