5-5
5 Designing Tasks
NJ-series CPU Unit Software User’s Manual (W501)
5-2 Task System
5
5-2-1 Overview of Tasks
Primary Periodic Task and Periodic Tasks
The CPU Unit periodically executes both the primary periodic task and periodic tasks.
(The interval in which the CPU Unit executes the primary periodic task or a periodic task is called the
task period.)
From 1 to 128 programs can be assigned to one task. The programs that are assigned to a task are
executed in the order that they are assigned. Execution of the all of the programs assigned to each task
is called user program execution.
Exchanging data with CJ-series Units or EtherCAT slaves is called I/O refreshing.
You can assign I/O refreshing for each slave and Unit to the primary periodic task or priority-16 periodic
task. By default, I/O refreshing for all slaves and Units is assigned to the primary periodic task.
Event Tasks
An event task is executed only once when the specified execution condition is met. There are the fol-
lowing two types of execution conditions for event tasks.
From 1 to 128 programs can be assigned to one task. The programs that are assigned to a task are
executed in the order that they are assigned.
Precautions for Correct UsePrecautions for Correct Use
• I/O refreshing and motion control are not executed in event tasks. This means that you cannot
assign programs to event tasks if the program performs I/O control or executes motion control
instructions.
• Event tasks are not executed repeatedly every task period. Therefore, you cannot assign a
program to an event task if that program contains an instruction whose execution is not
completed within one task period. Instructions that are executed over more than one task
period include some of the basic instructions, such as instructions for SD Memory Cards and
communications, all motion control instructions, and all simulation instructions. Refer to the
NJ-series Instructions Reference Manual (Cat. No. W502-E1-05 or later) for details on the
basic instructions that cannot be used in event tasks.
Execution condition Specification
Execution with an instruc-
tion
The event task is executed when the ActEventTask (Execute Event Task)
instruction is executed.
Execution when a condi-
tion for a variable is met
The event task is executed when the specified variable matches a predefined
condition.
Fixed
period
Primary periodic task
Fixed period (an
integer multiple of
the task period of
the primary
periodic task)
Periodic task
Task period
Task period
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