Structured text vs ladder logic decision in PLC Programming - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assign the value 1 to the output if the input is true.

PLC Programming

IF input THEN output := [1]; END_IF;

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

AFALSE

B0

CTRUE

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning 0 instead of 1 to turn the output ON.

Using TRUE or FALSE instead of numeric values.

In structured text, to turn an output ON, assign it the value 1 when the input condition is true.

2fill in blank

medium

Complete the ladder logic rung to turn ON the output coil when the input contact is closed.

PLC Programming

----[ [1] ]----( )----

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ATON

BNC

CNO

DCTU

Attempts:

3 left

💡 Hint

Common Mistakes

Using NC (Normally Closed) which is closed when input is false.

Confusing timer or counter instructions with contacts.

In ladder logic, NO (Normally Open) contact closes when input is true, allowing current to flow and energize the coil.

3fill in blank

hard

Fix the error in the structured text code to correctly toggle the output when the input rises.

PLC Programming

IF [1] THEN output := NOT output; END_IF;

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AR_TRIG(input)

Binput = TRUE

Cinput == FALSE

DF_TRIG(input)

Attempts:

3 left

💡 Hint

Common Mistakes

Using input = TRUE causes continuous toggling while input is true.

Using F_TRIG detects falling edge, not rising.

R_TRIG detects the rising edge of input, so the output toggles only when input changes from false to true.

4fill in blank

hard

Fill both blanks to create a structured text condition that checks if temperature is between 20 and 30 degrees.

PLC Programming

IF temperature [1] 20 AND temperature [2] 30 THEN heater := TRUE; END_IF;

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A>=

B<=

C<

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using > for the lower limit excludes 20.

Using <= for the upper limit includes 30, which may not be desired.

To check if temperature is between 20 and 30, it must be greater or equal to 20 and less than 30.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension in structured text that maps sensor names to their values if the value is above 50.

PLC Programming

sensor_map := { [1] : [2] FOR [3] IN sensors IF sensors[[3]] > 50 };

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

Asensor

Bsensors[sensor]

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using inconsistent variable names for keys and loop variable.

Using 'value' which is undefined in this context.

The comprehension maps each sensor name (sensor) to its value (sensors[sensor]) for sensors in the collection where the value is above 50.

Practice

1. Which programming style uses text commands like IF and ELSE to write logic in PLCs?

easy

A. Ladder Logic

B. Structured Text

C. Function Block Diagram

D. Sequential Function Chart

Solution

1. Step 1: Understand programming styles in PLCs

   Structured Text uses text-based commands like IF and ELSE to write logic clearly.

2. Step 2: Compare with Ladder Logic

   Ladder Logic uses graphical symbols resembling electrical circuits, not text commands.

3. Final Answer:

   Structured Text -> Option B

4. Quick Check:

   Text commands = Structured Text [OK]

Hint: Text commands like IF mean Structured Text [OK]

Common Mistakes:

• Confusing Ladder Logic with text commands
• Thinking graphical diagrams use IF/ELSE
• Mixing Function Block Diagram with text style

2. Which of the following is the correct way to write a conditional statement in Structured Text?

easy

A. MOVE input1 TO output;

B. LD input1; AND output;

C. SET input1; RESET output;

D. IF input1 THEN output := TRUE; END_IF;

Solution

1. Step 1: Identify Structured Text syntax

   Structured Text uses IF ... THEN ... END_IF; to write conditions.

2. Step 2: Check other options

   The other options use Ladder Logic or other instruction styles, not Structured Text.

3. Final Answer:

   IF input1 THEN output := TRUE; END_IF; -> Option D

4. Quick Check:

   IF ... THEN ... END_IF; = Structured Text condition [OK]

Hint: IF ... THEN ... END_IF; is Structured Text syntax [OK]

Common Mistakes:

• Using Ladder Logic commands in Structured Text
• Missing END_IF; in Structured Text
• Confusing assignment operator := with =

3. Given this Ladder Logic snippet:

--| |--| |--( )--

Which best describes its function compared to Structured Text?
IF input1 AND input2 THEN output := TRUE; ELSE output := FALSE; END_IF;

medium

A. Ladder Logic cannot represent conditional logic

B. Ladder Logic uses OR logic, Structured Text uses AND

C. Both represent the same AND logic controlling output

D. Structured Text cannot handle multiple inputs

Solution

1. Step 1: Understand Ladder Logic AND

   The two series contacts in Ladder Logic represent AND condition between inputs.

2. Step 2: Compare with Structured Text

   The Structured Text IF statement uses AND logic between input1 and input2 to set output TRUE or FALSE.

3. Final Answer:

   Both represent the same AND logic controlling output -> Option C

4. Quick Check:

   Series contacts = AND logic = Structured Text IF with AND [OK]

Hint: Series contacts in Ladder = AND condition [OK]

Common Mistakes:

• Thinking Ladder Logic only does OR logic
• Believing Structured Text can't handle multiple inputs
• Confusing parallel and series contacts

4. You wrote this Structured Text code but it does not compile:

IF input1 THEN output = TRUE; END_IF;

What is the error?

medium

A. Using = instead of := for assignment

B. Missing semicolon after TRUE

C. END_IF should be lowercase

D. IF statement cannot be used in Structured Text

Solution

1. Step 1: Check assignment operator

   Structured Text uses := for assignment, not = which is comparison.

2. Step 2: Verify other syntax

   Semicolon is present after TRUE; END_IF is case-insensitive.

3. Final Answer:

   Using = instead of := for assignment -> Option A

4. Quick Check:

   Assignment uses := not = in Structured Text [OK]

Hint: Use := for assignment, = is comparison [OK]

Common Mistakes:

• Using = instead of := for assignment
• Picking missing semicolon when it's present
• Thinking END_IF is case sensitive

5. You want to create a PLC program that is easy to read and modify by electricians familiar with circuit diagrams. Which programming style should you choose and why?

hard

A. Ladder Logic, because it visually resembles electrical circuits

B. Sequential Function Chart, because it shows steps

C. Function Block Diagram, because it uses blocks

D. Structured Text, because text is easier for electricians

Solution

1. Step 1: Consider electrician familiarity

   Electricians are used to reading electrical circuit diagrams.

2. Step 2: Match programming style to familiarity

   Ladder Logic visually looks like circuit diagrams, making it easier for electricians to read and modify.

3. Final Answer:

   Ladder Logic, because it visually resembles electrical circuits -> Option A

4. Quick Check:

   Electricians prefer Ladder Logic for circuit-like visuals [OK]

Hint: Choose Ladder Logic for circuit-like visuals [OK]

Common Mistakes:

• Assuming text is easier for electricians
• Confusing Function Block Diagram with Ladder Logic
• Ignoring user familiarity in choice