Rockwell Automation 1794-L34 FlexLogix Controller System User Manual - Manual (Page 83)

Publication 1794-UM001G-EN-P - January 2007
Develop Applications 83
the system overhead percentage does increase communications
performance, it also increases the amount of time it takes to execute a
continuous task - increasing overall scan time
The table below shows the ratio between the continuous task and the
system overhead functions:
At a time slice of 10%, system overhead interrupts the continuous task
every 9 ms (of continuous task time), as illustrated below.
The interruption of a periodic task increases the elapsed time (clock
time) between the execution of system overhead, as shown below.
At this time slice: The continuous tasks
runs for: And then overhead 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).
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 1ms
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)