User manual

Rockwell Automation Publication 1769-UM011I-EN-P - February 2013 111
Develop Applications Chapter 7
As the system overhead time slice percentage increases, time allocated to
executing the continuous task decreases. If there are 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.
At a time slice of 10%, system overhead interrupts the continuous task every 9 ms
of continuous task time.
The interruption of a periodic task increases the elapsed time (clock time)
between the execution of system overhead functions.
V15 and Lower V16 and Higher
Time Slice (SOTS) Comms Continuous Task Comms Continuous Task
10% 1 msec 9 msec 1 msec 9 msec
20% 1 msec 4 msec 1 msec 4 msec
33% 1 msec 2 msec 1 msec 2 msec
50% 1 msec 1 msec 1 msec 1 msec
66% 1 msec 0.5 msec 2 msec 1 msec
80% 1 msec 0.2 msec 4 msec 1 msec
90% 1 msec 0.1 msec 9 msec 1 msec
Legend:
Task executes.
Task is interrupted (suspended).
Periodic
1 ms 1 ms
System Overhead
9 ms 9 ms
Continuous Task
0 5 10 15 20 25
Elapsed Time (ms)
1 ms 1 ms 1 ms 1 ms 1 ms
Periodic Task
1 ms 1 ms
System Overhead
9 ms of Continuous Task Time 9 ms of Continuous Task Time
Continuous Task
0 5 10 15 20 25
Elapsed Time (ms)