There are a wide variety of PLC products, and their specifications and performance vary. The classification of PLCs is generally roughly classified according to the difference in their structural forms, functional differences, and the number of I/O points.

  1. Classified by I/O points

(1) Minicomputer. The total number of input and output of small PLC is generally below 256 points. Its function is mainly based on switching quantity control, and the user program memory capacity is below 4K words. Small PLCs are characterized by small size and low price, suitable for controlling single equipment and developing mechatronic products.

(2) Medium machine. The total number of input and output points of the medium-sized PLC is generally 256~2048 points, and the user program storage capacity reaches 2~8K words. The medium-sized PLC not only has the control functions of switching quantity and analog quantity, but also has stronger digital computing capability. Its communication function and analog processing capacity are more powerful, suitable for complex logic control systems and continuous production process control occasions.

(3) Mainframe. The total input and output points of large PLCs are above 2048 points, and the user program storage capacity is 8~16K words. It has the functions of calculation, control and regulation, and also has powerful network structure and communication networking capabilities. Its monitoring system uses a CRT display to represent the dynamic flow of the process. The mainframe is suitable for equipment automation control, process automation control and process monitoring systems.

  1. Classified by structure

According to the different PLC structure, PLC can be divided into two types: integral type and modular type.

(1) The overall structure. The monolithic type is also called a unit type or a box type, and its small size and low price, the small PLC generally adopts a monolithic structure.

The integral structure is characterized by compactly installing the basic components of the PLC, such as CPU modules, I/O modules, and power supplies, in a standard chassis to form a basic unit or expansion unit of the PLC. There is no expansion port on the base unit, and it is connected to the expansion unit through an extension cable to form different configurations of the PLC.

The integrated PLC is also equipped with a number of dedicated special function modules to extend the functionality of the PLC.

(2) Modular structure. The modular structure of the PLC consists of a number of modular units that can be inserted into the frame or on the substrate. The functions of each module are independent, the dimensions are uniform, and the inserted modules can be flexibly configured as needed. At present, large and medium-sized PLCs mostly adopt this structure.

  1. Classified by function

According to the different functions of the PLC, the PLC can be divided into three categories: low-end, mid-range and high-grade.

(1) Low-grade PLC: It has basic functions such as logic operation, timing, counting, shifting, self-diagnosis, monitoring, etc. It can also have a small number of analog input/output, arithmetic operation, data transmission and comparison, communication and other functions. Mainly used for logic control, sequential control or a small number of analog control single-machine control system.

(2) Mid-range PLC: In addition to the function of low-grade PLC, it also has strong functions of analog input/output, arithmetic operation, data transmission and comparison, digital system conversion, remote I/O, subroutine, communication networking. Some can also add interrupt control, PID control and other functions for complex control systems.

(3) High-grade PLC: In addition to the function of the mid-range machine, it also adds the functions of symbolic arithmetic operation, matrix operation, bit logic operation, square root operation and other special function functions, tabulation and table transfer functions. High-end PLC machines have stronger communication networking functions, which can be used for large-scale process control or to form a distributed network control system to achieve factory automation.

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