How a PLC Program Works¶

Before writing your first program, it helps to understand how a PLC application is structured. This chapter introduces the core idea that every PLC program is built around.

The Sense-Control-Actuate Cycle¶

Controllers operate in a continuous sense-control-actuate cycle:

Sense — read inputs from sensors (buttons, temperature probes, etc.).
Control — evaluate logic to decide what to do.
Actuate — write outputs to actuators (buzzers, motors, valves, etc.).

The runtime repeats this cycle on a fixed schedule — for example, every 100 milliseconds. Each repetition is called a scan cycle.

A Doorbell Example¶

A simple doorbell system illustrates the cycle. The system has a button (the sensor) and a buzzer (the actuator):

Pressing the button triggers the buzzer.¶

In each scan cycle, the PLC:

Senses whether the button is pressed.
Controls — decides the buzzer should sound when the button is pressed.
Actuates — turns the buzzer on or off.

Note

A real doorbell does not need a PLC. This example is deliberately simple to illustrate how PLC programs work.

What You Will Build¶

In this tutorial, you will write a doorbell program and progressively enhance it:

Write the control logic in Structured Text, the programming language defined by IEC 61131-3.
Run it directly from the editor and inspect the results.
Configure the application with a task schedule.
Connect it to hardware inputs and outputs.

Let’s start writing code.

Continue to Your First Program.