Rockwell Automation 1757-PROCESSLINX ProcessLinx OPC Server to Server Software User Manual - Manual (Page 47)

Publication 1757-UM002B-EN-P - January 2003
ProcessLinx Runtime Operation 4-3
Redundant OPC Servers /
Fail over ProcessLinx supports redundant OPC Servers using networked hosts. The
Primary and Secondary hosts are configured in the Pipe dialog box. When
runtime mode is started, each Pipe connection is attempted with the defined
primary host. If that connection attempt fails, a new attempt is sent to the
secondary host. If both attempts fail, ProcessLinx control mode is set to
Configure for the user to correct the situation.
After startup, if communication to an OPC Server fails, ProcessLinx
implements a “cold fail-over”. In this case, ProcessLinx disconnects all OPC
links to the “failed” Server and attempts a switch to the redundant Server.
In the first step of the fail-over, ProcessLinx verifies that a redundant Server is
configured. A network ping is sent to the new remote node to verify its
availability. A connection attempt is made to the specified OPC Server on that
node. If the connection attempt succeeds, the switchover is complete. If it
does not succeed, ProcessLinx attempts to re-connect for the “Number of
Retries” specified in the Utility
⇒
Configure Options menu.
After the number of retries has been exceeded, ProcessLinx attempts to fail
over to the first Server. This connection attempt / fail over process continues
until a successful connection has been established. Messages for each failed
attempt are logged to the ProcessLinx message pane and error log. Refer
to What Triggers a Failover on page 3-7
For redundancy to work properly, OPC Items must be configured identically in
the redundant OPC Server pair. If an OPC Item called “Device1.PV1” exists
in the primary OPC Server’s configuration, an OPC Item called
“Device1.PV1” must also exist in the secondary OPC Server’s configuration.
Missing OPC Items will fail and display Invalid OPC Item message. The OPC
Server still indicates running, but one or more OPC Items may not have valid
data.