PLC Foundation Roadmap
Use this page for familiarization: recognize the component, understand its purpose, and learn where it fits in a PLC control system.
1. Identify the PLC
- CPU / processor
- Power supply
- Digital and analog I/O
- Communication and expansion
2. Follow the Signal
- Input: sensor or switch
- PLC: program and logic
- Output: actuator or controlled device
3. Recognize the Panel
- Relays and contactors
- Overloads and breakers
- Terminal blocks
- 24 VDC control power
4. Learn the Language
- NO / NC contacts
- Coils and latches
- Timers and counters
- Analog signals and scaling
FIELD INPUT→
PLC INPUT→
PROGRAM / LOGIC→
PLC OUTPUT→
FIELD DEVICE
PLC Hardware & Architecture
Symbol & Schematics Reference
Foundation reference for recognizing PLC hardware, ladder instructions, field devices, actuators, motor-control components and common industrial signals. Touch or click any item for a simple explanation.
Ladder Logic & PLC Instructions
Switches, Sensors & Field Inputs
Outputs, Actuators & Controlled Equipment
Relays, Motor Control & Electrical Protection
Complete PLC Essentials
Core concepts every automation and electrical engineer must know.
The PLC Scan Cycle
- 1. Read Inputs: Reads physical input terminal voltages into Input Image RAM.
- 2. Execute Program: Evaluates ladder logic top-to-bottom, left-to-right.
- 3. Diagnostic & Comm: Checks memory integrity and network communication.
- 4. Update Outputs: Writes computed output memory to physical field terminals.
Sourcing vs. Sinking (DC)
- Sinking (NPN): Connects load to 0V. Current flows into the module.
- Sourcing (PNP): Connects load to +24VDC. Current flows out of the module.
- Rule: Sourcing outputs connect directly to Sinking inputs.
Industrial Protocols
- EtherNet/IP: Standard Ethernet for Allen-Bradley/Rockwell systems.
- PROFINET: High-speed Industrial Ethernet standard for Siemens S7.
- Modbus TCP/RTU: Universal open protocol for industrial meters and VFDs.
IEC 61131-3 Standard
- LAD (Ladder Diagram): Relay ladder-based visual programming.
- FBD (Function Block): Connected block diagrams for continuous process control.
- ST (Structured Text): High-level text programming for algorithms and data processing.