Configuring Redundancy

The OFSOPC driver supports fully transparent redundancy across the following architectures:

• Multiple I/O server redundancy, including support for load balancing across one or more computers

• Device redundancy, where a PLC is managed by more than one I/O server

• Communication redundancy, where each PLC is on a redundant network.

To facilitate a complex redundant architecture, the OFSOPC driver represents each PLC as a separate I/O device. This allows a switchover to occur on a per-PLC basis, as demonstrated by the diagram below.

Scenario one demonstrates the system in normal operating state, with the client connected to the three PLCs via the primary server. In scenario two, the link from the primary server to PLC2 is broken. Under these circumstances, the client is able to re-establish communication with PLC2 via the standby server.

The OFSOPC driver subscribes to #PLCQualStatus and system bit %S0 on each PLC to determine the current operational status of a device. If these tags report bad quality, the I/O device will be declared offline and a switchover to the standby will occur.

Note: It is also possible to specify a custom item value that the driver will check in addition to #PLCQualStatus. See Dynamic Status Item Parameters.