Allen-Bradley 1756-CNB/E, User Manual

Page: 76 / 186

Publication 1756-UM523F-EN-P - December 2006
76 Configure and Program the Controller
Minimize Scan Time
To minimize a project’s scan time, perform this procedure.
IMPORTANT
Do not try to get the scan time of a ControlLogix redundancy
project down below about 20 milliseconds. At very low scan
times, crossload data becomes a bigger performance burden.
This burden limits the minimum scan time.
Action Details
1. Use a few large programs instead of a lot
of small programs.
The controller stops at the end of every program to crossload data. So the more programs
that you have, the more the controller stops to crossload. And it often ends up
crossloading the same data many times.
To cut down the number of crossloads:
•
Use only one or a few programs.
•
Divide each program into whatever number of routines makes the most sense. A
routine does not cause a crossload.
•
Use the main routine of each program to call the other routines of the program.
•
If you want to use several tasks for different scan periods, put only one program in
each task. Remember that each program adds a crossload. So use only one or a few
tasks.
This is better Than this