What is PLC Programming? A Simple Explanation for Beginners
A beginner-friendly guide to PLC programming — what PLCs are, how they work, the five IEC 61131-3 programming languages, major manufacturers, and how PLC programming connects to electrical design in EPLAN.
What is PLC Programming? A Simple Explanation for Beginners
Every conveyor that moves, every motor that starts in sequence, every machine that stops safely when someone presses the emergency button — all of it is controlled by a PLC.
Programmable Logic Controllers are the brains of industrial automation. PLC programming is one of the most in-demand skills in engineering today. But for fresh engineers entering the field, the terminology, the programming environments, and the connection to the rest of the electrical system can feel overwhelming.
This guide explains PLCs and PLC programming clearly — from the hardware basics to the five standard languages — and shows you exactly how PLC programming connects to electrical design work.
What a PLC Actually Is (and What It Is Not)
The problem: Fresh engineers often conflate PLCs with regular computers or microcontrollers. The confusion leads to unrealistic expectations — and misunderstanding of why PLCs behave differently in an industrial environment.
What a PLC is: A PLC is an industrial computer purpose-built to control machines and processes in real time. Unlike a regular computer, it is designed to:
- Operate reliably in harsh environments — dust, heat, vibration, electrical noise
- Run continuously without crashes or scheduled restarts
- Respond to sensor inputs and control outputs in milliseconds
- Be programmed by engineers using graphical and structured languages, not traditional software development
What a PLC is not:
- A general-purpose computer — it does not run office software or connect to the internet by default
- A microcontroller — PLCs are industrial-grade, rated for 24/7 operation under harsh conditions
- A SCADA system — SCADA supervises multiple PLCs from a control room; the PLC does the actual control
The benchmark: A PLC scan cycle — one complete read-execute-write cycle — should complete in 1–20 milliseconds. If a machine requires faster response, a specialised real-time controller is needed instead.
How a PLC Works: The Scan Cycle
The problem: Engineers who do not understand the PLC scan cycle write programs that behave unexpectedly — outputs that seem to update randomly, timing that appears inconsistent, conditions that trigger once and then never again.
The three-step scan cycle:
READ Inputs → EXECUTE Program → WRITE Outputs → REPEAT-
Read Inputs — The PLC reads the current state of all connected sensors, switches, and signals. Is the door closed? Is the pressure above 5 bar? Is the start button pressed?
-
Execute Program — It runs through the entire user program and calculates the new state of all outputs based on current input states
-
Write Outputs — It applies the calculated states to all output devices — motors, valves, lights, alarms
This cycle repeats continuously, many times per second.
What this means for programming: Your program logic must account for the fact that inputs are only read at the start of each scan — not in real time during execution. Events that happen and resolve within a single scan may never be seen.
The Five IEC 61131-3 Programming Languages
The problem: Engineers who learn PLC programming from one vendor’s training often assume the programming language is proprietary to that vendor. In reality, IEC 61131-3 defines five standard languages that apply across all major PLC platforms.
1. Ladder Diagram (LD) — The most common starting point
Looks like electrical relay logic — horizontal rungs with contacts (inputs) and coils (outputs). Engineers who understand relay schematics find this the most intuitive language.
Best for: Discrete control, relay replacement logic, simple sequential control.
2. Function Block Diagram (FBD)
Graphical function blocks connected by signal lines. Standard blocks for PID controllers, timers, counters, and logic functions are pre-built.
Best for: Process control, analog signal processing, PID loops.
3. Structured Text (ST) — The most powerful language
Looks like Pascal or C programming. The most capable language for mathematical calculations, data processing, and complex algorithms.
IF Start_Button AND NOT Emergency_Stop THEN
Motor_Run := TRUE;
Speed_Setpoint := 1500;
END_IF;Best for: Complex calculations, string handling, data processing.
4. Instruction List (IL)
Low-level assembly-style language. Rarely used in modern projects — most PLC vendors are phasing it out.
5. Sequential Function Chart (SFC)
Visual flowchart language for sequential processes. Steps and transitions define the order of operations, making multi-step sequences clear and easy to audit.
Best for: Batch processes, washing cycles, filling sequences, multi-stage machine cycles.
The benchmark: Every automation engineer should be fluent in Ladder Diagram and Structured Text. Functional engineers who understand both can handle 95% of industrial automation requirements.
Major PLC Manufacturers and Their Platforms
The problem: Fresh engineers waste time learning a platform that is rarely used in the market they are targeting — and then find themselves at a disadvantage when applying for roles or working on client sites.
| Brand | Popular Platform | Programming Tool |
|---|---|---|
| Siemens | S7-1200, S7-1500 | TIA Portal |
| Allen-Bradley | CompactLogix, ControlLogix | Studio 5000 |
| Schneider Electric | Modicon M340, M580 | EcoStruxure Control Expert |
| Mitsubishi | MELSEC iQ-R, iQ-F | GX Works3 |
| Omron | NX, NJ series | Sysmac Studio |
| Beckhoff | CX series (PC-based) | TwinCAT 3 |
How PLC Programming Connects to Electrical Design
The problem: Engineering teams that treat electrical design (EPLAN) and PLC programming as separate disciplines create avoidable disconnects — I/O addresses that do not match terminal assignments, PLC programs that reference non-existent inputs, commissioning delays while engineers reconcile the two.
How the connection works:
- The electrical schematic in EPLAN defines all I/O connections — which sensor connects to which terminal on which PLC card
- The PLC I/O address list should come directly from the terminal plans and wiring designed in EPLAN
- Changes in EPLAN update the terminal diagrams and connection lists automatically
- Vidhasri Technology’s EPLAN automation tools can export structured I/O lists directly usable in Siemens TIA Portal and Allen-Bradley Studio 5000
Understanding both electrical design and PLC programming makes you the most valuable type of engineer in an automation team — the one who can trace a fault from the field sensor through the terminal strip through the PLC program without switching reference documents.
How Vidhasri Technology Bridges Electrical Design and PLC Integration
At Vidhasri Technology, we design the electrical systems that PLC programs run on — and we build the automation tools that connect EPLAN design data directly to PLC development environments.
Our services include:
- Electrical and Control System Design — Full EPLAN schematic design from panel power to PLC I/O wiring
- I/O List Generation — Automated export of structured I/O lists from EPLAN in TIA Portal and Studio 5000 compatible formats
- PLC Integration Support — Ensuring the electrical design and PLC program address lists are always in sync
- Automation Tool Development — Custom EPLAN add-ins and PLC integration utilities for engineering teams
Contact us to discuss your control system requirements — no commitment, just a conversation.
A Real-World Illustration
One of our clients — an automation integrator in the UAE — was spending two hours per project manually reconciling the EPLAN I/O terminal list with the PLC address assignment spreadsheet. Mismatches between the two were the most common cause of commissioning delays.
After a custom EPLAN export add-in that generated the PLC I/O list directly from the schematic, that reconciliation step was eliminated entirely. The PLC programmer imported the I/O list directly from EPLAN — same data, zero reconciliation.
The same engineering team. A connected workflow. Commissioning delays caused by I/O mismatches dropped to zero.
Ready to Start Your PLC Programming Journey?
Start with a free 30-minute consultation. Whether you are a fresh engineer planning your learning path or an engineering manager building your team’s PLC capability, we can give you a clear, practical roadmap.
Contact Vidhasri Technology — and start building the skills that automation teams in the UAE and GCC need most.
Summary: PLC Programming for Beginners
| Topic | Key Point | Where to Start |
|---|---|---|
| What a PLC is | Industrial computer for real-time machine control | Read the datasheet for Siemens S7-1200 |
| Scan cycle | Read inputs → Execute → Write outputs → Repeat | Simulate a simple I/O program in PLCSIM |
| IEC 61131-3 | 5 standard languages across all vendors | Start with Ladder Diagram |
| Best platform for UAE/GCC | Siemens TIA Portal | Download TIA Portal Trial + PLCSIM |
| Connection to electrical design | EPLAN I/O lists drive PLC address assignment | Learn to read terminal plans in EPLAN |