Rockwell Automation 1794-L34 FlexLogix Controller System User Manual - Manual (Page 104)

Publication 1794-UM001G-EN-P - January 2007
104 FlexLogix Back-Up on DeviceNet
How the Back-up Works Figure shows an example back-up system. In the back-up system, the
following occurs:
• Both controllers/scanners simultaneously receive all inputs.
• Both controllers execute in parallel but are NOT synchronized.
• Only the primary controllers sends output data to the I/O
devices. A virtual switch in the 1788-DNBO cards is used to
switch outputs between primary and secondary controllers.
• After failure or forced switchover, outputs are automatically
switched by the 1788-DNBO card 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.
43493
DeviceNet
Primary controller
Secondary controller All backed up devices must
operate on DeviceNet