Configuring Your Application

In the previous chapter, you compiled and ran a doorbell program. But it only ran once. On a real PLC, the sense-control-actuate cycle repeats continuously on a fixed schedule. In this chapter, you will add a configuration to schedule the program and a timer to make the buzzer pulse automatically.

Add a Timer

A doorbell that buzzes constantly is not very useful. Let’s add a timer so the buzzer pulses — turning on for a short duration, then turning off.

Open main.st and replace its contents with:

main.st — Doorbell with Timer

PROGRAM main
   VAR
      Button : BOOL;
      Buzzer : BOOL;
      PulseTimer : TON;
   END_VAR

   PulseTimer(IN := NOT Button, PT := T#500ms);
   Buzzer := PulseTimer.Q;

END_PROGRAM

What Changed

We added a timer on delay (TON) function block:

• PulseTimer : TON declares an instance of the standard TON function block. TON turns its output Q to TRUE after a specified delay.

• PulseTimer(IN := NOT Button, PT := T#500ms) calls the timer. IN is the enable input (TRUE when the button is not pressed), and PT is the delay duration (500 milliseconds).

• Buzzer := PulseTimer.Q reads the timer’s output. Q becomes TRUE once the timer has been running for 500 ms.

Add a Configuration Block

Now add a configuration to tell the runtime how to schedule the program. Add the following below the existing END_PROGRAM:

main.st — Complete Application

PROGRAM main
   VAR
      Button : BOOL;
      Buzzer : BOOL;
      PulseTimer : TON;
   END_VAR

   PulseTimer(IN := NOT Button, PT := T#500ms);
   Buzzer := PulseTimer.Q;

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

What the Configuration Does

The configuration block introduces three layers:

CONFIGURATION

The top-level container for your application. Every IEC 61131-3 application has exactly one configuration. Here it is named config.

RESOURCE

Represents a processing unit (a CPU or core). The resource named res runs ON PLC, where PLC is the name of the hardware target defined by the runtime environment.

TASK

Defines a scheduling policy. The task named plc_task runs every 100 milliseconds at priority level 1. This means the runtime will execute the sense-control-actuate cycle 10 times per second.

The line:

PROGRAM plc_task_instance WITH plc_task : main;

creates an instance of the main program, names it plc_task_instance, and binds it to plc_task. Every 100 ms, the runtime reads the inputs, runs main, and writes the outputs.

Tip

For a deeper look at how these layers fit together, see Program Organization.

Run the Updated Program

Now that the timer and configuration are in place, run the updated program:

1. Click Run Program above the PROGRAM main line.

2. The IronPLC Run output panel shows the variables updating in real time. After a moment, you should see output like:

   Scan cycle: 10
   ---
     Button : BOOL = FALSE
     Buzzer : BOOL = TRUE
     PulseTimer : TON = <TON>
   

   Buzzer is TRUE because the timer’s output Q turned on once Button had been FALSE for 500 ms. The panel lists the program’s variables; the timer instance appears as its type, and its members (Q, ET) are not listed.

3. Click Stop above the PROGRAM line to end execution.

The IronPLC Run panel showing the program variables after the TON fires.

The timer program in action: scan cycles advancing and PulseTimer variables updating.

The timer’s elapsed time (ET) shows how long it has been running. After enough scan cycles, Q becomes TRUE and the buzzer turns on.

Next Steps

Your main.st file now contains everything needed for a working IEC 61131-3 application. Before you grow it, take a look inside it while it runs.

Continue to Debugging Your Program.