4PLC, PAC, PCS

These kind of control systems are probably the most widely used types. Older German resources use the abbreviation SPS (Speicherprogrammierbare Steuerung). Main features of the programmable automate comprise the fact that they are programmed by a user, but user-friendly. Programming in graphical languages resembles drawing of diagrams (contact or logical diagrams, block diagrams, state diagrams). They are easy to learn, so they may be used by machine designers or designers of automated systems when programming basic tasks. Programming languages for PLC are standardized by an international norm IEC/EN 61131-3. It defines 4 types of programming languages. The Instruction List language (IL) is a text language which resembles the assembler language – respective commands are mnemonic abbreviations for respective instructions. A program written in the IL language is usually long and lacks transparency. The Structured Text language (ST) is a higher-level language. It somehow resembles the Pascal language. It is powerful and algorithm-oriented. Most advantage type of programming. The Ladder Diagram languages (LD) is a graphical language which looks like a line-diagram once used for connection of switchboards and relay systems. The LD language is suitable only for writing simple, logical programs (not complex!), which use only basic logical operations and which require intuitive approach. The graphical Function Block Diagram language (FBD) resembles logical diagram of a logical system with integrated circuits, or a more general block diagram. Similar languages of block diagrams are used as well. The program is very transparent, if it is not extremely complicated. If using libraries of specialized function blocks (standard or user libraries), the programming in FBD may be very efficient.

With respect to size and mechanical arrangement, we usually distinguish several types of PLCs.

Micro PLC

It is the lowest class of programmable automat with a typical range of 10 to 20 inputs and outputs.

Compact PLC

It is a mid-class device with a typical range of 20 to 80 inputs and outputs, sometimes even a little bit more.

Modular PLC

It is a high-class device – with respect to possible configurations (typical range of hundreds to thousands of inputs and outputs, various types and combinations, specialized peripheral modules, various types of central units and communication modules, industrial computers modules), and their computational power and possibilities of communication.