How do the large, medium, and small PLCs differentiate?
A wide range of PLC products have different specifications and performance. The classification of PLCs is generally classified according to the differences in their structural forms, differences in functions, and the number of I/O points.
1. Sort by I/O points
(1) Minicomputer. The total number of small PLC input and output points is generally below 256 points, its function is mainly controlled by the switch, the user program memory capacity is below 4K words. The small PLC is characterized by its small size and low price. It is suitable for controlling single devices and developing mechatronic products.
(2) Medium-sized machine. The input and output total points of the medium-sized PLC are generally 256~2048 points, and the user program storage capacity reaches 2~8K words. The medium-sized PLC not only has switching and analog quantity control functions, but also has stronger digital computing capabilities. Its communication functions and analog processing capabilities are more powerful. It is suitable for complex logic control systems and continuous production process control applications.
(3) Mainframe. The input and output total points of a large-scale PLC are more than 2048 points. The user program has a storage capacity of 8 to 16K words. It has the functions of calculation, control, and adjustment, and also has a powerful network structure and communication networking capability. Its monitoring system uses CRT display to represent the dynamic process of the process. The mainframe is suitable for equipment automation control, process automation control and process monitoring systems.
2. Classification by structure
According to the different PLC structure, PLC can be divided into two types: integral and modular.
(1) Monolithic structure. The integral type is also called a unit type or a box type. It has a small size and a low price. A small-sized PLC adopts a monolithic structure.
The monolithic structure is characterized by compactly mounting the PLC's basic components, such as CPU modules, I/O modules, and power supplies, into a standard enclosure to form a basic or expansion unit of the PLC. There is no expansion port on the basic unit and it is connected to the expansion unit through an extension cable to form a different configuration of the PLC.
The integral PLC is also equipped with a number of special special function modules to expand the functions of the PLC.
(2) Modular structure. The modular structure of the PLC consists of modular units that can be inserted on the frame or on the substrate. The functions of each module are independent, and the external dimensions are unified. The inserted modules can be flexibly configured as required. At present, large and medium-sized PLCs often use this type of structure.
3. Classification by function
According to the different functions of the PLC, the PLC can be divided into low-grade, mid-range, high-grade three categories.
(1) Low-grade PLC: It has basic functions such as logic operation, timing, counting, shifting, self-diagnosis, and monitoring. It also has a small amount of analog input/output, arithmetic operation, data transmission and comparison, and communication. Mainly used for logic control, sequence control or a small number of analog control single-machine control system.
(2) Mid-range PLC: In addition to the functions of low-grade PLC, it also has strong analog input/output, arithmetic operation, data transfer and comparison, digital system conversion, remote I/O, subroutine, communication networking and other functions. Some can also add interrupt control, PID control and other functions, suitable for complex control systems.
(3) High-end PLC: In addition to the functions of a mid-range machine, it also adds arithmetic operations such as signed arithmetic, matrix operations, bit logic operations, square root operations and other special function functions, tabulation, and table transfer functions. The high-end PLC machine has stronger communication networking function, which can be used for large-scale process control or constitute a distributed network control system to achieve factory automation.
