Rockwell Automation 5720 DriveLogix System - Manual (Page 238)

Publication 20D-UM002C-EN-P - November 2003
12-14 DriveLogix Back-Up on DeviceNet
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 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 back-up 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 to power-up or become available
on DeviceNet first is the primary.
• Unlike some back-up systems (i.e. PLC5), the primary controller still
maintain control of the I/O and switchover does NOT occur if the
primary controller is set to Program/Idle mode. The secondary
1788-DNBO scanner also indicates that it is in Idle Mode.
• By default, a switchover will NOT occur if the default fault routine or
user fault routine is executed in the primary controller. However, the
user fault routine can force a switchover if so desired.
• If an operator interface is on DeviceNet, then it can work without
knowledge which controller is primary or secondary.
• Online edits are not automatically performed on both Primary and
Secondary since no synchronization exists between Primary and
Secondary. Once an online edit occurs on the Primary, then the Primary
and Secondary will have different programs.
• DriveLogix Back-up on DeviceNet is not Hot Back-up. Hot Back-up
implies complete synchronization of program, program variables, and
I/O. Also, I/O switchover is completely bumpless is Hot Back-up.