Basic Structure Of PLC

Programmable logic controller is essentially a computer dedicated to industrial control. Its hardware structure is basically the same as that of a microcomputer. The basic composition is shown in Figure 1. The basic composition is described in detail as follows:
Power supply
The power supply is used to convert AC power into DC power required by the PLC. Most PLCs are powered by switching regulated power supplies.
Central processing unit
The central processing unit (CPU) is the control center of the PLC and the core component of the PLC. Its performance determines the performance of the PLC.
The central processing unit consists of a controller, an arithmetic unit, and registers. These circuits are concentrated on a chip and connected to the input/output interface circuit of the memory through the address bus and the control bus. The function of the central processing unit is to process and run user programs, perform logical and mathematical operations, and control the entire system to coordinate it.
Memory
The memory is a semiconductor circuit with memory function. Its function is to store system programs, user programs, logical variables and other information. Among them, the system program is a program that controls the PLC to implement various functions. It is written by the PLC manufacturer and solidified into the read-only memory (ROM), which cannot be accessed by users.
Input unit
The input unit is the input interface between the PLC and the controlled device. It is the bridge for the signal to enter the PLC. Its function is to receive the signal from the main command element and the detection element. The input types include DC input, AC input, and AC/DC input.
Output unit
The output unit is also the connecting component between the PLC and the controlled device. Its function is to transmit the output signal of the PLC to the controlled device, that is, to convert the weak current signal sent by the central processor into a level signal to drive the actuator of the controlled device. The output types include relay output, transistor output, and thyristor output.
In addition to the above parts, PLC also has a variety of external devices depending on the model. Its function is to help programming, realize monitoring and network communication. Commonly used external devices include programmers, printers, cassette recorders, computers, etc.