Reusable code patterns in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a reusable code pattern in PLC programming?

A reusable code pattern is a set of instructions or logic that can be used multiple times in different parts of a PLC program to save time and reduce errors.

beginner

Why should you use reusable code patterns in PLC programming?

Using reusable code patterns helps make programs easier to read, maintain, and update. It also reduces repeated work and mistakes.

intermediate

Name a common reusable code pattern in PLC programming.

Function blocks are a common reusable code pattern. They allow you to create a block of code that performs a task and can be used many times with different inputs.

intermediate

How do function blocks improve code reuse in PLC programs?

Function blocks package logic and data together. You can call the same block multiple times with different data, which saves time and keeps code consistent.

advanced

What is the difference between a function and a function block in PLC programming?

A function returns a value and does not keep data between calls. A function block can keep data (state) between calls, making it useful for tasks that need memory.

What is the main benefit of using reusable code patterns in PLC programming?

A Make the program harder to read

B Save time and reduce errors

C Use more memory

D Make the program run faster

Reusable code patterns help save time and reduce errors by avoiding repeated code.

Which PLC programming element is commonly used for reusable code?

A Function block

B Timer

C Counter

D Input

Function blocks are designed to be reusable pieces of code.

What does a function block in PLC programming do that a function does not?

A Runs faster

B Uses less memory

C Keeps data between calls

D Returns multiple values

Function blocks keep data (state) between calls, unlike functions.

Which of these is NOT a benefit of reusable code patterns?

A More code duplication

B Easier maintenance

C Consistent logic

D Faster development

Reusable code patterns reduce code duplication, not increase it.

How can reusable code patterns help when updating a PLC program?

A Cause more errors

B Require rewriting all code

C Make updates slower

D Update once and apply everywhere

Reusable code lets you update one place and have changes apply everywhere it is used.

Explain what reusable code patterns are and why they are useful in PLC programming.

Think about how repeating the same code can cause problems and how reusable patterns help.

Describe the difference between a function and a function block in PLC programming and when you might use each.

Consider if you need to remember information between calls.

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