PLC explained in plain language

A PLC (programmable logic controller) is an industrial computer that controls equipment according to a defined program. It reads signals from sensors, executes the logic an engineer has written, and issues commands to actuators: pumps, valves, drives, heaters. The PLC is the foundation of virtually any automation system. In this article, Atlas Scada engineers explain in plain language what a PLC is, how it works, and how to choose one.

Why a PLC is needed

Control logic used to be built with relay circuits: dozens of relays, timers, and contactors wired together. Any change to the algorithm meant reworking the circuit with a soldering iron and a screwdriver. The PLC replaced that hardwired logic with a program: to change how the system behaves, you change the code instead of rewiring. This is more reliable, more compact, and many times more flexible.

How a PLC works — the scan cycle

The controller runs cyclically, repeating the same scan cycle many times per second

• Reading inputs — polling the state of sensors and signals.
• Executing the program — computing the logic according to the defined algorithm.
• Writing outputs — issuing commands to the actuators.
• Housekeeping operations — communications, diagnostics, data exchange with SCADA.

One cycle (the scan cycle) typically takes a few to a few tens of milliseconds, so to the operator control appears instantaneous.

What a PLC consists of

• Central processing unit (CPU) — executes the program and stores it in memory.
• Input/output (I/O) modules — connect the controller to the field.
• Power supply.
• Communication interfaces — Ethernet, RS-485 (Modbus), USB.

Types of inputs and outputs

• Discrete (digital) inputs/outputs — on/off signals: a button pressed, a pump running, a valve open.
• Analog inputs/outputs — continuous values: temperature, pressure, 4–20 mA, 0–10 V signals.

The number and type of I/O points determine which controller suits the task, so they are counted as early as the design stage — with headroom for future growth.

How a PLC differs from an ordinary relay or microcontroller

• From a relay — by being programmable: the logic is changed in code rather than by rebuilding the circuit.
• From a consumer microcontroller (for example, Arduino) — by its industrial build: resistance to electromagnetic interference, temperature, and vibration; a service life of years of continuous operation; standard industrial interfaces and guaranteed support.

What languages PLCs are programmed in

The IEC 61131-3 standard defines several languages

• LD (Ladder Diagram) — relay-contact diagrams familiar to electricians.
• FBD (Function Block Diagram) — function blocks.
• ST (Structured Text) — a text-based language similar to Pascal.
• SFC, IL — for sequential processes and low-level tasks.

The development environments depend on the vendor: TIA Portal (Siemens), CODESYS and Owen Logic (Owen), EcoStruxure/Unity (Schneider Electric).

Popular PLC manufacturers

• Siemens — LOGO!, S7-1200, S7-1500.
• Owen — PLC110/160/210, PR200 (popular in the CIS, programmed in CODESYS).
• Schneider Electric — Modicon M221/M241/M340.
• Delta, Mitsubishi, Omron, and others.

How to choose a PLC — what to look at

• The number and type of inputs/outputs (discrete, analog) with headroom for growth.
• Processing power and memory capacity for the task.
• Communication interfaces (Modbus, OPC UA, Ethernet) for integration with SCADA.
• Operating conditions — temperature, humidity, dust, vibration.
• Availability and support on the Kazakhstan market, the presence of specialists and spare parts.
• Compatibility with the development environment and equipment already installed.

A common mistake is to choose a PLC with no spare I/O capacity: at the very first upgrade of the facility you have to replace the controller. Building in a 15–20% reserve is cheaper than re-equipping the panel.

PLC and supervisory control

On its own, a PLC controls equipment locally. To view data and control the facility remotely, the controller is connected to SCADA over Modbus or OPC UA. For example, through the Atlas supervisory server the equipment parameters are available online 24/7, and an engineer can analyze operation and respond to deviations remotely.

The key takeaways

A PLC is a reliable industrial computer that executes equipment control logic according to a program. It replaced relay circuits, runs cyclically, communicates with sensors and mechanisms through I/O modules, and integrates with SCADA. Choosing the right PLC and writing solid code is half the success of the entire automation system.

Atlas Scada programs Siemens, Owen, Schneider, and Delta controllers, refines existing projects, and carries out import substitution. If you need a controller programmed or modernized — tell us about your task.