Rockwell Automation 1794-L34 FlexLogix Controller System User Manual - Manual (Page 116)

Publication 1794-UM001G-EN-P - January 2007
116 FlexLogix Back-Up on DeviceNet
• 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 unsynchronized can
cause 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 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.
• FlexLogix 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.