Rockwell Automation Allen-Bradley DriveLogix 5730 User Manual page 20

1-10 What is DriveLogix5730?
periodic
system overhead
continuous task
periodic task
system overhead
continuous task
execute the continuous task. While increasing the system overhead
percentage decreases execution time for the continuous task, it does
increase communications performance. However, increasing the system
overhead percentage also increases the amount of time it takes to execute a
continuous task - increasing overall scan time.
The following table shows the ratio between the continuous task and the
system overhead functions:
At this time slice:
10%
20%
33%
50%
TIP: For typical DriveLogix applications, a setting of 20-33% is
recommended.
At the default time slice of 10%, system overhead interrupts the continuous
task every 9 ms (of continuous task time), as illustrated below.
Legend:
Task executes.
Task is interrupted (suspended).
9 ms
0 5
elapsed time (ms)
The interruption of a periodic task increases the elapsed time (clock time)
between the execution of system overhead, as shown below.
1 ms
9 ms of continuous task time
0 5
elapsed time (ms)
The continuous tasks runs for:
9 ms
4 ms
2 ms
1 ms
1 ms
9 ms
10 15
1 ms 1 ms
1 ms
9 ms of continuous task time
10 15
And then overhead occurs for up to:
1 ms
1 ms
1 ms
1 ms
1 ms
20
1 ms
1 ms
20
25
1ms
25