ISPSoft User Manual

Chapter 5 Program Organization Units and Tasks
The characteristic of the IEC 61131-3 architecture is that a program is divided into several POUs
which can be developed independently. When all POUs are compiled, they are rearranged and
combined into an execution code which can be scanned step by step. The POUs are rearranged
and combined according to the tasks to which the POUs are assigned.
In ISPSoft, a POU of the program type is assigned to a task. However, more one POU can be
assigned to the same task, and the order in which these POUs are executed can be specified.
Besides, after users add a POU of the program type, they must assign it to a default task.
There are three types of tasks. They are cyclic tasks, timed interrupt tasks, and conditional interrupt
tasks.
• Cyclic task
A POU assigned to a cyclic task is scanned back and forth. Owning to the fact that there is only
one cyclic task in a project for a DVP series PLC, the POUs which will be scanned cyclically are
assigned to this cyclic task. However, there are 32 cyclic tasks in a project for an AH500 series
CPU module. These cyclic tasks are numbered from 0 to 31. The smaller the task number is, the
earlier the task is executed. Besides, AH500 series CPU modules support the applied
instructions (TKON and TKOFF) which control tasks. Users can enable or disable a task during
the operation of an AH500 series CPU module.
A POU is which a sequential function chart can only be assigned to a cyclic task. It can not be
assigned to an interrupt task.
• Timed interrupt task
A POU of the program type which is assigned to a timed interrupt task is like a timed interrupt
subroutine. If the time when timed interrupts should occur is reached, the POUs assigned to the
timed interrupt task will be executed in order. Besides, the number of timed interrupt tasks
depends on a PLC selected. The number of interrupt sources that a PLC supports is the number
of timed interrupt tasks that the system has.
• Conditional interrupt task
There are several types of conditional interrupt tasks. For example, external interrupts, I/O
interrupts, counting interrupts, and etc. Different PLCs provide different conditional interrupt tasks.
Users have to make sure of the conditional interrupt tasks which are supported by the PLC before
they assign the POUs of the program type to tasks. A POU of the program type which is assigned
to a conditional interrupt task is like an interrupt subroutine. If the condition of an interrupt is met,
e.g. the value of a counter reaches the setting value, the POUs assigned the interrupt task will be
executed in order.
5.3.2 Tasks in the Project Management Area
The POUs which are assigned to tasks are listed in the Tasks section in the project management
area, and the POUs which are not assigned to tasks are not listed in the Tasks section. The order in
which the POUs listed in a task section are arranged is the order in which the POUs are executed.
Take the project for an AH500 series CPU module below for example. There are 9 POUs of the
program type. The assignment of the POUs of the program type to tasks is as follows.
Task name POU Execution
Not assigned
Prog2 and Prog3
Owning to the fact that Prog2 and Prog3 are
not assigned to tasks, Prog2 and Prog3 are
not executed.
Cyclic (0)
Prog6 and Prog4
The smaller the task number is, the earlier the
task is executed. As a result, the order in
Cyclic (1)
Prog5 and Prog7
which Prog6, Prog4, Prog5, and Prog7 are
scanned is Prog6Prog4Prog5 Prog7.
I/O interrupt (0) Prog1
If the condition of the interrupt is met, Prog1
will be executed once.
Timed interrupt 1 (253) Prog8 and Prog9
Timed interrupt 1 is triggered every specific
period of time. The order in which Pog8 and
Prog9 are executed is Prog8Prog9.
*. The setting of the condition for an interrupt will be introduced in section 5.5. Please refer to instructions for PLCs
for more information.
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