Safety PLC concepts in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a safety input in the PLC program.

PLC Programming

SafetyInput : [1];

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

ABOOL

BINT

CSTRING

DREAL

Attempts:

3 left

💡 Hint

Common Mistakes

Using numeric types like INT or REAL for safety inputs.

The safety input must be a Boolean type to represent ON/OFF states.

2fill in blank

medium

Complete the code to check if the safety input is active.

PLC Programming

IF SafetyInput [1] TRUE THEN
  SafetyOutput := TRUE;
END_IF;

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

A<>

B!=

C=

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' which is not valid in PLC ST.

Use '=' to compare the safety input to TRUE in PLC structured text.

3fill in blank

hard

Fix the error in the safety timer code to start timing when input is active.

PLC Programming

IF SafetyInput = TRUE THEN
  SafetyTimer.[1] := TRUE;
END_IF;

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

AET

BQ

CPT

DIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Q' which is the timer output, not input.

The 'IN' input starts the timer when set to TRUE.

4fill in blank

hard

Fill both blanks to define a safety relay output that activates when timer is done.

PLC Programming

IF SafetyTimer.[1] THEN
  SafetyRelay := [2];
END_IF;

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

AQ

BTRUE

CFALSE

DIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'IN' instead of 'Q' for timer output.

The timer's 'Q' output signals done timing; setting relay to TRUE activates it.

5fill in blank

hard

Fill all three blanks to create a safety check that resets the timer when input is off.

PLC Programming

IF SafetyInput = FALSE THEN
  SafetyTimer.[1] := FALSE;
  SafetyTimer.[2] := T#0s;
  SafetyRelay := [3];
END_IF;

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

AIN

BPT

CFALSE

DTRUE

Attempts:

3 left

💡 Hint

Common Mistakes

Setting relay to TRUE instead of FALSE when input is off.

Reset timer input 'IN' to FALSE, preset time 'PT' to zero, and deactivate relay by setting it to FALSE.

Practice

1. What is the main purpose of a Safety PLC in an industrial setting?

easy

A. To stop machines quickly when a safety issue is detected

B. To increase the speed of the machine

C. To control the temperature of the machine

D. To monitor the machine's production output

Solution

1. Step 1: Understand Safety PLC role

   Safety PLCs are designed to protect people by stopping machines if something unsafe happens.

2. Step 2: Compare options with purpose

   Only To stop machines quickly when a safety issue is detected matches the safety function of stopping machines quickly during safety issues.

3. Final Answer:

   To stop machines quickly when a safety issue is detected -> Option A

4. Quick Check:

   Safety PLC purpose = stop machines fast [OK]

Hint: Safety PLCs stop machines to protect people fast [OK]

Common Mistakes:

• Confusing safety PLC with speed control
• Thinking safety PLC controls production output
• Assuming safety PLC manages temperature

2. Which of the following is the correct way to declare a safety input in a PLC program?

easy

A. SafetyInput := BOOL;

B. SafetyInput BOOL;

C. BOOL SafetyInput;

D. SafetyInput = BOOL;

Solution

1. Step 1: Recall PLC variable declaration syntax

   In many PLC languages, variables are declared by type first, then name, like BOOL SafetyInput;.

2. Step 2: Check each option's syntax

   BOOL SafetyInput; matches the correct syntax. Others mix assignment or wrong order.

3. Final Answer:

   BOOL SafetyInput; -> Option C

4. Quick Check:

   Type before name = correct declaration [OK]

Hint: Type comes before variable name in PLC declarations [OK]

Common Mistakes:

• Using assignment operator '=' in declaration
• Putting variable name before type
• Adding ':=' which is for assignment, not declaration

3. Given this safety logic code snippet:

IF EmergencyStop = TRUE THEN
  MachineRunning := FALSE;
ELSE
  MachineRunning := TRUE;
END_IF;

What will be the value of MachineRunning if EmergencyStop is FALSE?

medium

A. Undefined

B. TRUE

C. Error

D. FALSE

Solution

1. Step 1: Analyze the IF condition

   If EmergencyStop is TRUE, MachineRunning is set to FALSE; otherwise, TRUE.

2. Step 2: Apply given value

   Since EmergencyStop is FALSE, the ELSE branch runs, setting MachineRunning to TRUE.

3. Final Answer:

   TRUE -> Option B

4. Quick Check:

   EmergencyStop FALSE means MachineRunning TRUE [OK]

Hint: False emergency stop means machine runs [OK]

Common Mistakes:

• Confusing TRUE and FALSE branches
• Assuming error when none exists
• Thinking variable is undefined

4. Identify the error in this safety PLC code snippet:

IF DoorClosed = TRUE THEN
  SafetyRelay := TRUE
ELSE
  SafetyRelay := FALSE;
END_IF;

medium

A. Missing semicolon after first assignment

B. Incorrect variable name 'DoorClosed'

C. Wrong IF syntax

D. No error in the code

Solution

1. Step 1: Check syntax line by line

   The first assignment line SafetyRelay := TRUE lacks a semicolon at the end.

2. Step 2: Verify other lines

   Other lines have correct semicolons and syntax.

3. Final Answer:

   Missing semicolon after first assignment -> Option A

4. Quick Check:

   Semicolon missing = syntax error [OK]

Hint: Every assignment needs a semicolon [OK]

Common Mistakes:

• Ignoring missing semicolon errors
• Assuming variable names are wrong without evidence
• Thinking IF syntax is incorrect

5. You want to ensure a safety door sensor input disables the machine immediately when opened. Which logic best achieves this in a Safety PLC?

hard

A. Use a normally open input from the door sensor to start the machine when opened

B. Ignore the door sensor and rely on emergency stop only

C. Use a timer to delay stopping the machine after door opens

D. Use a normally closed input from the door sensor to stop the machine when opened

Solution

1. Step 1: Understand safety door sensor behavior

   A normally closed (NC) sensor input means the circuit is closed (true) when door is closed, and opens (false) when door opens.

2. Step 2: Choose logic to stop machine immediately

   Using NC input ensures the machine stops immediately when door opens because input changes state instantly.

3. Step 3: Evaluate other options

   Use a normally open input from the door sensor to start the machine when opened starts machine on door open (unsafe), C delays stop (unsafe), D ignores sensor (unsafe).

4. Final Answer:

   Use a normally closed input from the door sensor to stop the machine when opened -> Option D

5. Quick Check:

   NC input stops machine immediately on door open [OK]

Hint: Normally closed sensor stops machine instantly [OK]

Common Mistakes:

• Using normally open sensor causing unsafe delays
• Delaying stop with timers
• Ignoring door sensor safety