The function of all programming languages is to allow the user to communicate with the programmable controller (PC) via a programming device. They all convey to the system, by means of instructions, a basic control plan.
Ladder diagrams, function blocks, and the sequential function chart are the most common types of languages encountered in programmable controller system design. Ladder diagrams form the basic PC languages, while function blocks and the sequential function charting are categorised as high-level languages. The basic programmable controller languages consist of a set of instructions that will perform the most common type of control functions like relay replacement, timing, counting, sequencing, and logic. However, the instruction set may be varied from one controller to another, because it depends on controller model, specification and requirements. It may be extended or enhanced to perform other basic operations.
The high-level languages have been brought about by a need to execute more powerful instructions that go beyond the simple timing, counting and ON/OFF control. High-level languages are used for analog control, data manipulation, reporting, and other functions that are not possible with the basic instruction sets.
The language used in a PC dictates the range of applications in which the controller can be applied. Depending on the size and capabilities of the controller, one or more languages may be used. Here are some typical combinations of the languages:
2. ladder diagrams and function blocks.
3. ladder and sequential function chart.
4. ladder, function blocks, sequential function chart.