Program organization units (POU) in PLC Programming - Mini Project: Build & Apply

Program Organization Units (POU) in PLC Programming

📖 Scenario: You are working on a simple PLC program to control a conveyor belt system in a factory. The system needs to start and stop the conveyor based on sensor inputs.To keep the program organized and easy to maintain, you will use Program Organization Units (POUs) to separate different parts of the logic.

🎯 Goal: Build a PLC program using POUs: a main program and a function block to control the conveyor belt start and stop logic.

📋 What You'll Learn

Create a main program POU named MainProgram.

Create a function block POU named ConveyorControl with input and output variables.

Use the function block inside the main program to control the conveyor.

Print or display the conveyor status.

💡 Why This Matters

🌍 Real World

POUs help organize PLC programs into smaller, manageable parts, making it easier to build and maintain automation systems like conveyor belts.

💼 Career

Understanding POUs is essential for PLC programmers working in manufacturing, automation, and industrial control systems.

1

Create the main program POU

Create a program organization unit (POU) named MainProgram as a program type. Inside it, declare a variable conveyorStatus of type BOOL and set it to FALSE.

PLC Programming

// Your code here to create MainProgram POU and declare conveyorStatus variable

Hint

Use PROGRAM MainProgram to start the main program POU and declare variables inside VAR ... END_VAR.

2

Create the ConveyorControl function block POU

Create a function block POU named ConveyorControl. Inside it, declare an input variable startSignal of type BOOL, an input variable stopSignal of type BOOL, and an output variable conveyorRunning of type BOOL.

PLC Programming

PROGRAM MainProgram
VAR
    conveyorStatus : BOOL := FALSE;
END_VAR
END_PROGRAM

// Your code here to create ConveyorControl function block with inputs and output

Hint

Use FUNCTION_BLOCK ConveyorControl to create the function block and declare inputs inside VAR_INPUT ... END_VAR and outputs inside VAR_OUTPUT ... END_VAR.

3

Add logic inside ConveyorControl function block

Inside the ConveyorControl function block, write logic so that conveyorRunning becomes TRUE when startSignal is TRUE, and becomes FALSE when stopSignal is TRUE.

PLC Programming

PROGRAM MainProgram
VAR
    conveyorStatus : BOOL := FALSE;
END_VAR
END_PROGRAM

FUNCTION_BLOCK ConveyorControl
VAR_INPUT
    startSignal : BOOL;
    stopSignal : BOOL;
END_VAR
VAR_OUTPUT
    conveyorRunning : BOOL;
END_VAR

// Your code here to add logic for conveyorRunning
END_FUNCTION_BLOCK

Hint

Use an IF ... ELSIF ... END_IF statement to set conveyorRunning based on startSignal and stopSignal.

4

Use ConveyorControl in MainProgram and display status

In the MainProgram, create an instance of ConveyorControl named conveyor. Set startSignal to TRUE and stopSignal to FALSE. Then assign conveyorStatus to conveyor.conveyorRunning. Finally, print the value of conveyorStatus.

PLC Programming

PROGRAM MainProgram
VAR
    conveyorStatus : BOOL := FALSE;
    conveyor : ConveyorControl;
END_VAR

// Your code here to set signals, update conveyorStatus, and print it
END_PROGRAM

FUNCTION_BLOCK ConveyorControl
VAR_INPUT
    startSignal : BOOL;
    stopSignal : BOOL;
END_VAR
VAR_OUTPUT
    conveyorRunning : BOOL;
END_VAR

IF startSignal THEN
    conveyorRunning := TRUE;
ELSIF stopSignal THEN
    conveyorRunning := FALSE;
END_IF
END_FUNCTION_BLOCK

Hint

Create an instance of the function block, set its inputs, update the main variable from its output, and print or display the status.

Practice

1. Which of the following is NOT a type of Program Organization Unit (POU) in PLC programming?

easy

A. MODULE

B. PROGRAM

C. FUNCTION

D. FUNCTION_BLOCK

Solution

1. Step 1: Recall POU types

   The three main POUs are PROGRAM, FUNCTION_BLOCK, and FUNCTION.

2. Step 2: Identify the incorrect option

   MODULE is not a standard POU type in PLC programming.

3. Final Answer:

   MODULE -> Option A

4. Quick Check:

   POUs = PROGRAM, FUNCTION_BLOCK, FUNCTION [OK]

Hint: Remember only three POUs: PROGRAM, FUNCTION_BLOCK, FUNCTION [OK]

Common Mistakes:

• Confusing MODULE with FUNCTION_BLOCK
• Thinking FUNCTION_BLOCK is not a POU
• Mixing hardware modules with POUs

2. Which syntax correctly declares a FUNCTION_BLOCK named MotorControl in IEC 61131-3 standard?

easy

A. PROGRAM MotorControl
END_PROGRAM

B. FUNCTION MotorControl
END_FUNCTION

C. FUNCTION_BLOCK MotorControl
END_FUNCTION_BLOCK

D. FUNCTION_BLOCK MotorControl() END_FUNCTION_BLOCK

Solution

1. Step 1: Identify correct POU declaration syntax

   FUNCTION_BLOCK declaration starts with 'FUNCTION_BLOCK Name' and ends with 'END_FUNCTION_BLOCK'.

2. Step 2: Check each option

   FUNCTION_BLOCK MotorControl END_FUNCTION_BLOCK matches correct syntax; PROGRAM and FUNCTION declare different POUs; the version with parentheses is invalid.

3. Final Answer:

   FUNCTION_BLOCK MotorControl\nEND_FUNCTION_BLOCK -> Option C

4. Quick Check:

   FUNCTION_BLOCK syntax = FUNCTION_BLOCK MotorControl END_FUNCTION_BLOCK [OK]

Hint: FUNCTION_BLOCK has no parentheses in declaration [OK]

Common Mistakes:

• Adding parentheses to FUNCTION_BLOCK
• Confusing FUNCTION and FUNCTION_BLOCK syntax
• Using PROGRAM keyword for function blocks

3. Given the following FUNCTION in a PLC program:

FUNCTION AddTwo : INT
VAR_INPUT
  a : INT;
  b : INT;
END_VAR
AddTwo := a + b;
END_FUNCTION

What is the output of calling AddTwo(3, 5)?

medium

A. 8

B. 35

C. Error: FUNCTION cannot be called like this

D. 0

Solution

1. Step 1: Understand FUNCTION behavior

   The FUNCTION AddTwo takes two INT inputs and returns their sum.

2. Step 2: Calculate the output for inputs 3 and 5

   3 + 5 = 8, so the function returns 8.

3. Final Answer:

   8 -> Option A

4. Quick Check:

   3 + 5 = 8 [OK]

Hint: Functions return values; sum inputs to get output [OK]

Common Mistakes:

• Concatenating inputs as strings
• Thinking FUNCTION_BLOCKs return values
• Assuming syntax error on call

4. Identify the error in this FUNCTION_BLOCK declaration:

FUNCTION_BLOCK Timer
VAR
  StartTime : TIME;
END_VAR
METHOD Start()
  StartTime := TIME();
END_METHOD
END_FUNCTION_BLOCK

medium

A. METHOD keyword is not allowed inside FUNCTION_BLOCK

B. TIME() is not a valid way to get current time

C. StartTime variable must be VAR_INPUT

D. FUNCTION_BLOCK must not have variables

Solution

1. Step 1: Check METHOD usage in FUNCTION_BLOCK

   METHODs are allowed inside FUNCTION_BLOCKs to define actions.

2. Step 2: Verify TIME() usage

   TIME() is not a standard function to get current time in PLC; this causes an error.

3. Final Answer:

   TIME() is not a valid way to get current time -> Option B

4. Quick Check:

   Invalid TIME() call = TIME() is not a valid way to get current time [OK]

Hint: Check if functions used are standard PLC functions [OK]

Common Mistakes:

• Assuming METHODs are disallowed
• Thinking all variables must be inputs
• Believing FUNCTION_BLOCKs can't have variables

5. You want to create reusable code that holds internal state and can be called multiple times with different inputs. Which POU type should you choose and why?

hard

A. FUNCTION, because it can be called multiple times with different inputs

B. PROGRAM, because it organizes the entire PLC project

C. FUNCTION, because it returns a value and holds state internally

D. FUNCTION_BLOCK, because it can hold data and be reused with different instances

Solution

1. Step 1: Understand POU types and their features

   FUNCTION_BLOCKs can hold internal data/state and be instantiated multiple times.

2. Step 2: Compare with other POUs

   FUNCTIONs return values but do not hold state; PROGRAMs organize code but are not reusable units with state.

3. Final Answer:

   FUNCTION_BLOCK, because it can hold data and be reused with different instances -> Option D

4. Quick Check:

   Reusable with state = FUNCTION_BLOCK [OK]

Hint: Use FUNCTION_BLOCK for reusable code with internal state [OK]

Common Mistakes:

• Choosing FUNCTION for holding state
• Confusing PROGRAM with reusable unit
• Ignoring difference between FUNCTION and FUNCTION_BLOCK