Redundancy and scan time – Rockwell Automation 1756-L6x_L7x ControlLogix System User Manual User Manual
Page 181
Rockwell Automation Publication 1756-UM001O-EN-P - October 2014
181
Redundant Systems
Chapter 12
Redundancy and Scan Time
At the end of each program, the primary controller synchronizes and crossloads
fresh data to the secondary controller. This process keeps the secondary
controller up-to-date and ready to take over. It also increases the scan time when
compared to a nonredundant system.
The amount of time a crossload consumes depends on how much data the
primary controller has to crossload:
•
The primary controller synchronizes and crossloads any tag to which an
instruction wrote a value, even the same value, since the last crossload.
•
Crossloading also requires a small amount of overhead time (1 ms per
crossload) to tell the secondary controller which program the primary
controller is executing.
Redundancy firmware revision 16.53, or later, limits which programs precede
synchronization and data crossloading. In many applications, changing this can
reduce the overall impact to the task scan time by reducing the number of times a
data area is synchronized. Removing a synchronization point results in 1 ms of
overhead time that is saved and any time that was used to crossload the data.
For complete details about the scan time of a redundant system, see the
ControlLogix Enhanced Redundancy System User Manual, publication
.