Vijeo citect introduction for online

Editor's Notes

• #18: Lets now take a closer look at what is offered by the redundancy components of Vijeo Citect.First we will have a look at the I/O server process data redundancy.So, here we have a SCADA system with display clients; a pair of I/O servers; and the field I/O devices.[]The clients will subscribe to the data points that they need to display.The I/O server process, will poll for the required data from the I/O devices.When the data changes are received in the I/O server process, the data point updates will be published to the clients.[^]Now we will take a look at what happens to the processing, if the primary process fails.
• #19: So, here we have the primary server in the failed condition.The clients will have detected the failure and transparently switched over to the next available I/O server process.The I/O device polling, is now handled by the standby I/O server process, and the published data is forwarded to the clients.[] The failure could also have occurred with regards to the network connection of the primary I/O server process.The result is the same;The I/O process is marked as failed for an I/O device in Vijeo Citect; if it is unable to process, or maintain successful communications with the I/O device.
• #20: On successfully restoring the primary IO server process, the communication path will switch back to the primary.The I/O communication redundancy, is designed to switch back, based on the redundancy priority of the server processes; with the default being, the primary, I/O server process.
• #21: Lets have a look now at what is available with regards to a system, that is using more than one standby I/O server process.[]Here we have three I/O servers.The first server is set as the primary server, and it is using the Ethernet network to communicate with the I/O device.The second server is set as the first standby server, and it is able to use the Ethernet, or use serial network number 1, to communicate with the I/O device.The third server is the last standby, and it has serial network number 2, available to communicate with the I/O device.By having multiple standby’s, it ensures connectivity to the data source, by being able to use multiple paths.Network connections, especially data connections to remote field devices, can be unreliable at times; so, by being able to transparently use multiple data paths, to access remote data, it keeps the control system functional for the operator.With the first server being the primary, the default communication path will use server 1.
• #22: So, if server 1 was to fail; server 2 would take over using the Ethernet communication path.In this setup example; within the server 2 configuration for the I/O device data paths; the Ethernet path has been given a higher priority than the serial network path, so that the Ethernet network is used first, if it is available for server 2, otherwise the server will use the serial path.[] Now, once we lose connection to the I/O device, over the Ethernet network; server 2 will not be able to communicate successfully using the Ethernet network; the data connection will move on server 2 to use serial network 1.[] This would be the same result if we only lost the Ethernet network connection to the I/O device; in this situation, both server 1 and server 2, would be unable to use the Ethernet network, to communicate with the I/O device; so then server 2 would use serial network 1.I should point out; in this example, we have server 1 and 3, only having a single data path option. Where with server 2, it has 2 data paths.We could also have a single PC setup that has all the 3 data paths, connected to a single I/O server process; with that I/O server process arbitrating the best data path to use.The flexibility is available, to offer you what ever data path configuration is required; to be able to offer the maximum data connectivity and reliability for your system designs.
• #23: Then in the last example; we now have both the Ethernet network connection, and serial network 1 failed;So server 3 will take over communication with the I/O device using serial network 2.Throughout all the network failure and I/O server switching; it is transparent to the client display, as the client is automatically following the active process.There is no function, or selection, that has to be made on the client, by the operator; the client change over, is automatic with the required subscription data being published to the client.As the communication networks are restored; the use of the data paths will activate, using the defined redundancy priority.