Allen-Bradley 1756-CNB/E, User Manual

Page: 18 / 186

Publication 1756-UM523F-EN-P - December 2006
18 ControlLogix Redundancy System Overview
Making Online Edits
Online edits automatically crossload to the secondary controller. They
become inactive if a switchover happens before you assemble them
into the project. This stops a mistake from faulting both the old and
new primary controllers.
Suppose you test an online edit and it causes the controller to fault. In
that case, a switchover happens. The new primary controller
automatically untests the edits and goes back to the original code.
You have the option to keep the edits active after a switchover (at the
risk of faulting both controllers).
Increasing Scan Time At the end of each program, the primary controller pauses its
execution to crossload the result of any output instruction that
executed in the program. This results in an increased program scan
time for a synchronized redundancy system.
Scan Time
Network Addresses During
a Switchover Each CNB, ENBT, or EWEB module in a redundant chassis shares a
pair of network addresses with its partner in the other chassis.
Execute Program in
Primary Controller
Start of Program End of Program
Crossload Results to
Secondary Controller
Program Scan Time