Publication 1756-UM523F-EN-P - December 2006
36 Design the System
Set aside 2 consecutive ControlNet addresses for each
pair of redundant chassis (for example, nodes 3 and 4).
• If each redundant chassis has multiple CNB modules, set aside a pair of
node numbers for each pair of CNB modules (one in each chassis).
• Do not configure any other device on the ControlNet network for either of
these addresses. For example, if you allocated nodes 3 and 4 for the
redundant chassis, then no other device should use those node numbers.
Know that the switchover time depends on the NUT of
the ControlNet network.
Use the network update time (NUT) of the ControlNet network to estimate
how long it takes your system to switchover.
Example 1
The chassis loses power and the NUT = 4 ms. In that case, the switchover
time is approximately 60 ms.
Example 2
The chassis loses power and the NUT = 10 ms. In that case, the switchover
time is approximately 80 ms.
Example 3
You unplug the CNB from the network and the NUT = 10 ms. In that case, the
switchover time is approximately 220 ms.
Use a NUT that is less than or equal to 90 ms. If you use a larger network update time (NUT), the controller could lose its
connection with a module during a switchover. This could cause outputs to
change state.
Guideline Details
Pair of CNB modules (one in each
redundant chassis)
Slot and node numbers
Slot # Primary node # Secondary node #
(primary node # + 1)
1st pair of CNB modules
2nd pair of CNB modules
3rd pair of CNB modules
4th pair of CNB modules
5th pair of CNB modules
If And the NUT is Then the switchover time is
The chassis loses power or
a module fails
6 ms or less 60 ms
7 ms or more 5 (NUT ) + MAX (2 (NUT ), 30)
A CNB module cannot
communicate with any
other node
⇒ 14 (NUT ) + MAX (2 (NUT ), 30) + 50
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