The core components of a PLC controller: the "body" and "brain" of a logic control system
Programmable Logic Controller (PLC) is the cornerstone of modern industrial automation. It is essentially a specialized computer system designed for harsh industrial environments to reliably execute control logic. Understanding its components is the foundation for mastering its working principle. A typical PLC system can be clearly divided into the following core parts, which work together to achieve closed-loop control of "perception decision execution":
Central processing unit:
Function: This is the "brain" and core command center of PLC.
Function: Responsible for executing user written control programs (ladder diagrams, instruction lists, functional block diagrams, etc.), performing logical operations, data processing, mathematical calculations, timing, counting, and other tasks. It reads the status of the input module, processes it according to program logic, and then updates the status of the output module. It also manages the internal communication, diagnosis, and operational modes (run, stop, program) of the system.
Key components: including microprocessor (CPU), system firmware (operating system), working memory (RAM, used to store runtime programs and data), and system bus interface.
Function: Serve as a "memory" system for PLC.
Function:
System Memory (ROM/Flash): Stores the firmware (operating system) of the PLC, responsible for startup, self-test, and basic function management. It is usually non erasable or erasable but non-volatile (not lost when powered off).
User program memory (usually Flash or battery backed RAM): stores control programs downloaded by users. It is necessary to ensure that the program will not be lost (non-volatile) after the PLC is powered off.
Working memory (RAM): The "workbench" where CPUs run programs. Used for storage:
Input/output mapping area (stores the current state of the input signal and the state to be output).
The intermediate calculation result of the program.
The current values of timers and counters.
Data registers (storing variables, parameters, etc.).
Data storage (usually Flash or battery backup RAM): used to store data that needs to be kept for a long time, such as formulas, set values, historical records, etc. (non-volatile).
Input/output module:
Function: This is the "sensory" and "hands-on" interaction between PLC and the external physical world (sensors, actuators, switches, buttons, etc.).
Function:
Input module: receives electrical signals (digital ON/OFF or analog such as 420mA/010V) from on-site devices (such as limit switches, photoelectric sensors, temperature transmitters, pressure sensors, buttons). The module processes these external signals through filtering, isolation, and level conversion, and converts them into safe and standard logic level signals (usually 5V or 24V DC) that can be recognized by the internal CPU of the PLC. The CPU periodically scans these input states and updates the input image area.
Output module: Receive output instructions calculated by the CPU based on program logic (stored in the output image area). The module converts the low-level logic signal of the CPU into the power level signal (such as 24V DC, 120/240V AC, or 420mA analog signal) required to drive on-site execution devices (such as contactor coils, relays, solenoid valves, indicator lights, motor drivers, regulating valves). Output modules typically also provide electrical isolation to protect PLCs.
Types: Divided into digital input/output (DI/DO), analog input/output (AI/AO), and special functional modules (such as high-speed counters, pulse outputs, motion control, communication protocol conversion, temperature measurement RTD/TC, etc.).
Power module:
Function: The "lifeline" and "energy center" of the PLC system.
Function: Convert and stabilize external AC power (such as 120/240V AC) or DC power (such as 24V DC) into low voltage, pure DC working power (usually 5V DC, 3.3V DC, ± 12V DC, 24V DC) required by various components inside the PLC (especially the electronic circuits of the CPU, memory, and I/O modules). High quality power modules provide voltage stabilization, filtering, overvoltage/overcurrent protection, and have certain anti-interference capabilities to ensure stable and reliable operation of the system. Sometimes isolated 24V DC power supply is also provided for field devices on some I/O modules.
Programming devices and communication interfaces:
Function: PLC's "programming window" and "communication bridge".
Function:
Programming equipment: typically a computer (PC, laptop) or handheld programmer installed with specialized programming software (such as Siemens TIA Portal, Rockwell Studio 5000, Schneider EcoStruxure Machine Expert, Mitsubishi GX Works, etc.). Used for writing, editing, compiling, and debugging user control programs, and downloading the programs to the CPU of the PLC. It can also monitor the real-time running status of the PLC (variable values, diagnostic information).
Communication interface: This is the physical and logical channel through which PLC exchanges information with the external world.
Programming port: used to connect programming devices (usually USB, Ethernet, or RS232/RS485).
Communication port/module: used for communication with other devices, such as:
Human Machine Interface (HMI), Supervisory Control and Data Acquisition (SCADA) system.
Other PLCs or intelligent devices (frequency converters, robots, instruments).
Upper computer management system (MES/ERP).
Supports multiple industrial network protocols, such as Ethernet/IP, PROFINET, Modbus TCP/RTU, CANopen, CCLink, etc.
Expansion bus: used to connect local or remote expansion I/O racks (such as backplane bus, Profibus DP, DeviceNet, EtherCAT, etc.).
Overview of Collaborative Workflow
Perception: The input module continuously collects the status of on-site sensors/switches (on/off, pressure value, temperature value, etc.) and converts it into digital signals.
Reading and processing: At the beginning of each scanning cycle, the CPU reads the status of the input module and copies it to the input image area.
Decision: The CPU performs logical operations, data processing, and other operations based on the state of the input image area and the control program stored in the user program memory.
Update output: The CPU writes the calculation result to the output image area.
Execution: At the end of each scanning cycle, the CPU sends the status of the output image area to the output module. The output module drives the corresponding actuators (such as opening valves, starting motors, and lighting indicator lights).
Communication: The CPU simultaneously processes communication requests (uploading/downloading programs, monitoring data) from programming devices or network devices.
Loop: The PLC repeats the scanning cycle of "read input>execute program>update output" repeatedly (usually a few milliseconds to tens of milliseconds).
In summary, a PLC controller is a compact and powerful industrial control computer driven by a central processing unit (CPU), relying on memory to store programs and data, sensing and controlling the physical world through input/output modules, providing stable energy through power modules, and utilizing programming equipment and communication interfaces for human-machine interaction and system integration. Each part is indispensable and together forms a solid foundation for automation control.