Ladder Diagram (LAD)
Features
Ladder Diagram is a graphical programming language that looks like a relay circuit diagram in an electrical control schematic. This language uses “ladders” to represent logical relationships and consists of elements such as left and right busbars, contacts, coils, and function blocks. It is visual and intuitive, and is easy to understand and master for electrical engineers familiar with relay control circuits.
For example, in a simple motor start/stop control program, a normally open contact represents the start button, a normally closed contact represents the stop button, and an output coil represents the motor contactor. When the start button is pressed (the normally open contact is closed) and the stop button is not pressed (the normally closed contact remains closed), the motor contactor coil is energized and the motor starts. This expression is very similar to traditional relay control circuits and is easy to read and maintain.
Application scenarios
It is widely used in various logic control and sequential control occasions. In the field of industrial automation, such as the control of automated production lines, including operations such as material transportation and start and stop of processing equipment; in building automation, it is used for simple logic control of elevator control and HVAC (heating, ventilation and air conditioning) systems, etc.
Instruction List Language (STL – Statement List)
Features
Instruction List language is a text-based programming language, which is similar to the assembly language of computers. It consists of a series of instructions, each of which corresponds to a basic operation of the PLC, such as logical operations (AND, OR, NOT), data transfer (MOV), timer and counter operations, etc. This language can accurately describe the control logic and has the characteristics of high execution efficiency.
For example, in a simple timing control program, the “LD” (load) instruction is used to load the state of a normally open contact into the accumulator, the “TON” (on delay timer) instruction is used to set a timer. When certain conditions are met, the timer starts timing, and the “OUT” (output) instruction sends the output state of the timer to an output coil.
Application scenarios
Suitable for occasions with high requirements for execution speed and program storage space. In some small PLC devices, due to limited resources, instruction list language can make more efficient use of these resources. At the same time, instruction list language is also commonly used when optimizing programs or processing complex arithmetic and logical operations.
Function Block Diagram (FBD)
Features
Function Block Diagram is also a graphical programming language that describes programs in function blocks. Each function block represents a specific function, such as arithmetic operations, logical operations, comparison operations, data processing, etc. Function blocks are connected by signal lines to represent the flow of data and the relationship between functions. This language has a clear structure and is easy to understand complex functional combinations.
For example, in an analog quantity processing program, an analog quantity input function block is used to convert the analog signal collected by the sensor into a digital signal, and then a comparison function block is used to compare this digital signal with the set value, and finally an output function block is used to control the actuator according to the comparison result.
Application scenarios
Commonly used in complex control systems, especially those involving collaborative control and data processing between multiple functional modules. In process control systems, such as production process control in chemical and pharmaceutical industries, it is used to control the complex adjustment process of parameters such as temperature, pressure, and flow of reactors.
Structured Text Language (ST – Structured Text)
Features
Structured Text language is a high-level programming language, which is similar to PASCAL or C language in computer programming languages. It is written in text form and has strong logic and structure. It can perform complex arithmetic operations, data processing, loops and conditional judgments. Programs written in this language are concise, efficient and can handle complex control tasks.
For example, in the inventory management program of an automated warehousing system, the use of structured text language can easily realize functions such as inventory quantity calculation, goods in and out of the warehouse recording and inventory. By defining variables, functions and procedures, the logical structure of the program can be clearly organized.
Application scenarios
Suitable for processing complex algorithms and large-scale control systems. In the control system of large automated factories, it is used for high-level control functions such as production scheduling, equipment management, and quality control. In intelligent transportation systems, it is used for complex control of traffic signals and flow optimization.
