PLC programming languages overview (IEC 61131-3) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a variable in Structured Text (ST) language.

PLC Programming

VAR myVar : [1]; END_VAR

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

ABOOL

BFLOAT

CSTRING

DINT

Attempts:

3 left

💡 Hint

Common Mistakes

Using STRING or BOOL when an integer is needed.

In Structured Text, variables are declared with their type. INT is a common integer type.

2fill in blank

medium

Complete the Ladder Diagram (LD) logic to represent an AND operation between two inputs.

PLC Programming

LD ([1]) AND (Input2) THEN Output END_LD

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AInput1

BCounter

CTimer

DOutput

Attempts:

3 left

💡 Hint

Common Mistakes

Using Output or Timer instead of an input variable.

In Ladder Diagram, inputs like Input1 and Input2 are used in logic operations.

3fill in blank

hard

Fix the error in this Function Block Diagram (FBD) snippet by completing the missing block type.

PLC Programming

FB1 : [1]; // Function block instance

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ATIMER

BAND

CCOUNTER

DOUTPUT

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing AND or COUNTER which are different block types.

Timers are common function blocks in FBD. TIMER is the correct block type here.

4fill in blank

hard

Fill both blanks to complete this Structured Text (ST) conditional statement checking if a variable is TRUE.

PLC Programming

IF [1] = [2] THEN
  Output := TRUE;
END_IF

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

AInputSignal

BTRUE

CFALSE

DOutput

Attempts:

3 left

💡 Hint

Common Mistakes

Using Output or FALSE in the condition incorrectly.

The condition checks if InputSignal equals TRUE to set the output.

5fill in blank

hard

Fill all three blanks to create a simple loop in Structured Text (ST) that counts from 1 to 5.

PLC Programming

FOR [1] := [2] TO [3] DO
  count := count + 1;
END_FOR

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

Ai

B1

C5

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using count as loop variable or wrong start/end values.

The loop variable i starts at 1 and goes to 5, incrementing count each time.

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