What Is a PLC? A Complete Beginner-to-Intermediate Guide

If you walk into any modern factory, one device silently controls almost everything —
motors, valves, conveyors, pumps, alarms, and interlocks.

That device is the PLC (Programmable Logic Controller).

Whether you are a student, maintenance engineer, or automation beginner, understanding PLCs is no longer optional — it’s a core skill.

This article explains PLCs in easy language, with clear logic, taking you from zero knowledge to solid understanding.

1. Why PLCs Exist (The Real Reason)

Before PLCs, industries used:

• Relays
• Timers
• Contactors
• Massive control panels

Problems with Relay Logic:

❌ Too many wires
❌ Difficult troubleshooting
❌ No flexibility
❌ Time-consuming changes.

Pic Courtesy- https://www.reddit.com/r/PLC/comments/1m7q85p/relay_logic/

• Industries needed something:
  ✔ Flexible
  ✔ Reliable
  ✔ Easy to modify
  ✔ Compact

  👉 That’s how PLCs were born.

2. What Is a PLC? (Simple Definition)

A PLC is an industrial computer used to:

• • Read inputs (switches, sensors)

• • Process logic (program)

• • Control outputs (motors, solenoids, valves)

In simple words:

PLC = Brain of industrial automation

3. Basic PLC Working Principle

PLC works in a continuous cycle, called the scan cycle.

PLC Scan Cycle:

1️⃣ Read Inputs
2️⃣ Execute Program
3️⃣ Update Outputs

This happens thousands of times per second.

So when a sensor changes, PLC reacts almost instantly.

4. Main Parts of a PLC

Every PLC has these core components:

4.1 Power Supply

• • Converts AC to DC

• • Supplies power to PLC electronics

4.2 CPU (Central Processing Unit)

This is the brain of the PLC. It includes

• • Executes logic

• • Performs calculations

• • Handles communication

• • Detects faults

4.3 Input Modules

These receive signals from field devices like:

• • Push buttons

• • Limit switches

• • Proximity sensors

• • Pressure switches

Inputs can be:

• • Digital (ON/OFF)

• • Analog (4–20 mA, 0–10 V)

4.4 Output Modules

These control field devices:

• • Contactors

• • Solenoid valves

• • Lamps

• • Relays

Outputs can also be:

• • Digital

• • Analog

5. Digital vs Analog Signals (Very Important)

http://youtube.com/watch?v=_20YjMi6p0c&list=PLPAl-AztVhyZHn4SAP4K8e-3mEaeJUHCS&index=3

Digital Signals:

• • Only two states → ON or OFF

• • Example: Motor start/stop

Analog Signals:

• • Continuous values

• • Example:

    • • 4–20 mA pressure signal
    • • 0–10 V temperature signal

PLC must be selected based on signal type requirements.

Want a video Learning- Please look at below video.

https://www.youtube.com/watch?v=ID_JvKYvE8Y&list=PLPAl-AztVhyZHn4SAP4K8e-3mEaeJUHCS&index=1&t=144s

6. PLC Programming Languages (IEC Standard)

PLC programming is standardized globally.

Common PLC Languages:

🔹 Ladder Logic (Most Popular)

• • Looks like electrical relay diagrams

• • Easy for electricians & beginners

• • Widely used in industries

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🔹 Function Block Diagram (FBD)

• • Block-based logic

• • Used in process automation

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🔹 Structured Text (ST)

• • Similar to programming languages

• • Used for complex calculations

👉 Start with Ladder Logic — always.

7. Basic PLC Ladder Logic Concepts

Understanding these makes PLC easy:

a) Contacts

• • NO (Normally Open)

• • NC (Normally Closed)

Used to represent input conditions.

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b) Coils

• • Output instruction

• • Turns devices ON/OFF

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c) Timers

• • TON (ON delay)

• • TOF (OFF delay)

Used for sequencing and delays.

d) Counters

Count operations or products

Up counter / Down counter

8. Example: Simple Motor Start-Stop Logic

Logic Description:

• • Start button → Motor ON

• • Stop button → Motor OFF

• • Motor stays ON using seal-in logic

This single example teaches:
✔ Contacts
✔ Coils
✔ Latching
✔ Real industrial logic

This is the foundation of PLC programming.

9. PLC Addressing (Beginner Friendly)

Each PLC uses addresses for I/O.

Example:

• • I0.0 → Input (Start button)

• • I0.1 → Input (Stop button)

• • Q0.0 → Output (Motor)

Addressing varies by brand:

• • Siemens

• • Allen-Bradley

• • Mitsubishi

• • Schneider

But concept remains same.

10.Communication in PLCs

PLCs communicate with:

• • HMIs

• • SCADA

• • Other PLCs

• • Drives & instruments

Common protocols:

• • Modbus

• • Profibus

• • Profinet

• • Ethernet/IP

Communication is a must-learn intermediate skill.

11.How to Learn PLC Step-by-Step

Recommended Path:

1️⃣ Basics of electricity
2️⃣ Digital logic
3️⃣ PLC hardware
4️⃣ Ladder logic
5️⃣ Timers & counters
6️⃣ Analog signals
7️⃣ Communication
8️⃣ SCADA integration

Practice matters more than theory.

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https://www.youtube.com/watch?v=aPeox5lvRic&list=PLPAl-AztVhyZHn4SAP4K8e-3mEaeJUHCS&index=49&t=8s