Program organization units (POU) in PLC Programming

Introduction

Program Organization Units (POUs) help you organize your PLC code into smaller, manageable parts. This makes your program easier to understand and maintain.

When you want to split a large PLC program into smaller sections.

When you need to reuse code blocks in different parts of your program.

When you want to separate different tasks like input reading, logic processing, and output control.

When you want to improve program clarity by grouping related instructions.

When you want to debug or test parts of your program independently.

Syntax

PLC Programming

POU_TYPE POU_NAME
  (* POU code here *)
END_POU

POU_TYPE can be PROGRAM, FUNCTION_BLOCK, or FUNCTION.

POU_NAME is the name you give to this unit, like 'MainProgram' or 'MotorControl'.

Examples

This defines a main program POU where you write the main logic.

PLC Programming

PROGRAM MainProgram
  (* Main program code here *)
END_PROGRAM

This defines a function block POU to control a motor with its own variables.

PLC Programming

FUNCTION_BLOCK MotorControl
  VAR
    Speed: INT;
  END_VAR
  (* Motor control logic here *)
END_FUNCTION_BLOCK

This defines a function POU that takes inputs and returns a value.

PLC Programming

FUNCTION CalculateSum : INT
  VAR_INPUT
    A: INT;
    B: INT;
  END_VAR
  CalculateSum := A + B;
END_FUNCTION

Sample Program

This program uses a main program POU that calls a function POU to add two numbers and store the result.

PLC Programming

PROGRAM MainProgram
  VAR
    Result: INT;
  END_VAR

  Result := CalculateSum(5, 7);
END_PROGRAM

FUNCTION CalculateSum : INT
  VAR_INPUT
    A: INT;
    B: INT;
  END_VAR
  CalculateSum := A + B;
END_FUNCTION

Important Notes

Each POU type serves a different purpose: PROGRAM for main logic, FUNCTION_BLOCK for reusable blocks with memory, FUNCTION for calculations.

POUs help keep your PLC code clean and easier to debug.

Summary

POUs organize PLC code into smaller parts.

Use PROGRAM, FUNCTION_BLOCK, or FUNCTION depending on your needs.

POUs make your code easier to read, reuse, and maintain.

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