Runtime Behavior

The OI Gateway starts with the active data source. The OI Server Engine switches to the standby data source when the active data source fails to communicate. The value of the $SYS$Status determines the communication failure.

• The value of the $SYS$Status of the standby data source must be TRUE in order to switch over to the standby data source. Otherwise, there will not be any failover.

When $SYS$Status shows a FALSE value at both active and standby data sources, the OI Server Engine considers a complete communication failure and mark all the items subscribed to the redundancy data source hierarchy with the current time and the appropriate OPC quality. The OI Server Engine activates the slow-poll mechanism to retry the communication to both data sources until either one of the Ping Items returns to a good quality and update its $SYS$Status item to TRUE.

When the OI Server Engine switches to the standby data source, the standby data source becomes active and the originally active data source becomes the standby.

When the active data source becomes the standby data source the Ping Item is not deleted from that the standby data source. 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.

The OI Gateway logs any failover activities. All other functionality such as diagnostics, enable/disable, and reset is performed exactly the same as it is performed for any other hierarchy node.

This feature allows the OI Gateway to provide fail over support by providing one node which switches between two other nodes. The Redundant Device Object 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 data source 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 data source groups defined in the regular server hierarchy.