Three classifications of programmable controller PLC

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. Classified by structure

According to the structure of PLC, PLC can be divided into two types: integral type and modular type. (1) Integral PLC The integral PLC integrates power supply, CPU, I/O interface and other components in a single chassis. It has the characteristics of compact structure, small size, and low price. Small PLCs generally use this monolithic structure. The integral PLC is composed of basic units (also called hosts) and expansion units with different I/O points. The basic unit has a CPU, an I/O interface, an expansion port connected to the I/O expansion unit, and an interface to a programmer or an EPROM writer. There are only I/Os, power supplies, and no CPUs in the expansion unit. Basic units and expansion units are generally connected by flat cables. Integral PLCs can also be equipped with special function units, such as analog units, position control units, etc., so that their functions can be expanded.

(2) Modular PLC Modular PLC is the PLC of the various components, were made into a number of separate modules, such as CPU modules, I / O modules, power modules (some included in the CPU module) and various functional modules. The modular PLC consists of a frame or base plate and various modules. The module is mounted on the socket of the frame or base plate. This modular PLC is characterized by flexible configuration and can be equipped with systems of different sizes according to needs, and is easy to assemble and easy to expand and maintain. Large and medium-sized PLCs generally adopt a modular structure.

There are also PLCs that combine the monolithic and modular features to form a so-called stacked PLC. The stack-mounted PLC has its own CPU, power supply, I/O interface, etc., but they are connected by cables, and the modules can be stacked one on top of the other. In this way, not only can the system be configured flexibly, but it can also be made compact.

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

(1) Low-grade PLC has basic functions such as logic operation, timing, counting, shifting, self-diagnosis, and monitoring. It can also have 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) In addition to the functions of the low-grade PLC, the mid-range PLC also has strong analog input/output, arithmetic operations, data transfer and comparison, digital system conversion, remote I/O, subroutines, and communication networking. Some can also add interrupt control, PID control and other functions, suitable for complex control systems.

(3) In addition to the functions of the mid-range machine, the high-end PLC also adds arithmetic operations such as signed arithmetic, matrix operation, bit logic operation, square root operation and other special function functions, tabulation, and table transmission functions. The high-end PLC machine has stronger communication networking function, can be used for large-scale process control or constitute a distributed network control system to achieve factory automation.

3. Sort by I/O points

According to the number of PLC I/O points, PLC can be divided into small, medium and large categories.

(1) Small PLCs Smaller PLCs with 256 or less I/O points are small PLCs. Among them, an I/O point less than 64 points is an ultra-small or micro PLC.

(2) Mid-sized PLCs with medium-sized PLC I/O points of 256 points or more and 2048 points or less.

(3) Large-scale PLC I/O points with a number of 2048 or more are large PLCs. Among them, an I/O point exceeding 8192 points is a very large PLC.

In practice, the strength of the general PLC function is related to the number of I/O points. That is, the stronger the PLC function, the more configurable I/O points. Therefore, usually referred to as the small, medium, large PLC, in addition to its I / O points are different, but also means that the corresponding function is low, mid-range, high-end.

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