The MULTIPROG programming system is your central tool in the development of automation applications. Systems are becoming increasingly complicated, and commissioning and maintenance tasks more demanding. With powerful and easy-to-operate tools, however, you can take control of even complex systems and applications.

The standard IEC 61131 has been established to standardize the multiple languages, sets of instructions and different concepts existing in the field of automation systems. The great variety of PLC concepts has led to an incompatibility between the different PLC platforms and manufacturers. The result was a great effort to be made for training, hard- and software investments.

IEC 61131 standardizes the programming languages, the interfaces between PLC and programming system, the sets of instructions and the handling and structuring of projects. The advantage of using IEC 61131 compliant PLCs and programming systems is a portability of all platforms and the use of same concepts reducing costs for automation systems.

The main changes that have come with IEC 61131-3 are:

Declaration of variables is similar to the variable declaration in higher programming languages.
Declaration of data types is possible.
Global and local data can be differentiated.
Programming means symbolic programming.

Tasks in IEC 61131-3
Tasks determine the time scheduling of the programs associated with them. This means that programs have to be associated to tasks. The settings of the task determine the time scheduling.

IEC 61131-3 describes different time scheduling models which results in different task types:

Default task. Each resource can contain one default task. The default task is the task with the lowest priority (lower than cyclic tasks) and is not time scheduled. After executing it completely the system waits for a defined idle time. If then no cyclic task or any other task with a higher priority has to be processed, the default task is executed again automatically. This means that the default task is always executed (apart from the necessary pre-defined idle period between two invocations), as long as no task with a higher priority runs.

Cyclic tasks are activated in a certain time interval and the program is executed periodically.
System tasks are called automatically by the PLC operating system if an error or a change of the operational state of the PLC occurs. They are also known as system programs or SPGs.

Event or interrupt tasks are activated if a certain event has happened.
Each task has a certain priority. In so called preemptive scheduling systems, an active task with low priority is interrupted immediately, when a task with higher priority becomes active due to a certain event. In systems with non-preemptive scheduling, task interruptions by tasks with higher priority are not possible.

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