Rockwell Automation 1769-SDN Compact I/O DeviceNet Scanner Module User Manual - Manual (Page 144)

144 Publication 1769-UM009E-EN-P - August 2009
Appendix B CompactLogix Backup on the DeviceNet Network
Development and
Debugging Tips When you implement the CompactLogix system backup on the DeviceNet
network, we recommend that you consider the following development and
debugging tips:
• Develop and debug the entire application with only the primary
controller and scanner module present. When the application is totally
verified, then download the program and exact same scanlist to the
secondary controller, without the primary controller present. Verify that
the secondary is also functioning properly, and then both primary and
secondary can be added to the network at the same time.
• No configuration parameters are entered from RSNetworx for
DeviceNet software or RSLogix 5000 programming software to enable
backup. All configuration occurs in the user program. The entire
application (except for a few ladder rungs) can be developed without
knowledge that the application will have a backup controller and
scanner module.
• Local I/O still works when this solution is used, but the local I/O is not
backed up.
• Switchover time depends on the user configurable heartbeat. Typically,
switchover occurs after two heartbeats, plus the time required to
complete network polling.
• The I/O during switchover is not bumpless. Since the programs and
I/O updates are not synchronized, it is possible for the secondary
controller to be either slightly faster or slower than the primary.
For example, if output changes during a switchover, the fact that the
primary and secondary controllers are not synchronized because the
output to momentarily switch between an older and newer value. If you
configure the switchover time slower than the program scan and I/O
update, the secondary lags behind the primary and eliminates this.
• State variables, such as counters or timers, are not synchronized. The
user program must synchronize the primary and secondary controllers,
typically over an EtherNet/IP or ControlNet network link between
controllers. If the outputs are dependent on a state variable, the lack of
synchronization can also cause a bumpy switchover.
• As with all backup and redundancy systems, the I/O must change at a
slower rate than the switchover time. If the inputs change faster than the
switchover, the change of state is lost.
• Either the user program or user action determine the primary controller.
In its simplest mode, the first scanner module to turn on or become
available on the DeviceNet network is the primary.