A complete 20-section interactive guide to programmable logic controllers — control system architecture, the scan cycle, hardware and I/O wiring, ladder logic and structured text programming, SCADA and network integration, and 5 real-world P&ID applications. Every reference diagram gets its own full-size slide so labels, tables, and code stay legible.
Chapter 1 lays out the three-tier PLC control system architecture (supervisory, control, field) and applies it to a tank-level control use case. Chapter 2 covers PLC fundamentals — what a PLC is, why PLCs replaced relay logic, basic hardware architecture, and the three IEC 61131-3 programming languages. Chapter 3 walks through the five-phase scan cycle every PLC repeats continuously. Chapter 4 covers digital I/O wiring, including the sink (NPN) and source (PNP) conventions. Chapters 5 and 6 cover ladder logic and structured text programming in depth, with 14 worked examples between them — start/stop circuits, interlocks, timers, counters, latches, state machines, PID tuning, and signal scaling. Chapter 7 covers network topologies, SCADA system architecture, HMI screen design, and standardized plant data models. Chapter 8 closes with 5 real-world P&ID diagrams spanning conveyor systems, tank level control, traffic signal control, packaging sorting, and HVAC air handling.
Use the Prev / Next buttons at the bottom, or press the arrow keys on your keyboard. Click the ☰ menu button in the top-right to open the table of contents and jump to any section. Each chapter's reference diagram(s) follow immediately after its text as their own dedicated slide, so you can view the full diagram at a readable size before moving to the next chapter.
Engineering students and technicians learning PLC fundamentals and ladder logic for the first time, automation and controls engineers wanting a structured refresher on I/O wiring and structured text, and anyone preparing for an industrial controls role who needs a visual, example-driven reference rather than a vendor manual.
The guide covers 8 chapters: PLC-based control system architecture, PLC fundamentals and why PLCs replaced relay logic, the scan cycle, I/O module wiring, ladder logic programming, structured text programming (IEC 61131-3), communications and SCADA integration, and 5 real-world P&ID applications.
The scan cycle is the continuous loop every PLC repeats: an input scan reads all field inputs, program execution runs the user logic using that data, an output update writes the results to physical outputs, housekeeping tasks run diagnostics and communications, and the cycle repeats — typically every 1–10 milliseconds.
In sink (NPN) wiring, a digital input module's common is tied to 0V, and current flows into the PLC when a field device turns on. In source (PNP) wiring, a digital output module's common is tied to +24V, and the PLC sources power out to the load. The two conventions must match between the PLC module and the connected field device.
Ladder logic (LD) is a graphical IEC 61131-3 language modeled on relay schematics, best suited to discrete on/off interlocking. Structured text (ST) is a high-level, Pascal-like text language best suited to math-heavy or algorithmic logic like PID tuning, state machines, and analog signal scaling.
A PLC is built for industrial control with high I/O capacity and harsh-environment reliability, typically programmed in ladder logic. A microcontroller is a general-purpose embedded device with lower I/O capacity, typically programmed in C/C++. A DCS (Distributed Control System) is built for very large plants with very high I/O capacity, using function-block programming across many distributed controllers.
Disclaimer: This guide summarizes general PLC and industrial control concepts for educational purposes only, using vendor-neutral diagrams and examples. Always consult the specific PLC manufacturer's documentation and applicable safety standards for actual system design and implementation.