What Is PLC? Introduction to Programmable Logic Controllers (2026)

What Is PLC? Introduction to Programmable Logic Controllers (2026) (Updated August 2026)

The AURIC manufacturing corridor is attracting ₹71,343 crore in industrial investment and creating 62,405 direct engineering jobs — and a Programmable Logic Controller (PLC) sits at the center of every automated production line running in those facilities. PLC engineers are among the most in-demand profiles at MIDC Pune, Waluj, Shendra, and Chakan industrial zones, with freshers earning ₹3.5–5 LPA and experienced professionals commanding ₹12–20 LPA at companies like Siemens, Bajaj Auto, and Endurance Technologies. This guide covers exactly what a PLC is, how it works, its 56-year history from the GM Model 084 to today's Siemens TIA Portal, and how ABC Trainings teaches it in Pune and Chhatrapati Sambhajinagar.

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What Is a PLC? (Simple Answer in Plain Words)

A PLC (Programmable Logic Controller) is a digitally operating electronic device — a combination of hardware and software — that controls industrial machinery, processes, and automation systems using a stored program. Instead of hardwired relay logic that required physical rewiring to change any function, a PLC stores its control logic as a software program that can be edited on a computer and downloaded in minutes. The PLC reads inputs from field devices (sensors, switches, push buttons), executes its ladder logic program, and drives outputs (motors, valves, lamps, contactors) — all in a continuous scan cycle running every 1–10 milliseconds. A modern PLC can control multiple digital inputs and outputs, analog signals, timers, counters, and communication networks simultaneously.

How Does a PLC Work — The Scan Cycle Explained

A PLC executes its control program in a continuous scan cycle: Step 1 — Input Scan: the PLC reads all digital and analog input terminals and stores the current values in the input image table (I memory). Step 2 — Program Execution: the CPU executes the ladder logic program from top to bottom, rung by rung, using the input image values to calculate new output values and stores them in the output image table (Q memory). Step 3 — Output Scan: the PLC writes the output image table values to the physical output terminals, energizing or de-energizing connected devices. Step 4 — Housekeeping: the PLC updates diagnostics, communication buffers, and resets the watchdog timer. The entire cycle repeats every 1–20ms — fast enough for most machine control applications.

History of PLC — From GM 1968 to Dick Morley's Microprocessor PLC

Industrial automation control began with manual relay panels in the 1950s — reliable but inflexible, requiring complete rewiring to change any control function. The first PLC was developed in 1968 by Bedford Associates for General Motors, which needed a more flexible replacement for relay logic on its automotive assembly lines. The Model 084, the first programmable controller, was installed at GM's Hydra-matic Division and could be reprogrammed without hardware changes. In 1969, Allen-Bradley introduced a hardware CPU with 1K of memory and 128 input points. Dick Morley, who developed the first microprocessor-based PLC in 1970 at his company Modicon, is universally recognized as the "Father of PLC." Today's PLCs from Siemens, Allen-Bradley, Mitsubishi, and Delta run programs of millions of instructions with Ethernet connectivity and remote diagnostics.

PLC Architecture — CPU, Power Supply, I/O Modules and Communication

A PLC consists of five main components: Power Supply (converts 230V AC or 24V DC supply to the internal voltages needed by the CPU and I/O modules), CPU Module (the processor — stores and executes the program, manages I/O, handles communication), Digital Input Module (DI — reads 24V DC signals from field devices, typically 16 or 32 inputs per module), Digital Output Module (DO — drives 24V DC or relay outputs, typically 16 outputs per module), and Analog Input/Output Modules (AI/AO — reads 4–20mA or 0–10V signals from instruments, drives analog control signals). The Siemens S7-300 Model 313C used in ABC Trainings' lab has: 24V DC supply, 24 digital inputs, 16 digital outputs, 5 analog inputs, 2 analog outputs, 64KB program memory, and USB/MPI communication for programming via STEP 7 or TIA Portal.

PLC vs Relay Logic — Why Industry Switched to Programmable Controllers

Relay logic (hardwired relay panels) was the standard control method in Indian factories through the 1980s — each control function was wired as a fixed circuit on a relay panel. Changing any function required physically rewiring the panel, which could take days. A single relay panel for a complex machine could have hundreds of relays, thousands of wires, and took weeks to troubleshoot a single fault. PLCs replaced relay panels because: a program change takes minutes, not days; the same hardware handles any control function; faults are diagnosed instantly with online monitoring; the program is documented and reproducible; and PLCs handle timers, counters, math, and communication that relay logic cannot. A 12-rung relay panel can be replaced by 12 rungs of ladder logic on a PLC worth ₹15,000 — with complete online diagnostic visibility.

PLC SCADA Training at ABC Trainings — Siemens Hardware, Batches and Fees

ABC Trainings runs a complete PLC and SCADA course at Wagholi (Pune), Hadapsar (Pune), CIDCO (Chhatrapati Sambhajinagar), and Osmanpura (Chhatrapati Sambhajinagar). Students learn on real Siemens S7-300 and S7-1200 hardware, programming in TIA Portal (ladder logic, function block, statement list), with labs covering input/output wiring, motor control, latching, interlocking, timers, counters, analog I/O, SCADA with WinCC, HMI design, and a final live plant simulation project. Weekday batches run Mon–Fri (2–3 hrs/day, 3–4 months); weekend batches run Sat–Sun. CMYKPY Maharashtra students receive ₹6,000–₹10,000/month government stipend during training. Call 7039169629 or WhatsApp 7774002496 for the next batch start date and syllabus.

FAQs

What is a PLC in simple words?

A PLC (Programmable Logic Controller) is a rugged industrial computer that controls machinery and automated processes. It reads signals from sensors and switches (inputs), runs a stored control program (ladder logic), and drives motors, valves, and lights (outputs) based on that program. Unlike a general-purpose computer, a PLC is designed to operate reliably in harsh factory environments — dust, vibration, extreme temperatures — and runs its program continuously in a deterministic scan cycle of 1–20ms.

Who invented the PLC and when was it first used in industry?

The first PLC was developed in 1968 by Bedford Associates (later Modicon) for General Motors Corporation, which needed a programmable replacement for hardwired relay logic panels on automotive assembly lines. The Model 084 was the first unit installed industrially. Dick Morley, who developed the first microprocessor-based PLC at Modicon in 1970, is universally called the "Father of PLC." By the 1980s, PLC manufacturers including Allen-Bradley, Siemens (via AEG), and Mitsubishi had established the product category that now runs virtually every automated factory worldwide.

What is the difference between PLC and relay logic?

Relay logic uses physical electromechanical relays hardwired in panels to implement control logic — changing any function requires physically rewiring the panel, which takes days or weeks. A PLC implements the same logic as a software program (ladder diagram) stored in memory — changing a function means editing the program on a laptop and downloading in minutes. PLCs also provide timers, counters, analog control, and communication that relay logic cannot implement without additional hardware. For any system requiring flexibility or future changes, PLC is always the better choice.

Which PLC does ABC Trainings use for training in Pune?

ABC Trainings uses Siemens S7-300 (CPU 313C) and Siemens S7-1200 PLCs for hands-on training, programmed using TIA Portal V15/V16. These are the same PLC families used at Bajaj Auto, Endurance Technologies, Forbes Marshall, and other major manufacturers in Pune's MIDC zones. Students learn both hardware wiring (I/O modules, field devices, power supply) and software programming (ladder logic, function blocks, HMI design in WinCC). Training is available at Wagholi, Hadapsar, CIDCO, and Osmanpura centres.