Lenze DDS v2.0 - Manual (Page 5)

Introduction to IEC 61131-3 programming
The software model
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DDS-IEC61131 EN 2.0
2 The software model
The software model of IEC 61131-3 describes the concepts of configuration, resource, task,
program, function block and function and their connection.
For the definition of these terms, the standard is based on a maximum powerful PLC providing the
following features:
•
Mulit-processor can be used
•
Multi-tasking is possible
•
Unlimited number of analog and digital inputs and outputs
•
Communication with other PLCs and PCs is possible
2.1 Resources within a configuration
The highest level in the software model is the configuration ( configuration), which defines the unit
structure. This unit can be, for instance, a PLC with several CPUs connected.
A configuration contains one or several resources ( resources), which form a CPU.
The programs of the resource are controlled by tasks which represent an executable program unit.
Configuration
Resource
Task
Resource Resource
Task Task
DDS001
Fig. 1 A configuration with several resources which can contain independent tasks.
2.1.1 Tasks
Tasks can be processed periodically or because of a certain event. They have a priority which
defines the assignment of CPU times within the resource.
There are several task types:
•
Cyclic tasks
•
Time-controlled tasks ( INTERVAL tasks)
•
Event-controlled tasks ( EVENT tasks)
•
Interrupt tasks
A task declaration consists of the task name, its priority, and a condition on which the task is to be
executed.
The condition can be a time interval, an event (rising signal at a digital input or FALSE/TRUE transition
of a global variable) or an interrupt.
Every task can be assigned to several programs which are to be activated by the task. The programs
are processed in the sequence indicated.
The following rules apply to the execution of a task:
•
The task with the condition that has been met will be executed when, for instance, the interval
time indicated is over or the variable addressed changes from FALSE to TRUE.
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