There are many types of PLC products, and their specifications and performance also vary. PLCs are usually classified according to their different structural forms, differences in functions, and the number of I/O points.
2.1 Classification by structural form
According to the structural form of PLC, PLC can be divided into two categories: integral type and modular type.
(1) Integral PLC
The integral PLC is a centralized power, CPU, I/O interface and other components in a chassis, which has the characteristics of compact structure, small size and low price. Small PLCs generally adopt this integral structure.
The integral PLC is composed of a basic unit (also known as a host) and an expansion unit with different I/O points, and the basic unit has a CPU, an I/O interface, an expansion port connected with the I/O expansion unit, and an interface connected with a programmer or EPROM writer. There is only I/O and power supply in the expansion unit, but no CPU.
The basic unit and the extension unit are generally connected by a flat cable. The integral PLC can generally be equipped with special functional units, such as analog units, position control units, etc., so that its functions can be expanded.
(2) Modular PLC
Modular PLC makes each component of PLC into several individual modules, such as CPU module, I/O module, power module (some are included in CPU module) and various functional modules. The modular PLC is composed of a frame or substrate and various modules, and the modules are mounted on the socket of the frame or substrate, as shown in the figure.
This modular PLC is characterized by flexible configuration, which can be configured with different sizes of systems as needed, and is easy to assemble for expansion and maintenance. Large and medium-sized PLCs generally adopt a modular structure.
There are also PLCs that combine the characteristics of integral and modular to form the so-called stacked PLC.
The CPU, power supply, and I/O interfaces of stacked PLCs are also independent modules, but they are connected by cables, and each module can be stacked layer by layer. In this way, not only can the system be flexibly configured, but it can also be made small.
2.2 Classification by function
According to the different functions of PLC, PLC can be divided into three categories: low-grade, mid-range and high-end.
(1) Low-grade PLC
Low-grade PLC has basic functions such as logic operation, timing, counting, shifting, self-diagnosis, monitoring, etc., and can also have a small number of analog input/output, arithmetic operation, data transmission and comparison and communication functions, mainly used for logic control, sequence control or a small amount of analog control system stand-alone control system.
(2) Mid-range PLC
In addition to the functions of low-grade PLC, the mid-range PLC also has strong functions such as analog input/output, arithmetic operation, data transmission and comparison, number system conversion, remote I/O, subroutine and communication networking. Some of them can also be equipped with interrupt control, PID control and other functions, which are suitable for complex control systems.
(3) High-grade PLC
In addition to the functions of mid-range PLC, high-end PLC also adds the operation of symbolic arithmetic operation, matrix operation, bit logic operation, square root operation and other special functional functions, tabulation and table transmission functions.
High-grade PLC has stronger communication networking function, which can be used for large-scale process control or to form a distributed network control system to realize factory automation.
2.3 Classification by I/O points
According to the number of I/O points of the PLC, the PLC can be divided into three categories: small, medium and large.
(1) Small PLC
The compact PLC has less than 256 I/O points, a single CPU, an 8-bit or 16-bit processor, and a user memory capacity of 4KB or less.
(2) Medium-sized PLC
Medium-sized PLCs have I/O points ranging from 256 to 2048, with dual CPUs and user memory capacity of 2 to 8 KB.
(3) Large PLC
Large PLCs have more than 2048 I/O points, multiple CPUs and 16-bit or 32-bit processors, and user memory capacities of 8-16KB.
In the world, PLC products can be divided into three major schools by region, one is United States products, one is European products, and the other is Japan products.
United States and European PLC technologies are independently researched and developed in isolation from each other, so there are obvious differences between United States and European PLC products.
Japan's PLC technology was introduced by the United States, and there is a certain inheritance to United States' PLC products, but Japan's main products are positioned on small PLCs. United States and Europe are known for large and medium-sized PLCs, while Japan is known for small PLCs.
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