NOT logic with NC contact in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to represent a NOT logic using a normally closed (NC) contact in ladder logic.

PLC Programming

IF [1] THEN
  OUTPUT := TRUE;
END_IF;

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

AInput1 OR Input2

BInput1 AND Input2

CNOT Input1

DInput1

Attempts:

3 left

💡 Hint

Common Mistakes

Using the input directly without NOT, which represents a normally open contact.

Combining inputs unnecessarily.

Using 'NOT Input1' simulates the behavior of a normally closed contact, which is true when Input1 is false.

2fill in blank

medium

Complete the ladder logic rung to turn ON the motor when the NC contact is active.

PLC Programming

MOTOR := [1];

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

AInput2

BNOT Input1

CInput1

DNOT Input2

Attempts:

3 left

💡 Hint

Common Mistakes

Using the input directly without NOT.

Using the wrong input variable.

The motor should turn ON when the normally closed contact (NOT Input1) is active.

3fill in blank

hard

Fix the error in the code to correctly represent the NC contact logic.

PLC Programming

IF [1] Input1 THEN
  LIGHT := TRUE;
END_IF;

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

AAND

BOR

CXOR

DNOT

Attempts:

3 left

💡 Hint

Common Mistakes

Using AND or OR which do not invert the input.

Using XOR which is unrelated here.

The NOT operator is needed to invert Input1 to simulate the NC contact behavior.

4fill in blank

hard

Fill both blanks to create a rung that energizes the alarm when the NC contact is open or the emergency stop is pressed.

PLC Programming

IF [1] OR [2] THEN
  ALARM := TRUE;
END_IF;

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

ANOT NC_Contact

BNC_Contact

CEmergency_Stop

DNOT Emergency_Stop

Attempts:

3 left

💡 Hint

Common Mistakes

Using NC_Contact directly instead of NOT NC_Contact.

Using NOT Emergency_Stop which is incorrect.

The alarm triggers if the NC contact is open (NOT NC_Contact) or if Emergency_Stop is pressed.

5fill in blank

hard

Fill all three blanks to create a dictionary mapping inputs to their NOT logic states.

PLC Programming

not_states = {"NC1": [1], "NC2": [2], "NC3": [3]

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

Anot Input1

Bnot Input2

Cnot Input3

DInput1

Attempts:

3 left

💡 Hint

Common Mistakes

Using inputs directly without NOT.

Mixing input variables incorrectly.

Each NC contact state is the NOT of its input signal.

Practice

1. What does a Normally Closed (NC) contact represent in PLC logic?

easy

A. The circuit is ON when the switch is NOT pressed

B. The circuit is ON only when the switch is pressed

C. The circuit is always OFF

D. The circuit toggles ON and OFF automatically

Solution

1. Step 1: Understand NC contact behavior

   An NC contact means the circuit is closed (ON) when the switch is not pressed.

2. Step 2: Compare options with NC contact logic

   Only The circuit is ON when the switch is NOT pressed correctly states the circuit is ON when the switch is NOT pressed.

3. Final Answer:

   The circuit is ON when the switch is NOT pressed -> Option A

4. Quick Check:

   NC contact = ON when NOT pressed [OK]

Hint: NC means circuit closed when switch is NOT pressed [OK]

Common Mistakes:

• Confusing NC with Normally Open (NO) contact
• Thinking circuit is ON when switch is pressed
• Assuming NC contact toggles automatically

2. Which of the following is the correct PLC ladder logic symbol for a Normally Closed (NC) contact?

easy

A. --|&|--

B. --| |--

C. --|/|--

D. --|X|--

Solution

1. Step 1: Recall PLC contact symbols

   Normally Open (NO) contact is --| |--, Normally Closed (NC) contact is --|/|--.

2. Step 2: Match symbol to NC contact

   --|/|-- shows --|/|--, which is the correct NC contact symbol.

3. Final Answer:

   --|/|-- -> Option C

4. Quick Check:

   NC contact symbol = --|/|-- [OK]

Hint: NC contact symbol has a slash / inside brackets [OK]

Common Mistakes:

• Choosing NO contact symbol instead of NC
• Confusing symbols with AND/OR logic
• Using invalid or made-up symbols

3. Given this PLC ladder logic snippet with an NC contact labeled Stop controlling output Motor:

--|/|--(Stop)----( )--(Motor)

What happens to Motor when the Stop button is pressed?

medium

A. Motor turns OFF

B. Motor toggles ON and OFF rapidly

C. Motor turns ON

D. Motor state does not change

Solution

1. Step 1: Understand NC contact effect when pressed

   Pressing the NC contact opens the circuit, breaking the path.

2. Step 2: Determine output state

   Since the circuit is open, the Motor output coil is not energized and turns OFF.

3. Final Answer:

   Motor turns OFF -> Option A

4. Quick Check:

   Pressing NC contact = circuit open = output OFF [OK]

Hint: Pressing NC opens circuit, output turns OFF [OK]

Common Mistakes:

• Assuming pressing NC turns output ON
• Thinking output toggles automatically
• Ignoring NC contact behavior

4. A PLC program uses an NC contact named EmergencyStop to stop a conveyor motor. The motor never stops when the emergency button is pressed. What is the most likely error?

medium

A. The PLC is powered off

B. The NC contact is wired as Normally Open (NO)

C. The emergency button is not pressed long enough

D. The output coil is missing

Solution

1. Step 1: Analyze symptom

   Motor never stops when emergency button pressed means circuit does not open.

2. Step 2: Identify wiring error

   If NC contact is wired as NO, pressing button closes circuit instead of opening it, so motor stays ON.

3. Final Answer:

   The NC contact is wired as Normally Open (NO) -> Option B

4. Quick Check:

   Wrong contact type wiring = motor won't stop [OK]

Hint: Check contact type wiring if stop fails [OK]

Common Mistakes:

• Blaming output coil without checking input wiring
• Assuming button press duration causes issue
• Ignoring PLC power status

5. You want to design a safety stop circuit using an NC contact SafetySwitch and an output coil MachineRun. Which ladder logic rung correctly implements the NOT logic with the NC contact to stop the machine when the switch is pressed?

hard

A. --| |--(SafetySwitch)----( )--(MachineRun)

B. --| |--(MachineRun)----( )--(SafetySwitch)

C. --|/|--(MachineRun)----( )--(SafetySwitch)

D. --|/|--(SafetySwitch)----( )--(MachineRun)

Solution

1. Step 1: Identify correct NC contact usage

   Safety stop uses NC contact so circuit is closed (MachineRun ON) when switch NOT pressed.

2. Step 2: Match ladder logic rung

   --|/|--(SafetySwitch)----( )--(MachineRun) uses NC contact --|/|-- controlling MachineRun output correctly.

3. Final Answer:

   --|/|--(SafetySwitch)----( )--(MachineRun) -> Option D

4. Quick Check:

   NC contact before output coil = safety stop logic [OK]

Hint: Use NC contact before output coil for safety stop [OK]

Common Mistakes:

• Using NO contact instead of NC for safety stop
• Reversing contact and coil positions
• Confusing contact labels with output coil