Function block definition in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a function block named 'Counter'.

PLC Programming

FUNCTION_BLOCK [1]
VAR
  count : INT;
END_VAR
END_FUNCTION_BLOCK

Drag options to blanks, or click blank then click option'

ACounter

Bcounter

CCount

DCOUNTER

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase or all uppercase letters for the function block name.

Misspelling the function block name.

The function block name must start with a capital letter and match the definition. 'Counter' is the correct name.

2fill in blank

medium

Complete the code to declare an integer variable named 'count' inside the function block.

PLC Programming

FUNCTION_BLOCK Counter
VAR
  [1] : INT;
END_VAR
END_FUNCTION_BLOCK

Drag options to blanks, or click blank then click option'

Acounter

Bcount

CCount

DCOUNTER

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase or capitalized variable names incorrectly.

Misspelling the variable name.

Variable names are case sensitive. 'count' matches the variable name exactly.

3fill in blank

hard

Fix the error in the function block header to correctly define the function block.

PLC Programming

FUNCTION_BLOCK [1]
VAR
  count : INT;
END_VAR
END_FUNCTION_BLOCK

Drag options to blanks, or click blank then click option'

ACounter

Bcounter

CCOUNTER

DFuncBlock

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase or all uppercase names.

Using a different name than intended.

Function block names are case sensitive and should match the declaration. 'Counter' is correct.

4fill in blank

hard

Fill both blanks to declare a boolean input named 'Enable' and an integer output named 'Result' inside the function block.

PLC Programming

FUNCTION_BLOCK Counter
VAR_INPUT
  [1] : BOOL;
END_VAR
VAR_OUTPUT
  [2] : INT;
END_VAR
END_FUNCTION_BLOCK

Drag options to blanks, or click blank then click option'

AEnable

Benable

CResult

Dresult

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase names for inputs or outputs.

Mixing input and output variable names.

Input and output variable names are case sensitive. 'Enable' and 'Result' are correct.

5fill in blank

hard

Fill all three blanks to define a function block 'Timer' with an input 'Start' (BOOL), an output 'Done' (BOOL), and a variable 'Elapsed' (TIME).

PLC Programming

FUNCTION_BLOCK [1]
VAR_INPUT
  [2] : BOOL;
END_VAR
VAR_OUTPUT
  [3] : BOOL;
END_VAR
VAR
  Elapsed : TIME;
END_VAR
END_FUNCTION_BLOCK

Drag options to blanks, or click blank then click option'

ATimer

BStart

CDone

DTimerBlock

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect names or wrong case for inputs and outputs.

Misspelling the function block name.

The function block name is 'Timer'. Input is 'Start' and output is 'Done', all case sensitive.

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