Rockwell Automation 5720 DriveLogix System - Manual (Page 37)

Publication 20D-UM002C-EN-P - November 2003
What Is DriveLogix? 2-3
Defining tasks
A task provides scheduling and priority information for a set of one or more
programs. You can configure tasks as either continuous or periodic. The
DriveLogix controller supports as many as 8 tasks, only one of which can
be continuous.
A task can have as many as 32 separate programs, each with its own executable
routines and program-scoped tags. Once a task is triggered (activated), all the
programs assigned to the task execute in the order in which they are grouped.
Programs can only appear once in the Controller Organizer and cannot be
shared by multiple tasks.
Specifying task priorities
Each task in the controller has a priority level. The operating system uses the
priority level to determine which task to execute when multiple tasks are
triggered. There are 15 configurable priority levels for periodic tasks that range
from 1-15, with 1 being the highest priority and 15 being the lowest priority. A
higher priority task will interrupt any lower priority task. The continuous task
has the lowest priority and is always interrupted by a periodic task.
The DriveLogix controller uses a dedicated periodic task at priority 7 to
process I/O data. This periodic task executes at the fastest RPI (Requested
Packet Interval) you have scheduled for the DriveLogix system. Its total
execution time is as long as it takes to scan the configured I/O modules.
How you configure your tasks affects how the controller receives I/O data.
Tasks at priorities 1-6 can starve the dedicated I/O task; tasks at priority 8-15
can be starved by the dedicated I/O task.
TIP
For typical applications, the periodic task priority should be
set at a priority level lower than 7. (Remember, priority
levels are ranked in ascending order–a priority lower than 7
means a priority level of 8-15.) The default priority of 10
should be sufficient for most applications.
If a periodic task must execute without interruption from
the I/O update task, set the priority level higher than 7 (i.e.
1-6).