11. Composition of PLC Systems

Composition of PLC Systems

1. Core Concept: Understanding PLC Systems through Human Metaphor

Human Organ	PLC Component	Functional Correspondence
🧠 Brain	Central Processing Unit (CPU)	Center for system thinking and decision-making
💭 Memory	Memory	Stores system programs and user programs
👀👋 Senses and Limbs	Input/Output Interfaces	Perceives the environment and executes actions
❤️ Heart	Power Module	Supplies energy to the system
📚 Educational Tools	Programming Devices	Imparts knowledge instructions to the system
🌐 Neural Network	Communication Interface	Channel for internal and external information transfer

2. Detailed Composition of PLC System Hardware

1. Central Processing Unit (CPU) – 🧠 System Brain

Core Functions:

Program Execution: Cycles through user control programs according to a predetermined scan periodExample: Just as the brain processes billions of neural signals per second, the PLC’s CPU scans programs at millisecond speeds
System Diagnostics: Monitors the status of each module in real-time to ensure stable system operation
Coordinated Management: Oversees and schedules all hardware modules to work together

Performance Indicators:

Scan Period: Typically 1-100 milliseconds (higher-end PLCs have faster scan speeds)
Memory Capacity: Determines program complexity and data processing capability

2. Memory – 💭 System Memory

Dual Storage Structure:

Storage Type	Functional Analogy	Characteristics	Practical Application Example
System Memory	Operating System (e.g., Windows)	Factory-fixed, user read-only	Maintains basic operational functions of the PLC
User Memory	Personal Hard Disk Data	Read-write, stores user programs	Stores device control logic and parameters

User Memory Breakdown:

Program Area: Stores control logic such as ladder diagrams and instruction listsExample: The action flow of a packaging machine – Detect object → Robotic arm grabs → Positioning → Packaging
Data Area: Records real-time device status dataExample: Records production line output, equipment running time, temperature set values, etc.

3. Input/Output Interfaces – 👀👋 Senses and Limbs

Input Interfaces (Sensing Organs):

Function: Converts field signals into PLC-recognizable signalsExamples:Button pressed → 24V DC signal → PLC recognizes as “Start Command”Temperature sensor → 4-20mA signal → PLC reads specific temperature value
Electrical Isolation: Uses opto-isolation to effectively prevent field interference

Output Interfaces (Executing Limbs):

Comparison of Three Output Methods:

Output Type	Characteristics	Applicable Scenarios	Practical Cases
Relay Output	Universal for AC and DC, limited lifespan	Devices with low action frequency	Control indicator lights, alarms
Transistor Output	Fast switching speed, long lifespan	High-frequency pulse control	Stepper motor control, high-speed counting
Thyristor Output	Suitable for AC large loads	High-power devices	Heating element control, large motor start/stop

4. Power Module – ❤️ System Heart

Energy Conversion Function:

Input: AC 220V or DC 24V
Output: Stable DC 5V/24V (internal working voltage of PLC)
Key Role: Ensures stable operation of the system during power grid fluctuations

5. Programming Devices – 📚 Knowledge Imparting Tools

Modern Programming Methods:

Software Platforms: TIA Portal (Siemens), Studio 5000 (Rockwell)
Programming Languages: Ladder Diagram (main), Function Block Diagram, Structured Text, etc.
Online Monitoring: Real-time viewing of program running status, quick fault diagnosis

6. Rack/Backplane – 🦴 Skeleton and Blood Vessels

Core Support of Modular PLC:

Provides module slots and installation base
Backplane bus enables high-speed data exchange between modules
Supports hot swapping (high-end PLCs support module replacement without power interruption)

3. Composition of PLC System Software

1. System Program – Operating System Layer

Stored in system memory, includes:

System Diagnostic Module
Instruction Interpreter
Communication Driver Management
Input Output Scheduler

2. User Program – Application Logic Layer

Engineers write according to process requirements, typical programming languages:

Ladder Diagram Example:

[Start Button]—-[Stop Button]—-(Motor Contactor) | | |–[Self-locking Contact]–|

Explanation: Achieves self-locking control of the motor, the motor runs after pressing the start button until the stop button is pressed

4. Detailed Explanation of PLC Working Process

Cycle Scanning Working Mode

Step 1: Input Sampling (Sensing Phase)

CPU reads the status of all input points
Like human senses collecting environmental information simultaneously
Converts physical signals into memory data

Step 2: Execute User Program (Thinking and Decision Making)

Scans user program line by line
Performs logical operations based on input status
Updates the status of the output image area

Step 3: Output Refresh (Action Execution)

Transfers computation results to output modules
Drives external actuators to act
Like the brain directing limbs to execute actions after making decisions

Step 4: Internal Processing (Self-diagnosis and Maintenance)

System self-diagnosis
Communication processing
Prepares for the next scan cycle

Real-time Assurance

Scan Period: Typical value 1-100ms
Determinism: Ensures all operations are completed within a fixed time
Priority Management: Urgent tasks can interrupt normal scanning

5. Explanation of Practical Application Cases

Case: Automatic Filling Production Line

Input Signals (Sensing): Bottle detection sensor (bottle present/absent), emergency stop button status, liquid level detection signal
CPU Processing (Decision): Determines whether filling conditions are met, calculates filling time, coordinates the action sequence of each actuator
Output Control (Execution): Controls the opening and closing of the filling valve, drives the conveyor motor, triggers the labeling machine