What is a PLC, how does it work and what is it used for?
A PLC (programmable logic controller) is an industrial computer that reads sensors and commands, executes a control program and activates motors, valves, cylinders, alarms and other actuators.
What is a PLC or programmable logic controller?
Un PLC is a programmable industrial control device. It receives signals from sensors and operator commands, executes defined logic and controls the outputs that operate the machine or process. The acronym comes from the English term programmable logic controller, and TERMCAT lists the Catalan term programmable logic controller.
The modern PLC concept emerged in the late 1960s as an electronic alternative to large hardwired relay panels, which had to be rewired to change a sequence. Schneider Electric traces the origin of the Modicon 084 to 1968, developed in response to the need to replace this rigid logic with reprogrammable control.
Its role is not limited to simply “switching on and off”. A PLC can manage sequences, timing, counting, analog control, recipes, alarms, communications, positioning, traceability and diagnostics, provided the model, modules and software are suitable for the application.
The family of standards IEC 61131 places these controllers and their peripherals within the control and command of machinery, automated manufacturing and discrete, batch or continuous industrial processes.
What is a PLC used for in an industrial installation?
The PLC is the point where the physical conditions of the process become control decisions. It receives information, compares it with the programmed logic, generates commands and continuously supervises the system.
From sensor to actuator, with a programmed decision in between
The PLC function can be summarized in four blocks that repeat during operation.
Physical world
The controller receives the actual state of the installation.
Control decision
The CPU executes the program, updates memory, checks conditions and coordinates tasks.
Physical action
Decisions become commands applied to the process.
What parts make up a PLC?
A compact PLC can combine almost all functions in a single enclosure. In a modular system, those same functions are distributed among a power supply, CPU, input and output modules, communications and specialized modules.
Power supply
Converts and distributes the power required by the controller. In many projects, the field power supply for sensors and actuators is sized separately.
CPU and memory
The CPU executes the program and manages data, tasks, timing, communications and diagnostics. Memory stores the project, variables and retentive information according to the configuration.
Inputs and outputs
The I/O adapts process signals to the controller electronics. It can be integrated, local, remote, digital, analog or special-purpose.
Bus, rack and communications
The internal bus connects the modules. Communication interfaces integrate HMI, drives, distributed I/O, robots, SCADA and data systems.
Technology modules
Encoder resolution, high-speed counting, positioning, temperature control, weighing, motion, special communications or other functions not covered by basic I/O.
Engineering software
Allows you to configure the hardware, write the program, download it, monitor variables, force signals, diagnose faults and document the system.
How does a PLC scan cycle work?
The PLC repeats the control process continuously. The general idea is to update process information, execute the logic, apply the results and handle system functions. The exact details depend on the platform, tasks, interrupts and how the I/O is updated.
Follow a cycle step by step
The visualization shows a teaching sequence. Some manufacturers describe the internal order differently, and certain I/O or tasks may be updated asynchronously or by interrupt.
Update inputs
The controller loads into memory the states or values that the program will use, according to its I/O architecture.
Execute the program
Instructions, blocks and tasks process the data and calculate the new control commands.
Update outputs
The calculated results are applied to the output channels or sent to the corresponding devices.
System services
Diagnostics, communications, task management, timer updates and other internal activities.
The total response time is not just the program execution time: sensor filtering, the I/O cycle or bus, task scheduling, communication and actuator response time also contribute.
What are PLC inputs and outputs?
Inputs tell the PLC what is happening. Outputs transmit the PLC’s decisions to the process. Before selecting modules, you need to know the signal type, voltage or current, isolation, speed, load and wiring diagram.
Inputs
Information to the PLCThey capture operator commands, machine states, presence, position, temperature, pressure, level, flow or data from other equipment.
0 / 10–10 V · 4–20 mATC · Pt100pulsesOutputs
Commands from the PLCThey command switching devices or transmit setpoints. The output type must be suited to the load, switching frequency and required protection.
relay · transistor0–10 V · 4–20 mAPTO · PWMindustrial busCheck voltage, current, inrush current, resistive or inductive load characteristics, switching frequency, protections and electrical life. Interface relays, contactors, protection modules or drives are often used.
How is a PLC programmed?
IEC 61131-3 standardizes the syntax and semantics of the main controller programming languages. The fourth edition, published in May 2025, defines Structured Text, Ladder Diagram and Function Block Diagram, and retains Sequential Function Chart elements for structuring programs and blocks.
Four ways to represent logic
Select an option to see the approach and a simplified example.
Ladder Diagram: logic inspired by relay schematics
It is especially readable for Boolean logic, interlocks, permissives, timers and control-circuit diagnostics.
Function Block Diagram: functions connected by signal flow
Represents the program using blocks, inputs and outputs. It is common in control loops, signal processing, timers and reusable functions.
Temperature ─────┐
├──► [ PID ] ───► Drive_Setpoint
Setpoint ─────────┘
Permissive ─────────────► [ AND ] ───► Enable_Control
No_Alarm ───────► [ ]Structured Text: high-level textual language
It is well suited to calculations, data manipulation, iterations, recipes, algorithms and logic that would be very extensive in graphical form.
(* Pump command with process permissives *)
IF Start_PB AND Level_OK AND NOT Overload THEN
Pump_Command := TRUE;
ELSIF Stop_PB OR Overload THEN
Pump_Command := FALSE;
END_IF;Sequential Function Chart: structure based on steps and transitions
It helps describe sequential processes: each step activates actions and transitions determine when the process advances to the next step.
[ STEP 0 · Wait ]
│ Start AND Condicions_OK
▼
[ STEP 1 · Clamp part ]
│ Clamp_Sensor
▼
[ STEP 2 · Process ]
│ Cycle_Finalitzat
▼
[ STEP 3 · Release ] ───► return to waitWhat equipment does a PLC communicate with?
A modern PLC is usually part of a broader architecture. It can exchange commands, states, alarms, recipes and production data with field devices, supervisory systems and management platforms.
Protocol compatibility is a project decision
Not all PLCs include the same ports or protocols. Some integrate them into the CPU; others require modules, gateways, licenses or specific configurations.
What types of PLC are there?
The categories overlap and manufacturers use their own terminology. The following classification helps explain the scale, architecture and primary function of the controller.
Micro or compact
Integrates a CPU and a defined number of I/O points in a small footprint. It is common in small machines, auxiliary services and standalone automation.
- Lower cost and compact implementation
- Limited or moderate expansion
- Programming and communications depend on product range
Modular
Allows CPUs, power supplies, I/O, communications and technology modules to be combined. It makes scalability and adaptation to medium or complex projects easier.
- Tailored configuration
- Greater expansion capacity
- Advanced diagnostics and options
With distributed I/O
The CPU coordinates input and output stations located close to the process, reducing field wiring and enabling a decentralized architecture.
- Industrial bus or Ethernet
- Less point-to-point wiring
- Diagnostics by station and channel
Safety PLC
Certified controller for implementing the logical part of safety functions within an architecture designed and validated for the corresponding risk.
- Safety I/O and communications
- Diagnostics and fault-control measures
- Specific programming and validation
Redundant or high-availability
Uses redundant controllers, power supplies, networks or other components to reduce interruptions in processes where downtime has a high impact.
- Service continuity
- Synchronization and switchover
- Greater engineering complexity
Software PLC
The controller function runs on a compatible industrial computing or virtualized platform. Real-time performance, availability, cybersecurity and maintenance must be assessed.
- Integration with industrial computing
- Flexible deployment
- Platform and architecture dependency
Where are PLCs used?
Programmable controllers are used in discrete automation, batch processes, continuous control, infrastructure and buildings. Selection depends more on the function and risk than on the industry alone.
Machines and lines
Conveyors, assembly, handling, packaging, labeling, palletizing and cell coordination.
Water, food and chemicals
Pumping, dosing, temperature, pressure, flow, level, recipes, cleaning and batch sequences.
Auxiliary installations
Management of pump groups, compressors, industrial HVAC, utility panels and energy monitoring.
Buildings and transport
Ventilation, technical lighting, tunnels, water treatment, doors, conveyors, industrial lifts and support systems.
PLC, programmable relay, PAC, DCS or industrial PC?
The table summarizes the typical focus of each solution so you can compare flexibility, scalability, diagnostics and use context before defining the control architecture.
| Criterion | Hardwired relays | Programmable relay | PLC | PAC / advanced controller | DCS | Industrial PC / soft PLC |
|---|---|---|---|---|---|---|
| Primary focus | Simple contact logic | Small automation systems | Machine and process control | Multidisciplinary control and data | Distributed process control | Control and computing on a PC platform |
| Logic modification | Requires rewiring | Using basic software | Using an engineering environment | Advanced environment and libraries | Integrated plant engineering | Software, operating system or runtime |
| Scalability | Low | Low or moderate | From compact to high | High | Very high at process scale | Depends on hardware and architecture |
| Diagnostics | Visual and electrical | Basic | Broad, depending on product range | Advanced | Integrated at system level | Can be very extensive, with greater complexity |
| When it makes sense | Very simple and stable function | Few signals and limited logic | Most industrial automation systems | Motion, data or complex architecture | Distributed process plants | Vision, analytics, virtualization or PC-based control |
The terms PLC and PAC are used differently depending on the manufacturer. The decision should be based on actual capabilities: cycle time, tasks, memory, I/O, communications, motion, redundancy, safety, tools and support.
Why is a PLC used?
What does it not solve by itself?
Official sources
TERMCAT: programmable logic controller
Catalan term, PLC acronym and definition for use in industrial automation and monitoring.
View TERMCATIEC 61131-1: general information
Scope of PLCs and peripherals intended for the control and command of machines and industrial processes.
View IEC 61131-1IEC 61131-2: equipment and tests
Functional requirements, electromagnetic compatibility and tests for industrial control equipment.
View IEC 61131-2IEC 61131-3:2025
Fourth edition of the programming language standard: ST, LD and FBD, with SFC for structuring programs and blocks.
View IEC 61131-3PLCopen: status of IEC 61131-3
Public summary of the fourth edition, the current languages and changes from the previous edition.
View PLCopenSiemens S7-1200 G2 System Manual
Manufacturer documentation on cycle, processing, inputs, outputs, tasks and controller functions.
Open the manualOMRON: Technical Explanation for PLC
Explanation of CPU, memory, power supply, inputs, outputs, programming and actuator control.
Open the guideSchneider Electric: history of Modicon
Origin of the Modicon 084 as an electronic replacement for hardwired relay systems in the late 1960s.
View the official historyFrequently asked questions about PLCs
What does PLC stand for?
They stand for programmable logic controller. TERMCAT lists the Catalan term programmable logic controller.
What is the main function of a PLC?
To read process information, execute programmed logic and control outputs in order to automate and monitor a machine or installation.
Is a PLC a computer?
It is a computing device specialized in industrial control. Its architecture, execution system, I/O, diagnostics and environmental requirements are designed for machines and processes.
Can it operate without an HMI?
Yes. The PLC can execute control without a screen. The HMI makes operation, visualization, setpoints, alarms and diagnostics easier, but it is not required in every application.
What is the difference between a digital and an analog input?
A digital input typically represents two states. An analog input represents a continuous value within a range, such as 0–10 V or 4–20 mA, which is then scaled to a physical quantity.
Which IEC 61131-3 languages are current?
The fourth edition from 2025 defines Structured Text, Ladder Diagram and Function Block Diagram. It also defines Sequential Function Chart for structuring the internal organization of programs and blocks.
Does a PLC cycle always follow the same order?
Not necessarily. The conceptual model is reading, execution, writing and services, but the internal order, process image, interrupts and asynchronous updating depend on the controller.
When is a modular PLC appropriate?
When you need to expand I/O, add protocols, distribute signals, use technology modules, separate functions or prepare the system for future developments.
Can a PLC control a variable frequency drive?
Yes, using digital signals, analog setpoints or industrial communication, depending on the capabilities of the PLC and the drive.
Can a PLC replace all electrical protection devices?
No. Short-circuit, overload and earth-fault protection, isolation and safety functions require specific devices and architectures.
