Function block definition in PLC Programming - Step-by-Step Execution

Concept Flow - Function block definition

Start

↓

Define Function Block

↓

Declare Inputs and Outputs

↓

Write Internal Logic

↓

Use Function Block in Program

↓

Execute Function Block

↓

End

This flow shows how a function block is defined, inputs and outputs declared, logic written, then used and executed in a PLC program.

Execution Sample

PLC Programming

FUNCTION_BLOCK FB_Add
VAR_INPUT
  A : INT;
  B : INT;
END_VAR
VAR_OUTPUT
  Sum : INT;
END_VAR
Sum := A + B;
END_FUNCTION_BLOCK

Defines a function block named FB_Add that adds two integers A and B and outputs their sum.

Execution Table

Step	Action	Input Values	Internal Calculation	Output Value
1	Function block FB_Add is called	A=3, B=5	Sum := 3 + 5	Sum=8
2	Function block FB_Add is called	A=10, B=7	Sum := 10 + 7	Sum=17
3	Function block FB_Add is called	A=0, B=0	Sum := 0 + 0	Sum=0
4	Function block FB_Add is called	A=-2, B=4	Sum := -2 + 4	Sum=2
5	Execution ends	-	-	-

💡 Function block execution ends after processing inputs and producing output.

Variable Tracker

Variable	Start	After Call 1	After Call 2	After Call 3	After Call 4	Final
A	-	3	10	0	-2	-2
B	-	5	7	0	4	4
Sum	-	8	17	0	2	2

Key Moments - 2 Insights

Why do we declare inputs and outputs separately in a function block?

Inputs are values given to the function block to work with, outputs are results it produces. This separation is clear in the execution_table where inputs A and B are used to calculate output Sum.

What happens if we call the function block multiple times with different inputs?

Each call uses the new inputs to calculate a new output independently, as shown in execution_table rows 1 to 4 where Sum changes based on inputs.

Visual Quiz - 3 Questions

Look at the execution table, what is the output Sum when inputs A=10 and B=7?

A 17

B 7

C 10

D 0

💡 Hint

Check row 2 in the execution_table where inputs A=10 and B=7 produce Sum=17.

At which step does the function block output Sum equal zero?

A Step 4

B Step 1

C Step 3

D Step 2

💡 Hint

Look at execution_table row 3 where inputs are zero and output Sum is zero.

If input A changes to 5 in the first call, what will be the new output Sum?

A 8

B 10

C 5

D Unknown

💡 Hint

Sum is A + B, so with A=5 and B=5 (from first call), Sum=10. See execution_table row 1 for reference.

Concept Snapshot

Function Block Definition in PLC:
- Use FUNCTION_BLOCK keyword to start
- Declare VAR_INPUT and VAR_OUTPUT separately
- Write logic inside block
- Call block with inputs to get outputs
- Each call is independent and returns output
- Helps reuse code with clear input/output structure

Full Transcript

A function block in PLC programming is a reusable code block with inputs and outputs. First, you define the block with FUNCTION_BLOCK keyword. Inside, you declare inputs (VAR_INPUT) and outputs (VAR_OUTPUT). Then you write the logic that uses inputs to calculate outputs. When the block is called with specific inputs, it runs the logic and produces outputs. Each call is independent and uses the current inputs to produce results. This makes code organized and reusable. The execution table shows how inputs A and B are added to produce Sum output in each call.

Practice

1. What is the main purpose of a Function Block in PLC programming?

easy

A. To group inputs, outputs, and code for repeated tasks

B. To store data permanently in the PLC

C. To create a graphical user interface

D. To connect the PLC to external devices

Solution

1. Step 1: Understand the role of function blocks

   Function blocks group inputs, outputs, and code to perform tasks repeatedly in PLC programs.

2. Step 2: Compare options with the definition

   Only To group inputs, outputs, and code for repeated tasks describes grouping inputs, outputs, and code for reuse, which matches the function block purpose.

3. Final Answer:

   To group inputs, outputs, and code for repeated tasks -> Option A

4. Quick Check:

   Function block purpose = Group inputs, outputs, and code [OK]

Hint: Function blocks bundle inputs, outputs, and code for reuse [OK]

Common Mistakes:

• Confusing function blocks with data storage
• Thinking function blocks create user interfaces
• Assuming function blocks connect hardware directly

2. Which of the following is the correct way to start defining a function block named MotorControl in IEC 61131-3 structured text?

easy

A. PROGRAM MotorControl

B. FUNCTION MotorControl

C. VAR MotorControl

D. FUNCTION_BLOCK MotorControl

Solution

1. Step 1: Recall syntax for function block definition

   In IEC 61131-3, function blocks start with the keyword FUNCTION_BLOCK followed by the name.

2. Step 2: Check each option

   FUNCTION_BLOCK MotorControl uses FUNCTION_BLOCK MotorControl, which is correct. Options A, B, and D use other keywords for different constructs.

3. Final Answer:

   FUNCTION_BLOCK MotorControl -> Option D

4. Quick Check:

   Function block syntax = FUNCTION_BLOCK Name [OK]

Hint: Function blocks start with FUNCTION_BLOCK keyword [OK]

Common Mistakes:

• Using FUNCTION instead of FUNCTION_BLOCK
• Confusing PROGRAM with function block
• Using VAR keyword to define function blocks

3. Given this function block code snippet, what will be the output of FB1.Output after calling FB1.Run() twice?

FUNCTION_BLOCK FB1
VAR_INPUT
  In: BOOL;
END_VAR
VAR_OUTPUT
  Output: INT;
END_VAR
VAR
  Counter: INT := 0;
END_VAR

METHOD PUBLIC Run
IF In THEN
  Counter := Counter + 1;
END_IF
Output := Counter;
END_METHOD

END_FUNCTION_BLOCK

// Usage:
FB1.In := TRUE;
FB1.Run();
FB1.Run();

medium

A. 2

B. 1

C. 0

D. TRUE

Solution

1. Step 1: Understand the counter increment logic

   Each call to Run checks if In is TRUE, then increments Counter by 1.

2. Step 2: Trace two calls with In = TRUE

   First call increments Counter from 0 to 1, second call increments from 1 to 2. Output is set to Counter.

3. Final Answer:

   2 -> Option A

4. Quick Check:

   Counter increments twice with In=TRUE = 2 [OK]

Hint: Count increments each Run call if input TRUE [OK]

Common Mistakes:

• Assuming Output resets each call
• Confusing BOOL and INT output types
• Forgetting Counter is retained between calls

4. Identify the error in this function block definition:

FUNCTION_BLOCK FB_Error
VAR_INPUT
  Start: BOOL;
END_VAR
VAR_OUTPUT
  Status: BOOL;
END_VAR
VAR
  Count: INT;
END_VAR

METHOD Run
IF Start THEN
  Count := Count + 1
  Status := TRUE;
END_IF
END_METHOD
END_FUNCTION_BLOCK

medium

A. Function block name cannot contain underscore

B. Missing semicolon after Count := Count + 1

C. Output variable Status should be input

D. Incorrect keyword METHOD used

Solution

1. Step 1: Check syntax inside the method

   The line Count := Count + 1 is missing a semicolon at the end, which is required in structured text.

2. Step 2: Verify other parts

   METHOD keyword is valid, output variable can be named Status, and underscores are allowed in names.

3. Final Answer:

   Missing semicolon after Count := Count + 1 -> Option B

4. Quick Check:

   Semicolon missing after assignment line [OK]

Hint: Check each line ends with semicolon in structured text [OK]

Common Mistakes:

• Ignoring missing semicolons
• Thinking METHOD keyword is invalid
• Assuming variable naming rules are stricter

5. You want to create a function block TimerFB that counts seconds when Enable input is TRUE and resets when Reset input is TRUE. Which internal variable type should you use to store the count so it keeps its value between calls?

hard

A. VAR_INPUT

B. VAR_OUTPUT

C. VAR (internal variable)

D. VAR_TEMP

Solution

1. Step 1: Understand variable types in function blocks

   VAR_INPUT and VAR_OUTPUT are for inputs and outputs, not for storing internal state. VAR_TEMP is temporary and does not keep value between calls.

2. Step 2: Identify variable type that retains value

   VAR variables inside function blocks keep their value between calls, acting like memory.

3. Final Answer:

   VAR (internal variable) -> Option C

4. Quick Check:

   Use VAR for internal memory in function blocks [OK]

Hint: Use VAR for internal memory that persists between calls [OK]

Common Mistakes:

• Using VAR_TEMP which resets each call
• Confusing inputs/outputs with internal storage
• Assuming VAR_INPUT can store internal state