System Overhead Time Slice - Allen-Bradley Compact GuardLogix 5370 User Manual

Chapter 8 Elements of a Control Application

System Overhead Time Slice

138
The Compact GuardLogix 5370 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.
Table 22 - Ratio between Continuous Task and Service Communication
At this time slice
10%
20%
25%
33%
50%
66%
75%
80%
90%
As shown in Table 22, 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.
Rockwell Automation Publication 1769-UM022C-EN-P - June 2018
The continuous task runs Service communication occurs for up to
9 ms 1 ms
4 ms 1 ms
3 ms 1 ms
2 ms 1 ms
1 ms 1 ms
1 ms 2 ms
1 ms 3 ms
1 ms 4 ms
1 ms 9 ms