The PLC control cabinet is essentially a computer dedicated to industrial control, and its hardware structure is basically the same as a microcomputer.

Basic structure of PLC control cabinet

The PLC control cabinet is essentially a computer dedicated to industrial control, and its hardware structure is basically the same as a microcomputer.

1. Central processing unit (CPU)

The central processing unit (CPU) is the control center of the PLC control cabinet. It receives and stores the user program and data typed from the programmer according to the functions given by the PLC system program: checks the status of power supply, memory, I/O and warning timer, and can diagnose syntax errors in the user program. When the PLC is put into operation When running, it first receives the status and data of each input device on site in a scanning manner, and stores them in the I/O image area respectively. Then it reads the user program one by one from the user program memory, and executes it according to the instructions after interpreting the commands. The results of logical or arithmetic operations are sent to the I/O image area or data register. After all user programs are executed, each output status of the I/O image area or the data in the output register is transferred to the corresponding output device, and this cycle continues until the operation stops.

In order to further improve the reliability of PLC control cabinets, in recent years, dual CPUs have been used to form a redundant system for large PLCs, or a three-CPU voting system has been adopted. In this way, even if one CPU fails, the entire system can still operate normally.

2. Memory

The memory that stores system software is called system program memory.

The memory that stores application software is called user program memory.

(1) Commonly used memory types in PLC

1)RAM (Random Assess Memory)

This is a read/write memory (random access memory) with fast access speed and is supported by lithium batteries.

2)EPROM (Erasable Programmable Read Only Memory)

This is a type of erasable read-only memory in which all contents of the memory remain unchanged when the power is turned off. (Memory contents can be erased under continuous UV irradiation)

3)EEPROM (Electrical Erasable Programmable Read Only Memory)

This is an electrically erasable read-only memory. The stored content can be easily modified using a programmer.

(2) Allocation of storage space in PLC control cabinet

Although the large addressing space of the CPU of various PLCs is different, according to the working principle of the PLC, its storage space generally includes the following three areas:

System program storage area

System RAM storage area (including I/O image area and system software devices, etc.)

User program storage area

1) System program storage area

The system program equivalent to the computer operating system is stored in the system program storage area. Including monitoring programs, management programs, command interpreters, functional subroutines, system diagnostic subroutines, etc. It is solidified in EPROM by the manufacturer and cannot be accessed directly by the user. Together with the hardware, it determines the performance of the PLC.

2) System RAM storage area

The system RAM storage area includes the I/O image area and various soft devices, such as:

Logic coil

Data register

Timer

Counter

Index register

Accumulator

Waiting for memory