282 Rockwell Automation Publication 1769-UM021G-EN-P - October 2015
Chapter 10 Develop Applications
System Overhead Time Slice
The controller communicates with other devices at a specified rate (scheduled) or
when there is processing time available to service the communication.
The system overhead time slice specifies the percentage of time a controller
devotes to service communication. If you have a continuous task, the System
Overhead Time Slice entered in the Advanced tab of the Controller Properties
dialog box specifies continuous task/service communication ratio. However, if
there is no continuous task, the overhead time slice has no effect.
The table shows the ratio between the continuous task and service
communication at various system overhead time slices.
As shown in Tab le 36, if the system overhead time slice is less than or equal to
50%, the duration stays fixed at 1 ms. The same applies for 66% and higher,
except there are multiple 1 ms intervals. For example, at 66% there are two 1 ms
intervals of consecutive time and at 90% there are nine 1 ms intervals of
consecutive time.
Table 36 - Ratio between Continuous Task and Service Communication
At this time slice The continuous task runs Service communication occurs for up to
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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