Reusable code patterns in PLC Programming - Step-by-Step Execution

Concept Flow - Reusable code patterns

Identify repeated logic

↓

Create reusable block (Function/Function Block)

↓

Define inputs and outputs

↓

Use reusable block in main program

↓

Pass different inputs

↓

Get outputs

↓

Repeat usage as needed

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End

This flow shows how to identify repeated logic, create a reusable block with inputs and outputs, then use it multiple times in the main program.

Execution Sample

PLC Programming

FUNCTION_BLOCK FB_Add
VAR_INPUT
  A : INT;
  B : INT;
END_VAR
VAR_OUTPUT
  Sum : INT;
END_VAR

METHOD Calculate : VOID
  Sum := A + B;
END_METHOD
END_FUNCTION_BLOCK

Defines a reusable function block that adds two integers and outputs the sum.

Execution Table

Step	Action	Inputs (A, B)	Output (Sum)	Notes
1	Call FB_Add with A=3, B=5	3, 5	8	Sum calculated as 3+5=8
2	Call FB_Add with A=10, B=20	10, 20	30	Sum calculated as 10+20=30
3	Call FB_Add with A=0, B=0	0, 0	0	Sum calculated as 0+0=0
4	No more calls	-	-	Program ends

💡 No more calls to the reusable block, program ends.

Variable Tracker

Variable	Start	After Call 1	After Call 2	After Call 3	Final
A	-	3	10	0	-
B	-	5	20	0	-
Sum	-	8	30	0	-

Key Moments - 2 Insights

Why do we use inputs and outputs in reusable blocks?

Inputs allow us to give different values each time we use the block, and outputs give back the result. See execution_table rows 1-3 where different inputs produce different sums.

Can the reusable block change variables outside itself?

No, reusable blocks only change their outputs and internal variables. They do not directly change variables outside unless designed to do so. This keeps code safe and predictable.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output Sum when inputs A=10 and B=20?

A 20

B 8

C 30

D 10

💡 Hint

Check execution_table row 2 for inputs 10 and 20 and their output.

At which step does the program stop calling the reusable block?

A Step 4

B Step 2

C Step 3

D Step 1

💡 Hint

Look at the exit_note and execution_table last row.

If we change input A to 7 and B to 8 in the first call, what would Sum be?

A 8

B 15

C 7

D 0

💡 Hint

Sum is calculated as A + B, so add 7 + 8.

Concept Snapshot

Reusable code patterns in PLC use Function Blocks.
Define inputs and outputs.
Write logic once inside the block.
Call block multiple times with different inputs.
Outputs give results for each call.
This saves time and avoids repeating code.

Full Transcript

Reusable code patterns help us write a piece of logic once and use it many times. In PLC programming, we create Function Blocks with inputs and outputs. Inputs let us send different values each time. Outputs give back results. For example, a Function Block that adds two numbers takes inputs A and B, then outputs their sum. We call this block multiple times with different inputs, and each time it returns the correct sum. This way, we avoid rewriting the same code again and again. The execution table shows each call with inputs and outputs. Variables A, B, and Sum change as we call the block. Using reusable blocks keeps programs clean, easy to read, and maintain.

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