Rockwell Automation Logix5000 Controllers Tasks, Programs, and Routines Programming Manual - Manual (Page 30)

Chapter 1 Manage tasks
A Logix5000 controller communicates with other devices, I/O modules,
controllers, HMI terminals, and so forth, at either a specified rate (scheduled) or
when there is processing time available to service the communication
(unscheduled).
This type of communication Is
Update I/O data (not including block-transfers) Scheduled
Communication
Produce or consume tags
Communicate with programming devices (that is, the Logix Designer application)
Service Communication
Communicate with HMI devices
Run Message (MSG) instructions, including block-transfers
Respond to messages from other controllers
Synchronize the secondary controller of a redundant system
Re-establish and monitor I/O connections (such as Removal and Insertion Under Power
conditions); this does not include normal I/O updates that occur during the running of logic
Service communication is any communication that you do not configure through
the I/O configuration folder of the project.
The system-overhead time slice specifies the percentage of time a controller
devotes to service communication. However, if there is no continuous task, the
overhead time slice has no effect. If you have both a periodic and a continuous
task, the value selected on the Advanced tab of the Controller Properties dialog
box determines the ratio of running the continuous task and service
communication.
The following table shows the ratio between the continuous task and service
communication at various system overhead time slices.
At this time slice The continuous tasks 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
As shown in the table, for version 16 and later of the application, the system
overhead time slice at 50% stays fixed at 1 ms.
Adjust the system-overhead
time slice
30 Rockwell Automation Publication 1756-PM005-EN-P - October 2014