Attention

These docs are a bit ambitious. The steps described are accurate but IronPLC cannot yet run programs.

Hello, World!

Now that you’ve installed IronPLC, it’s time to write a first program.

If you’ve ever learned another programming language, you likely started by writing a “Hello, World” program to display that text. Don’t worry if you haven’t learned another programming language or haven’t written a “Hello, World” program.

IEC 61131-3 is designed for real-time automation controllers that often do not have a display. In other words, there is often no place to show “Hello, World”. Other solutions to get feedback, such as creating a file are also unusual.

In short, a “Hello, World” program in IEC 61131-3 is different.

The Sense-Control-Actuate Cycle

Controllers normally operate as part of a sense-control-actuate cycle. We’ll start with a simple example to illustrate the idea: a door bell system. Our door bell system contains a button (the sensor) and a buzzer (the actuator).

Pressing the button triggers the buzzer.

We desire that the buzzer makes noise when the button is pressed. To do that, we use a controller to check the button state and if pressed then enable the buzzer.

Note

It is possible to design a simpler door bell system. This example designed to illustrate how to use IEC 61131-3.

Create an Application with Structured Text

This section will show you how to create a IEC 61131-3 application.

In a Terminal, enter the commands in Create Project Directory to create a new folder and start Visual Studio Code.

Create Project Directory

mkdir helloworld
cd helloworld
code .

In Visual Studio Code, follow the steps to create a new Structured Text file.

1. In the main menu, select File ‣ New File….

2. In the New File… dialog, select the Structured Text File option.

3. Enter the code in Hello World into the Editor.

   Hello World

   PROGRAM main
      VAR
         Button AT %IX1: BOOL;
         Buzzer AT %QX1: BOOL;
      END_VAR
   
      Buzzer := NOT Button;
   
   END_PROGRAM
   
   CONFIGURATION config
      RESOURCE res ON PLC
         TASK plc_task(INTERVAL := T#100ms, PRIORITY := 1);
         PROGRAM plc_task_instance WITH plc_task : main;
      END_RESOURCE
   END_CONFIGURATION
   

4. Save the file with the name main.st.

Anatomy of the Hello, World Application

Let’s review this application. IEC 61131-3 applications are structured from blocks called Program Organization Units (POUs). The PROGRAM is a top level block and similar to the main function in other languages. The section indicated by

PROGRAM main

END_PROGRAM

defines a PROGRAM having the name main.

Unlike the main function in other languages, a program does not run by default. We need to tell the PLC runtime how we want to run the program. The piece indicated by

CONFIGURATION config

END_CONFIGURATION

defines how we want the program to run. The configuration declares we want to execute the main program once every 100 ms and this task is the highest priority task. This task executed on the hardware element named res.

We want our program to enable (or disable) the buzzer based on whether the button is pressed. The piece indicated by

VAR

END_VAR

defines two variables that will contain the state of the button and buzzer. We can then use the variable containing the state of the button to control the variable containing the desired state of the buzzer. The statement indicated by

Buzzer := NOT Button;

does just that. In plain English, the statement says “assign the value of Buzzer to be the boolean inverse of the value of Button.”

From the perspective of the program, there is no specific meaning to the names Buzzer and Button. We could have called them foo and bar, but we choose names that were indicative of their purpose.

Our program needs to associate the variables Buzzer and Button with digital input/output. We do this by declaring these as directly represented variables. Directly represented variable have specific physical or logical locations, for example, being associated with a digital input pin. The declarations

AT %IX1

associates the variable Button with a 1-bit (Boolean) input.

The net result of these elements is to define a program that every 100 ms, reads from an input device, evaluates the logical inverse, and assign the result to an output device.

Working with Multiple Files

You can define your IEC 61131-3 application in multiple files and IronPLC will combine them into a single unit.

In Visual Studio Code, follow the steps to create a new Structured Text file.

1. In the main menu, select File ‣ New File….

2. In the New File… dialog, select the Structured Text File option.

3. Enter the code in Hello World - Configuration into the Editor.

   Hello World - Configuration

   CONFIGURATION config
      RESOURCE res ON PLC
         TASK plc_task(INTERVAL := T#100ms, PRIORITY := 1);
         PROGRAM plc_task_instance WITH plc_task : main;
      END_RESOURCE
   END_CONFIGURATION
   

4. Save the file with the name config.st.