NOT logic with NC contact in PLC Programming - Step-by-Step Execution

Concept Flow - NOT logic with NC contact

Input Signal

↓

NC Contact (Normally Closed)

↓

NOT Logic Applied

↓

Output Signal

The NC contact inverts the input signal: when input is OFF, output is ON; when input is ON, output is OFF.

Execution Sample

PLC Programming

Input = 0  # OFF
Output = not Input  # Using NC contact logic
print(Output)

This code simulates NOT logic using an NC contact: output is the opposite of input.

Execution Table

Step	Input	NC Contact State	NOT Logic Result	Output
1	0 (OFF)	Closed (passes signal)	NOT 0 = 1	1 (ON)
2	1 (ON)	Open (blocks signal)	NOT 1 = 0	0 (OFF)

💡 Execution stops after testing both input states 0 and 1 to show NOT logic behavior.

Variable Tracker

Variable	Start	After Step 1	After Step 2
Input	undefined	0	1
NC Contact State	undefined	Closed	Open
Output	undefined	1	0

Key Moments - 2 Insights

Why does the output turn ON when the input is OFF?

Because the NC contact is normally closed, it passes the signal when input is OFF (0), so the NOT logic inverts 0 to 1, turning output ON as shown in execution_table row 1.

What happens to the output when the input is ON?

The NC contact opens, blocking the signal, so the NOT logic inverts 1 to 0, turning output OFF as shown in execution_table row 2.

Visual Quiz - 3 Questions

Look at the execution table, what is the output when input is 0?

A 1 (ON)

B 0 (OFF)

C Undefined

D Same as input

💡 Hint

Check execution_table row 1 under Output column.

At which step does the NC contact open?

A Step 1

B Never

C Step 2

D Both steps

💡 Hint

Look at NC Contact State column in execution_table.

If input stays at 1, what will the output be?

A 1 (ON)

B 0 (OFF)

C Toggle between 0 and 1

D Undefined

💡 Hint

Refer to variable_tracker Output value after Step 2.

Concept Snapshot

NOT logic with NC contact:
- NC contact is normally closed, passing signal when input is OFF.
- Output is the inverse of input.
- Input 0 → Output 1; Input 1 → Output 0.
- Used to invert signals in PLC logic easily.

Full Transcript

This visual execution shows how NOT logic works using a Normally Closed (NC) contact in PLC programming. When the input signal is OFF (0), the NC contact is closed, allowing the signal to pass, so the output becomes ON (1). When the input is ON (1), the NC contact opens, blocking the signal, so the output becomes OFF (0). The execution table traces these steps clearly, showing input, NC contact state, NOT logic result, and output. The variable tracker records how each variable changes after each step. Key moments clarify why output inverts input and how the NC contact state affects the output. The quiz tests understanding by asking about output values and NC contact states at different steps. This concept is useful for beginners to understand signal inversion in PLC logic using NC contacts.

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