Emergency stop implementation in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the emergency stop input.

PLC Programming

EM_STOP := [1];

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AOutput1

BInput1

CTimer1

DCounter1

Attempts:

3 left

💡 Hint

Common Mistakes

Using an output or timer instead of an input.

Confusing input and output variables.

The emergency stop is connected to an input, so we assign it to Input1.

2fill in blank

medium

Complete the code to stop the motor when emergency stop is pressed.

PLC Programming

IF [1] = TRUE THEN
  Motor := FALSE;
END_IF;

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AStart_Button

BMotor

CEM_STOP

DTimer1

Attempts:

3 left

💡 Hint

Common Mistakes

Checking the motor variable instead of the emergency stop input.

Using the start button variable by mistake.

The motor should stop when the emergency stop input (EM_STOP) is TRUE (pressed).

3fill in blank

hard

Fix the error in the emergency stop logic to ensure motor stops immediately.

PLC Programming

IF EM_STOP [1] TRUE THEN
  Motor := FALSE;
END_IF;

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A<>

B==

C!=

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using ':=' which assigns instead of compares.

Using '!=' or '<>' which check inequality.

In PLC programming, the equality operator is = to compare values.

4fill in blank

hard

Fill both blanks to create a safety check that resets the motor only if emergency stop is not pressed and start button is pressed.

PLC Programming

IF EM_STOP [1] FALSE AND Start_Button [2] TRUE THEN
  Motor := TRUE;
END_IF;

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A==

B=

C!=

D<>

Attempts:

3 left

💡 Hint

Common Mistakes

Using assignment ':=' instead of comparison '='.

Mixing equality and inequality operators.

Use = to compare both signals to their expected states.

5fill in blank

hard

Fill all three blanks to implement a complete emergency stop logic: stop motor if emergency stop pressed, else start motor if start button pressed and safety sensor is OK.

PLC Programming

IF EM_STOP [1] TRUE THEN
  Motor := FALSE;
ELSIF Start_Button [2] TRUE AND Safety_Sensor [3] TRUE THEN
  Motor := TRUE;
END_IF;

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

A==

B=

C!=

D<>

Attempts:

3 left

💡 Hint

Common Mistakes

Using ':=' instead of '=' in conditions.

Using inequality operators incorrectly.

All conditions must use the equality operator = to check the signals correctly.

Practice

1. What is the main purpose of an emergency stop in a PLC program?

easy

A. To immediately stop the machine for safety reasons

B. To start the machine automatically

C. To increase the machine speed

D. To reset the machine after a fault

Solution

1. Step 1: Understand emergency stop function

   The emergency stop is designed to stop machines immediately to protect people from harm.

2. Step 2: Identify correct purpose

   Stopping the machine immediately for safety is the key role of the emergency stop.

3. Final Answer:

   To immediately stop the machine for safety reasons -> Option A

4. Quick Check:

   Emergency stop = immediate machine stop [OK]

Hint: Emergency stop always stops machine immediately [OK]

Common Mistakes:

• Thinking emergency stop starts the machine
• Confusing emergency stop with reset
• Assuming it controls speed

2. Which of the following is the correct way to check an emergency stop button input named ESTOP in ladder logic?

easy

A. Timer instruction on ESTOP input

B. Normally Open contact of ESTOP input

C. Normally Closed contact of ESTOP input

D. Counter instruction on ESTOP input

Solution

1. Step 1: Understand emergency stop wiring

   Emergency stop buttons are usually wired as Normally Closed (NC) contacts for safety, so pressing opens the circuit.

2. Step 2: Use Normally Closed contact in logic

   In ladder logic, use the Normally Closed contact of ESTOP input to detect if the button is pressed (circuit open).

3. Final Answer:

   Normally Closed contact of ESTOP input -> Option C

4. Quick Check:

   ESTOP uses NC contact in ladder [OK]

Hint: Emergency stops use Normally Closed contacts [OK]

Common Mistakes:

• Using Normally Open contact instead of Normally Closed
• Using timers or counters incorrectly
• Ignoring safety wiring standards

3. Given this ladder logic snippet for emergency stop:

--|/|--[ESTOP]----( )--[MOTOR_RUN]

What happens to MOTOR_RUN when the emergency stop button is pressed?

medium

A. MOTOR_RUN coil de-energizes and motor stops

B. MOTOR_RUN coil toggles state

C. MOTOR_RUN coil energizes and motor runs

D. No change to MOTOR_RUN coil

Solution

1. Step 1: Interpret Normally Closed contact symbol

   The symbol |/| means Normally Closed contact of ESTOP input; it passes power when ESTOP is not pressed.

2. Step 2: Effect of pressing ESTOP

   Pressing ESTOP opens the NC contact, breaking the circuit and de-energizing MOTOR_RUN coil, stopping the motor.

3. Final Answer:

   MOTOR_RUN coil de-energizes and motor stops -> Option A

4. Quick Check:

   Press ESTOP = MOTOR_RUN off [OK]

Hint: NC contact breaks circuit when pressed [OK]

Common Mistakes:

• Confusing Normally Closed with Normally Open
• Assuming coil toggles instead of stops
• Ignoring contact state changes

4. This ladder logic is intended to implement an emergency stop:

--| |--[ESTOP]----( )--[MOTOR_RUN]

What is wrong with this implementation?

medium

A. Missing timer to delay motor stop

B. ESTOP input should be a counter

C. MOTOR_RUN coil is not connected

D. Using Normally Open contact for ESTOP can fail to stop motor

Solution

1. Step 1: Check contact type used for ESTOP

   The logic uses Normally Open contact | | for ESTOP, which means motor runs only when ESTOP is pressed, opposite of safety design.

2. Step 2: Understand safety wiring

   Emergency stops must use Normally Closed contacts to ensure motor stops immediately when pressed.

3. Final Answer:

   Using Normally Open contact for ESTOP can fail to stop motor -> Option D

4. Quick Check:

   ESTOP must use NC contact, not NO [OK]

Hint: Emergency stop must use NC contact, not NO [OK]

Common Mistakes:

• Using Normally Open contact for emergency stop
• Adding unnecessary timers
• Misusing counters for inputs

5. You want to implement an emergency stop that overrides all other start commands for a motor. Which ladder logic snippet correctly achieves this?

--|/|--[ESTOP]----+----( )--[MOTOR_RUN]
                  |
--| |--[START]----+

hard

A. Use NO ESTOP contact in series with START to control MOTOR_RUN coil

B. Use NC ESTOP contact in series with START to control MOTOR_RUN coil

C. Use NC ESTOP contact in parallel with START to control MOTOR_RUN coil

D. Use NO ESTOP contact in parallel with START to control MOTOR_RUN coil

Solution

1. Step 1: Analyze override logic

   Emergency stop must override start commands, so ESTOP NC contact should be in series to break circuit if pressed.

2. Step 2: Check ladder logic structure

   NC ESTOP contact in series with START contact means if ESTOP is pressed, circuit breaks and MOTOR_RUN coil de-energizes regardless of START.

3. Final Answer:

   Use NC ESTOP contact in series with START to control MOTOR_RUN coil -> Option B

4. Quick Check:

   NC ESTOP in series overrides START [OK]

Hint: Put NC ESTOP contact in series to override start [OK]

Common Mistakes:

• Placing ESTOP contact in parallel, losing override
• Using NO contact for ESTOP
• Not ensuring ESTOP breaks circuit immediately