Allen-Bradley 1756-CNB/E, User Manual

Page: 120 / 186

Publication 1756-UM523F-EN-P - December 2006
120 Maintain and Troubleshoot the System
Acceleration of Unscheduled Communication
Choose a Greater System Overhead Time Slice
The system overhead time slice specifies the percentage of time
(excluding the time for periodic tasks) that the controller devotes to
unscheduled communication. The controller performs unscheduled
communication for up to 1 ms at a time and then resumes the
continuous task.
This table shows the ratio between the continuous task and
unscheduled communication at various system overhead time slices.
Continuous Task and Unscheduled Communication Ratios
At a system overhead time slice of 20 % (default), unscheduled
communication occurs every 4 ms of continuous task time for 1 ms.
If an RSLogix 5000 project contains Then See Page
Only a continuous task and no other tasks
(This is the default task configuration.) Choose a Greater System Overhead Time
Slice 120
More than one task (for example, at least 1
periodic task) Make All Your Tasks Periodic 122
At this time slice The continuous task
runs for And unscheduled
communication occurs
for up to
10% 9 ms 1 ms
20% 4 ms 1 ms
33% 2 ms 1 ms
50% 1 ms 1 ms
Legend:
Task executes.
Task is interrupted (suspended).
1 ms 1 ms 1 ms 1 ms 1 ms
Unscheduled
Communication
4 ms 4 ms 4 ms 4 ms 4 ms
Continuous Task