Rockwell Automation 5720 DriveLogix System - Manual (Page 226)

Publication 20D-UM002C-EN-P - November 2003
12-2 DriveLogix Back-Up on DeviceNet
How the Back-up Works Figure 11.2 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.
Figure 11.2
PanelView 300
DeviceNet
Primary controller
Secondary controller
All backed up devices must
operate on DeviceNet