164 Rockwell Automation Publication 1756-UM535D-EN-P - November 2012
Chapter 7 Program the Redundant Controller
Specify a Larger System Overhead Time Slice
The system overhead time slice specifies the percentage of time the controller
devotes to servicing communication, excluding the time for periodic tasks. The
controller interrupts the continuous task to service communication, and then
resumes the continuous task.
This table shows the ratio between executing the continuous task and servicing
communication at various overhead time slices. Consider the following:
• When the system overhead time slice setting is between 10% and up to
50%, the time allocated for servicing communication is fixed at 1 ms, and
the continuous task time slice changes to produce the desired ratio.
• When the system overhead time slice is greater than 50…90%, the time
allocated to the continuous task is fixed at 1 ms, and the time allocated to
servicing communication changes to produce the desired ratio.
Table 28 - Overhead Time Slice
At this time slice The continuous tasks runs for And service communication occurs for as
long as
10% 9 ms 1 ms
20% 4 ms 1 ms
25% 3 ms 1 ms
33% 2 ms 1 ms
50% 1 ms 1 ms
66% 1 ms 2 ms
75% 1 ms 3 ms
80% 1 ms 4 ms
90% 1 ms 9 ms
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