PLC programming languages overview (IEC 61131-3) - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Identify the IEC 61131-3 language for graphical ladder logic

Which IEC 61131-3 programming language uses a graphical representation similar to electrical relay logic?

A Instruction List (IL)

B Ladder Diagram (LD)

C Structured Text (ST)

D Function Block Diagram (FBD)

💡 Hint

Think about the language that looks like relay circuits.

Ladder Diagram (LD) is a graphical language that resembles relay logic, making it intuitive for electricians and engineers familiar with electrical schematics.

💻 Command Output

intermediate

Output of a simple Structured Text program snippet

What is the output value of variable 'result' after running this Structured Text code snippet?

VAR
  a : INT := 5;
  b : INT := 3;
  result : INT;
END_VAR

result := a * b + 2;

PLC Programming

VAR
  a : INT := 5;
  b : INT := 3;
  result : INT;
END_VAR

result := a * b + 2;

A 11

B 15

C 17

D 8

💡 Hint

Calculate multiplication first, then addition.

The expression multiplies 5 by 3 to get 15, then adds 2, resulting in 17.

📝 Syntax

advanced

Identify the syntax error in this Instruction List code

Which option contains the syntax error in this Instruction List (IL) snippet?

LD 0
AND 1
OR 0
ST result

PLC Programming

LD 0
AND 1
OR 0
ST result

A

LD 0
AND 1
OR
ST result

B

LD 0
AND 1
OR 0
ST result

C

LD 0
AND 1
OR 0
STresult

D

tluser TS
0 RO
1 DNA
0 DL

💡 Hint

Look for missing operands in instructions.

Option A has 'OR' without an operand, which is a syntax error in Instruction List language.

🔧 Debug

advanced

Find the logic error in this Function Block Diagram description

A Function Block Diagram (FBD) has an AND block connected to two inputs: Input1 is TRUE, Input2 is FALSE. The output is unexpectedly TRUE. What is the most likely cause?

A The output wire is connected to the output of an OR block instead of the AND block.

B Input2 is actually TRUE but mislabeled as FALSE.

C The AND block is configured as an OR block by mistake.

D The AND block is malfunctioning and outputs TRUE regardless of inputs.

💡 Hint

Check the wiring and block connections carefully.

If the output is TRUE despite one FALSE input, the output might be connected to an OR block output, which outputs TRUE if any input is TRUE.

🚀 Application

expert

Determine the number of rungs in this Ladder Diagram

Given a Ladder Diagram with the following rungs: 1. Start button connected to a coil 'Motor' 2. Motor output connected to a normally open contact in rung 3 3. Stop button connected in series with the contact from rung 2 controlling coil 'Alarm' How many rungs does this Ladder Diagram have?

A 2

B 1

C 4

D 3

💡 Hint

Count each separate horizontal line with contacts and coils as one rung.

Each described line is a rung: one for Motor coil, one for the contact controlling Alarm coil, and one for the Stop button in series. Total is 3 rungs.

Practice

1. Which of the following is NOT one of the five main PLC programming languages defined by IEC 61131-3?

easy

A. Function Block Diagram

B. Python

C. Structured Text

D. Ladder Diagram

Solution

1. Step 1: Recall IEC 61131-3 languages

   IEC 61131-3 defines Ladder Diagram, Function Block Diagram, Structured Text, Instruction List, and Sequential Function Chart.

2. Step 2: Identify the language not listed

   Python is a general-purpose programming language, not part of IEC 61131-3.

3. Final Answer:

   Python -> Option B

4. Quick Check:

   IEC 61131-3 excludes Python [OK]

Hint: Remember IEC 61131-3 languages exclude general programming languages [OK]

Common Mistakes:

• Confusing general programming languages with PLC languages
• Assuming all text-based languages are included
• Mixing up graphical and text-based languages

2. Which syntax correctly represents a simple assignment in Structured Text (ST)?

easy

A. Output := Input + 5;

B. Output = Input + 5

C. Output == Input + 5;

D. Output <- Input + 5;

Solution

1. Step 1: Understand Structured Text assignment

   In ST, the assignment operator is ':=' and statements end with a semicolon.

2. Step 2: Check each option

   Output := Input + 5; uses ':=' and ends with ';' correctly. Others use wrong operators or miss semicolons.

3. Final Answer:

   Output := Input + 5; -> Option A

4. Quick Check:

   ST assignment uses ':=' and ';' [OK]

Hint: Look for ':=' operator and semicolon in Structured Text [OK]

Common Mistakes:

• Using '=' instead of ':=' for assignment
• Omitting semicolon at statement end
• Confusing equality '==' with assignment

3. Given the following Ladder Diagram logic:
--[ ]--( )--
where the input contact is normally open and connected to output coil, what is the output when the input is OFF?

medium

A. Output is undefined

B. Output is ON

C. Output toggles ON and OFF

D. Output is OFF

Solution

1. Step 1: Understand Ladder Diagram symbols

   The normally open contact [ ] closes when input is ON, allowing current to flow to the coil ( ).

2. Step 2: Analyze input OFF condition

   When input is OFF, the contact remains open, so no current flows to the coil, output stays OFF.

3. Final Answer:

   Output is OFF -> Option D

4. Quick Check:

   Open contact means no output current [OK]

Hint: Open contact means output coil is OFF [OK]

Common Mistakes:

• Assuming output is ON when input is OFF
• Confusing normally open with normally closed contacts
• Thinking output toggles without input change

4. Identify the error in this Structured Text snippet:

IF Input = TRUE THEN
  Output := TRUE
END_IF

medium

A. Missing semicolon after Output assignment

B. Incorrect IF syntax, should be IF (Input = TRUE) THEN

C. Assignment operator should be '=' not ':='

D. END_IF should be lowercase

Solution

1. Step 1: Check Structured Text syntax rules

   Each statement must end with a semicolon ';' in ST.

2. Step 2: Inspect the code snippet

   The line 'Output := TRUE' lacks a semicolon at the end, causing a syntax error.

3. Final Answer:

   Missing semicolon after Output assignment -> Option A

4. Quick Check:

   Statements end with ';' in ST [OK]

Hint: Always end ST statements with semicolon [OK]

Common Mistakes:

• Forgetting semicolons after assignments
• Misusing '=' for assignment
• Assuming IF needs parentheses

5. You need to implement a control logic that turns ON an output only if two inputs are ON simultaneously, using Function Block Diagram (FBD). Which block combination correctly represents this?

hard

A. Two OR blocks connected in parallel

B. Two AND blocks connected in series

C. One AND block with both inputs connected to it

D. One NOT block with inputs connected

Solution

1. Step 1: Understand AND logic in FBD

   AND block outputs TRUE only if all inputs are TRUE simultaneously.

2. Step 2: Choose correct block setup

   One AND block with both inputs connected directly checks if both inputs are ON together.

3. Final Answer:

   One AND block with both inputs connected to it -> Option C

4. Quick Check:

   AND block with multiple inputs = simultaneous ON check [OK]

Hint: Use single AND block with all inputs for simultaneous ON [OK]

Common Mistakes:

• Using OR blocks instead of AND
• Connecting AND blocks in series unnecessarily
• Using NOT block incorrectly for AND logic