Rockwell Automation 1769-SDN Compact I/O DeviceNet Scanner Module User Manual - Manual (Page 130)

130 Publication 1769-UM009E-EN-P - August 2009
Appendix B CompactLogix Backup on the DeviceNet Network
How the Backup Works The following figure shows an example backup system. In the backup system,
the following occurs:
• Both controllers/scanners simultaneously receive all inputs.
• Both controllers execute in parallel but are not synchronized.
• Only the primary controllers send output data to the I/O devices. A
virtual switch in the 1769-SDN scanner module is used to switch
outputs between primary and secondary controllers.
• After failure or forced switchover, outputs are automatically switched by
the 1769-SDN scanner module from the primary controller to
secondary. When the switch occurs, the secondary controller becomes
the primary controller.
The switchover occurs so quickly that the I/O devices do not timeout;
these devices are unaware that redundant controllers/scanners exist and
are unaware of the switchover.
Primary Controller
Secondary Controller
Primary Scanner Module
Secondary Scanner Module
DeviceNet Network