Around the end of the 1980s, a development started that made Programmable Logic Controls (PLCs) central components of automation technology. The PLC systems have various advantages compared with conventional relay technology, for example, they can be adjusted quickly in case of process changes and are easily extendable. The function and structure of the systems are independent of the size of the PLC. A PLC consists of hardware and software, with the hardware comprising a processor with storage components and further electronics for the connection of input and output systems. Modular PLC systems consist of various components such as component carriers with system bus, power unit, central processing unit and application storage, digital inputs and outputs and intelligent components for analogue data processing or drive control. For the input and output level, a standardised voltage of 24 V has been set leading to a resistance complying with industry requirements.

The software comprises the operating system and the application program. The operating system manages the system resources and the organisational functions and in addition, ensures a controlled start after the operating voltage is switched on. It is responsible for error management, enabling the exchange of information via communication groups and also coordinates the execution of the application program which maps the logic course of the control task.

A PLC starts processing the application program immediately after it is switched on (in the so-called RUN mode). Directly after the system is switched on, all non-remanent memories are reset and the processing cycle starts scanning in the input image after which the complete PLC instruction list is processed. In the next step, the output image created through the instruction list is sent to the outputs and the program cycle starts again with scanning in the input image. The cycle time is probably the most important speed criterion for a PLC, typically ranging from some ms to several hundred ms.

The simple flow structure shows its advantages when data and programs are manipulated in ongoing operation. Almost all programming environments allow for the change of all variables during a processing cycle. Due to the set program structure, program sequences can be loaded at run time (Online Change), since there are always exact starting points at the beginning and the end of a cycle. When these new systems were launched, the producers had to make their customers, who were used to contactors, acquainted with the programming of the new systems. For that reason, numerous but mostly short-lived attempts were made to find a programming language that facilitated the change from the traditional detailed wiring diagram to the PLC program. The programming languages, Function Block Diagram, Ladder Diagram and Instruction List, were then developed. At that time, however, developers failed to define a universally binding standard for programming PLC systems. As a consequence, each producer had specific characteristics in the programming languages Function Block Diagram, Ladder Diagram and Instruction List. Therefore, company standards emerged in various geographic regions, such as Siemens STEP 5 in Europe, Allen Bradley in the U.S. and MITSUBISHI in the Far East.