Comprehensive Guide to PLC Fundamentals: Architecture, Operation & Selection Criteria
Comprehensive Guide to PLC Fundamentals: Architecture, Operation & Selection Criteria
Core PLC Components & Interfaces
Beyond CPU, memory, and communication ports, PLCs feature critical industrial interfaces:
Input Interfaces
Receive signals from field devices via opto-couplers and input circuits.
Electrically isolates and conditions sensor/logic signals (e.g., switches, sensors).
Output Interfaces
Execute control commands through opto-couplers and output components:
Relays: Handle AC/DC loads (≤2A), slower response (10ms)
Transistors: DC loads only, high-speed switching (0.2ms)
Thyristors: AC loads only, medium speed (1ms)
Structural PLC Classifications
| Type | Characteristics | Use Cases |
| Unitary | Integrated CPU, I/O, power supply | Compact systems |
| Modular | Customizable rack-mounted modules | Medium/large systems |
| Stackable | Hybrid design; modular components with cable links | Space-constrained apps |
PLC Operating Principles
Scan Cycle Workflow
Internal Processing (Diagnostics)
Communication Services (Data exchange)
Input Sampling (Read all inputs)
Program Execution (Run logic)
Output Refresh (Update actuators)
Scan duration depends on:
CPU speed (µs/instruction)
Program complexity
I/O module count
I/O Response Lag Solutions
Direct I/O access modules
Interrupt-driven processing
High-speed counters (>100kHz)
Key Selection Criteria
Hardware Configuration
Structure: Unitary for simplicity vs modular for scalability
I/O Modules: ≥20% spare capacity for future expansion
Memory Requirements
Estimate: (I/O points × 10) + (Timers × 5) = Minimum program steps
Specialized Modules
Analog I/O (4-20mA, ±10V)
Motion control (stepper/servo)
Communication (PROFINET, EtherCAT)
Technical FAQ
Q: What defines a PLC?
*A: Industrial digital computer with programmable memory for:
Logic/sequence control
Real-time I/O management
Continuous process automation*
Q: Critical internal relays?
Input/Output Relays (X/Y)
Auxiliary Relays (M)
Timers (T), Counters (C)
Data Registers (D)
Q: Contactor operation?
Electromagnetic coil energizes → Movable contacts close
Arc chutes suppress sparks during disconnection
Industry Outlook
"Modern PLCs bridge legacy relay logic with Industry 4.0 through deterministic control, cybersecurity features, and IIoT integration. Their evolution continues to democratize industrial automation."
— Industrial Automation Specialist