Reusable code patterns in PLC Programming

Introduction

Reusable code patterns help you write parts of your PLC program once and use them many times. This saves time and makes your program easier to understand and fix.

When you have a task that repeats in different parts of your PLC program.

When you want to make your program shorter and clearer.

When you want to avoid mistakes by reusing tested code.

When you want to update a behavior in one place and have it change everywhere.

When you want to organize your program into smaller, manageable pieces.

Syntax

PLC Programming

FUNCTION_BLOCK BlockName
VAR_INPUT
  input1 : BOOL;
  input2 : INT;
END_VAR
VAR_OUTPUT
  output1 : BOOL;
END_VAR
(* Your code here *)
END_FUNCTION_BLOCK

Function blocks are reusable code units in PLC programming.

You define inputs and outputs to control how the block works.

Examples

This block can be reused to create timers with different settings.

PLC Programming

FUNCTION_BLOCK TimerBlock
VAR_INPUT
  Start : BOOL;
  TimeSet : TIME;
END_VAR
VAR_OUTPUT
  Done : BOOL;
END_VAR
(* Timer logic here *)
END_FUNCTION_BLOCK

This block controls a motor and can be used for many motors by changing inputs.

PLC Programming

FUNCTION_BLOCK MotorControl
VAR_INPUT
  Run : BOOL;
  Speed : INT;
END_VAR
VAR_OUTPUT
  Status : BOOL;
END_VAR
(* Motor control logic here *)
END_FUNCTION_BLOCK

Sample Program

This program uses the SimpleCounter block twice. Counter1 counts when Start1 is TRUE, Counter2 counts when Start2 is TRUE.

PLC Programming

FUNCTION_BLOCK SimpleCounter
VAR_INPUT
  Increment : BOOL;
END_VAR
VAR_OUTPUT
  Count : INT := 0;
END_VAR
IF Increment THEN
  Count := Count + 1;
END_IF
END_FUNCTION_BLOCK

PROGRAM MainProgram
VAR
  Counter1 : SimpleCounter;
  Counter2 : SimpleCounter;
  Start1 : BOOL := TRUE;
  Start2 : BOOL := FALSE;
END_VAR
(* Simulate incrementing counters *)
Counter1(Increment := Start1);
Counter2(Increment := Start2);
(* Output counts *)
END_PROGRAM

Important Notes

Reusable blocks help keep your PLC program clean and easy to fix.

Test your blocks separately before using them in the main program.

Use clear names for blocks and variables to understand their purpose.

Summary

Reusable code patterns save time and reduce errors.

Function blocks are a common way to create reusable code in PLC programming.

Use inputs and outputs to control how your blocks work in different situations.

Practice

1. What is the main benefit of using FUNCTION_BLOCK in PLC programming?

easy

A. It creates a graphical interface for the PLC.

B. It makes the program run faster by using less memory.

C. It automatically fixes errors in the code.

D. It allows you to reuse code and simplify program management.

Solution

1. Step 1: Understand the purpose of FUNCTION_BLOCK

   FUNCTION_BLOCK is designed to package code that can be reused multiple times in a PLC program.

2. Step 2: Identify the main benefit

   Reusing code saves time and makes the program easier to manage and maintain.

3. Final Answer:

   It allows you to reuse code and simplify program management. -> Option D

4. Quick Check:

   Reusable code = Simplify management [OK]

Hint: FUNCTION_BLOCK means reusable code blocks [OK]

Common Mistakes:

• Thinking FUNCTION_BLOCK speeds up execution
• Believing it fixes errors automatically
• Confusing it with user interface creation

2. Which of the following is the correct way to declare a FUNCTION_BLOCK named Counter in PLC programming?

easy

A. FUNCTION Counter
VAR_INPUT
Count: INT;
END_VAR
END_FUNCTION

B. FUNCTION_BLOCK Counter
VAR_INPUT
Count: INT;
END_VAR
END_FUNCTION_BLOCK

C. FUNCTION_BLOCK Counter()
VAR_INPUT
Count: INT;
END_VAR
END_FUNCTION_BLOCK

D. FUNCTION_BLOCK Counter
VAR_OUTPUT
Count: INT;
END_VAR
END_FUNCTION_BLOCK

Solution

1. Step 1: Identify correct FUNCTION_BLOCK syntax

   FUNCTION_BLOCK declaration starts with 'FUNCTION_BLOCK Name' and ends with 'END_FUNCTION_BLOCK'. Inputs are declared inside VAR_INPUT.

2. Step 2: Check each option

   FUNCTION_BLOCK Counter VAR_INPUT Count: INT; END_VAR END_FUNCTION_BLOCK correctly declares FUNCTION_BLOCK Counter with VAR_INPUT and ends properly. FUNCTION Counter VAR_INPUT Count: INT; END_VAR END_FUNCTION uses FUNCTION instead of FUNCTION_BLOCK. FUNCTION_BLOCK Counter() VAR_INPUT Count: INT; END_VAR END_FUNCTION_BLOCK adds parentheses which are not used. FUNCTION_BLOCK Counter VAR_OUTPUT Count: INT; END_VAR END_FUNCTION_BLOCK uses VAR_OUTPUT instead of VAR_INPUT for input variable.

3. Final Answer:

   FUNCTION_BLOCK Counter VAR_INPUT Count: INT; END_VAR END_FUNCTION_BLOCK -> Option B

4. Quick Check:

   Correct FUNCTION_BLOCK syntax = FUNCTION_BLOCK Counter VAR_INPUT Count: INT; END_VAR END_FUNCTION_BLOCK [OK]

Hint: FUNCTION_BLOCK uses VAR_INPUT for inputs, no parentheses [OK]

Common Mistakes:

• Using FUNCTION instead of FUNCTION_BLOCK
• Adding parentheses after FUNCTION_BLOCK name
• Declaring inputs under VAR_OUTPUT

3. Given this FUNCTION_BLOCK code snippet, what will be the output value of Result after execution?

FUNCTION_BLOCK AddTwo
VAR_INPUT
A: INT;
B: INT;
END_VAR
VAR_OUTPUT
Result: INT;
END_VAR
Result := A + B;
END_FUNCTION_BLOCK

// Usage
AddTwoInstance: AddTwo;
AddTwoInstance.A := 3;
AddTwoInstance.B := 5;
// Execute block
// What is AddTwoInstance.Result?

medium

A. 8

B. 35

C. 0

D. Error: Result not assigned

Solution

1. Step 1: Understand the FUNCTION_BLOCK logic

   The block adds inputs A and B and stores the sum in Result.

2. Step 2: Substitute input values and calculate

   A = 3, B = 5, so Result = 3 + 5 = 8.

3. Final Answer:

   8 -> Option A

4. Quick Check:

   3 + 5 = 8 [OK]

Hint: Add inputs to get output Result [OK]

Common Mistakes:

• Concatenating numbers as strings
• Assuming Result is zero by default
• Thinking Result is not assigned automatically

4. Identify the error in this FUNCTION_BLOCK code snippet:

FUNCTION_BLOCK Multiply
VAR_INPUT
X: INT;
Y: INT;
END_VAR
VAR_OUTPUT
Product: INT;
END_VAR
Product = X * Y;
END_FUNCTION_BLOCK

medium

A. Incorrect use of '=' instead of ':=' for assignment.

B. VAR_OUTPUT should be declared before VAR_INPUT.

C. Missing semicolon at the end of the assignment line.

D. FUNCTION_BLOCK name must be lowercase.

Solution

1. Step 1: Check assignment syntax in FUNCTION_BLOCK

   In PLC programming, assignment uses ':=' not '='.

2. Step 2: Verify other syntax elements

   Semicolon is present. VAR_INPUT before VAR_OUTPUT is correct. FUNCTION_BLOCK names are case-sensitive but uppercase is allowed.

3. Final Answer:

   Incorrect use of '=' instead of ':=' for assignment. -> Option A

4. Quick Check:

   Use ':=' for assignment [OK]

Hint: Use ':=' for assignments in PLC code [OK]

Common Mistakes:

• Using '=' instead of ':='
• Thinking semicolon is optional
• Believing VAR_OUTPUT must come first

5. You want to create a reusable FUNCTION_BLOCK that counts how many times an input boolean Trigger changes from FALSE to TRUE. Which approach correctly implements this behavior?

hard

A. Reset the counter to zero every time Trigger is TRUE.

B. Increment the counter every time the FUNCTION_BLOCK runs regardless of Trigger state.

C. Use a VAR to store previous Trigger state and increment a counter only when Trigger changes from FALSE to TRUE.

D. Use VAR_INPUT to store the counter and increment it inside the block.

Solution

1. Step 1: Understand the counting condition

   The counter should increase only when Trigger changes from FALSE to TRUE (rising edge).

2. Step 2: Implement edge detection logic

   Store previous Trigger state in a VAR variable. Compare current and previous states. Increment counter only if previous was FALSE and current is TRUE.

3. Final Answer:

   Use a VAR to store previous Trigger state and increment a counter only when Trigger changes from FALSE to TRUE. -> Option C

4. Quick Check:

   Edge detection needs previous state storage [OK]

Hint: Store previous input to detect rising edge [OK]

Common Mistakes:

• Incrementing counter every cycle
• Resetting counter incorrectly
• Using VAR_INPUT for internal state