Why Function block definition in PLC Programming? - Purpose & Use Cases

The Big Idea

What if you could fix a bug once and have it fixed everywhere instantly?

The Scenario

Imagine you have to control multiple machines on a factory floor, each needing the same set of instructions repeated over and over. You write the same code again and again for each machine.

The Problem

This manual repetition is slow and boring. If you find a mistake, you must fix it everywhere. It's easy to miss one and cause errors. Changing the logic means rewriting many parts, risking inconsistencies.

The Solution

Function blocks let you write the control logic once and reuse it for every machine. You just call the block with different inputs. This saves time, reduces errors, and makes updates simple and safe.

Before vs After

✗ Before

IF sensor1 THEN motor1 := TRUE; END_IF;
IF sensor2 THEN motor2 := TRUE; END_IF;

✓ After

FUNCTION_BLOCK MotorControl
VAR_INPUT
  sensor: BOOL;
END_VAR
VAR_OUTPUT
  motor: BOOL;
END_VAR
motor := sensor;
END_FUNCTION_BLOCK

What It Enables

It enables you to build reliable, reusable control logic that scales effortlessly across many machines.

Real Life Example

In a bottling plant, the same filling process runs on dozens of lines. Using function blocks, engineers write the filling logic once and apply it to all lines, ensuring consistent operation.

Key Takeaways

Manual repetition of code is slow and error-prone.

Function blocks package logic for reuse and easy updates.

This leads to safer, faster, and more maintainable automation programs.

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