GE 531X121PCRALG1 output module

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The basic components of PLC

1. Central processing unit

The central processing unit (CPU) consists of a controller, arithmetic unit, and registers integrated into a single chip. The CPU is connected to memory, input/output interfaces, programmers, and power supplies through data bus, address bus, control bus, and power bus.

The CPU of small PLCs uses 8-bit or 16 bit microprocessors or microcontrollers, such as 8031, M68000, etc., which are very affordable; The CPU of medium-sized PLC adopts 16 bit or 32-bit microprocessors or microcontrollers, such as 8086, 96 series microcontrollers, etc. The main characteristics of these chips are high integration, fast calculation speed, and high reliability; However, large PLCs require the use of high-speed chip microprocessors.

The CPU commands the PLC control system to complete various work tasks according to the functions assigned by the PLC internal system program.

2. Memory

The memory inside the PLC is mainly used to store system programs, user programs, and data.

1) System program memory

The PLC system program determines the basic functions of the PLC. This part of the program is written by the PLC manufacturer and solidified in the system program memory, mainly including the system management program, user instruction interpretation program, and functional program and system program calls.

The system management program mainly controls the operation of the PLC, making it work in the correct order; The user instruction interpreter converts PLC user instructions into machine language instructions and transmits them to the CPU for execution; Functional programs and system program calls are responsible for calling different functional subroutines and their management programs.

The system program belongs to important data that needs to be stored for a long time, so its memory uses ROM or EPROM. ROM is a read-only memory that can only read content and cannot write content. ROM is non-volatile, meaning that it can still store stored content even after the power is disconnected.

EPEROM is an electrically erasable read-only memory that requires ultraviolet light to illuminate the lens window on the chip in order to erase the written content. Electrically erasable programmable read-only memory also includes E2PROM, FLASH, etc.

2) User program memory

The user program memory is used to store the PLC application programs loaded by the user. In the early stage of loading, the user program needs to be modified and debugged, so it is called the user debugging program. It is stored in the random access memory RAM that can be read and written randomly for convenient modification and debugging by the user.

The program that has been modified and debugged is called a user executed program. As there is no need for further modification and debugging, the user executed program is fixed in EPROM for long-term use.

3) Data storage

During the operation of PLC, intermediate result data (such as status data of input/output components, preset and current values of timers and counters, etc.) and configuration data (such as input/output configuration, setting input filtering, pulse capture, output table configuration, defining storage area retention range, analog potentiometer setting, high-speed counter configuration, high-speed pulse output configuration, communication configuration, etc.) need to be generated or called

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