Allen-Bradley controllogix 1756-L55M12 User Manual page 80

80 Develop Applications
Periodic
System Overhead
Continuous Task
Publication 1756-UM001G-EN-P - January 2007
4. Click OK.
The controller performs system overhead functions for up to 1 ms at a
time. If the controller completes the overhead functions in less than
1 ms, it resumes the continuous task.
As the system overhead percentage increases, time allocated to
executing the continuous task decreases. If there is no communication
for the controller to manage, the controller uses the communication
time to execute the continuous task. While increasing the system
overhead percentage does increase communication performance, it
also increases the amount of time it takes to execute a continuous
task, increasing overall scan time.
Ratio between the Continuous Task and System Overhead Functions
Time Slice
10%
20%
33%
50%
At a time slice of 10%, system overhead interrupts the continuous task
every 9 ms (of continuous task time).
Legend:
Task executes.
Task is interrupted (suspended).
9 ms
0 5
Elapsed Time (ms)
Continuous Task Length
9 ms
4 ms
2 ms
1 ms
1 ms
9 ms
10 15
Max Overhead Function Time
1 ms
1 ms
1 ms
1 ms
1 ms
20 25