User manual

Publication 1768-UM001C-EN-P - November 2007
Develop Applications 81
c. From the System Overhead Time Slice pull-down menu, choose the
percentage of time your controller spends running its system task,
relative to running its user tasks.
3. 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.
IMPORTANT
System overhead tasks include:
• communication with programming and HMI devices
(such as RSLogix 5000 programming software).
• responding to messages.
• transmission of messages.
Ratio between the Continuous Task and System Overhead Functions
Time Slice Continuous Task Length Overhead Function Time, Max
10% 9 ms 1 ms
20% 4 ms 1 ms
33% 2 ms 1 ms
50% 1 ms 1 ms