Five standard programming languages for PLC

PLC user program is designed according to the process control requirements of the control system, the design of the PLC programming language. Industrial control programming language standard (IEC1131-3) based on the international electric commission. PLC programming language includes the following five kinds: ladder diagram language (LD), instruction list language (IL), function module diagram language (FBD), sequential function flow chart language (SFC) and structural text language (ST).




1, ladder diagram language (LD)

Ladder diagram language is the most commonly used programming language in PLC programming. It is a programming language similar to that of a relay circuit. Because the electrical design personnel are familiar with the relay control, the ladder diagram programming language has been widely welcomed and applied.
The ladder diagram programming language characteristic is: with the electric operation principle diagram corresponds, has the intuition and the correspondence; and the original relay control is consistent, the electrical design personnel is easy to grasp.
Ladder programming language and the original relay control point is different, the current flow in the trapezoid diagram is not practical, the internal relay is not the actual existence of relay application, need to treat with the original concept of relay control difference.

2, instruction list language (IL)

The instruction list programming language is a programming language similar to assembly language mnemonic, and assembly language consists of opcodes and operands. In the case of the computer for the PLC handheld programmer compile user program. At the same time, the programming language of the instruction list corresponds to the ladder diagram programming language, which can be converted to each other under the PLC programming software. 

The characteristics of instruction table programming language is used to represent mnemonic functions, is easy to remember, easy to grasp; in the handheld programmer on the keyboard using the mnemonic representation, easy to operate, can be programmed in computer applications; should have a one-to-one relationship with the ladder. Its characteristics are basically consistent with the ladder diagram language.

3, function block diagram language (FBD)

Functional block diagram language is a kind of PLC programming language, which is similar to digital logic circuit. The function module is used to represent the function of the module, and the different function modules have different functions.

The characteristics of function module chart programming language features: function module diagram of program design language is: to the function module for the unit, analysis of control scheme is simple and easy to understand; function module is the expression function of a graphical, intuitive, for design a digital logic circuit based programming is very easy to grasp; on a large scale, the control system of complex logical relationship, the function module chart is able to clearly express functional relationship, the programming and debugging time is greatly reduced.

4, sequential function flow chart language (SFC)

Sequential function flow chart language is designed to satisfy the sequential logic control. When programming, the process of sequential flow action is divided into steps and conditions. According to the transfer conditions, the functions of the control system are allocated according to the function flow sequence. Each step represents a control function, represented by the box. In the box, the ladder logic is used to complete the task of the corresponding control function. This programming language makes the program structure clear, easy to read and maintain, greatly reduce the workload of programming, shorten the programming and debugging time. Used in the system of the size of the school, procedures for more complex occasions. Figure 5 is a schematic diagram of a simple functional programming language.

The characteristics of sequential function chart programming language: to function as the main line, in accordance with the order of distribution, flow clear, easy to understand for the user program; to avoid the defects of ladder diagram or other language can not be of the order of actions, but also avoid the sequence of dynamic programming for ladder diagram language, because of the mechanical interlocking defects caused by the user the program structure is complicated and difficult to understand; the user program scan time is greatly shortened.

5, structured text language (ST)

Structured text language is a programming language that describes a program with a structured description of the text. It is a programming language similar to a high level language. In large and medium-sized PLC systems, structured text is often used to describe the relationship between the variables in the control system. Mainly used for other programming languages more difficult to achieve the user program.
Structured text programming language uses the computer description method to describe the system in a variety of variables between the various computing relationships, complete the desired function or operation. Most PLC manufacturers use structured text programming language and BASIC language, PASCAL language or C language is similar, but in order to facilitate the application, the expression method of statements and sentence types have been simplified.

The characteristics of structured text programming language: using advanced programming language, can complete the control of more complex operations; need to have some knowledge of the computer language and programming skills, higher requirements for the engineering design personnel. Poor visibility and operability.

Different types of PLC programming software for the above five kinds of programming language support is different, the early PLC only supports ladder programming language and instruction list programming language. The current PLC on the ladder diagram (LD), instruction list (STL), function block diagram (FBD) programming language to support. For example, STEP7 MicroWIN V3.2 SIMATIC.


In the design of PLC control system, it is not only to understand the hardware performance of PLC, but also to understand the type of PLC programming language support.

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