Allen-Bradley 1756-CNB/E, User Manual

Page: 74 / 186

Publication 1756-UM523F-EN-P - December 2006
74 Configure and Program the Controller
Estimate the Crossload
Time of a Program
You can estimate the crossload time of a program in a redundant
controller.
Perform this procedure to gauge the time a project spends
crossloading data.
Before You Begin
Crossload Time Estimations
IMPORTANT
By lowering the amount of time you spend crossloading data,
you can reduce your scan time.
Consideration Details
The controller crossloads data at the end of
every program. The primary controller stops at the end of every program to crossload fresh data to the
secondary controller. This 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 crossload time depends on how much
data changed. The length of time for the crossload depends on the how much data the primary controller
has to crossload.
•
The primary controller 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 to tell the secondary
controller which program the primary controller is executing.
In a redundant system, a ControlLogix5561,
5562, or 5563 controller is up to 30% faster
than a ControlLogix5555 controller. The scan time improvement of ControlLogix5561, 5562, and 5563 controllers is less in a
redundant system is than in a nonredundant system.
•
Even though the ControlLogix5561, 5562, and 5563 controllers execute logic faster,
they must still crossload data.
•
Given the same project and redundant system, a ControlLogix5561, 5562, or 5563
controller is up to 30% faster than a ControlLogix5555 controller.
Execute Program in
Primary Controller
Start of Program
End of Program
Crossload Results to
Secondary Controller
Scan Time of Program