Alarm system with latching in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a simple latching alarm logic

Given the following PLC ladder logic pseudocode for an alarm system with latching, what is the output state of the alarm after the input sensor is activated and then deactivated?

PLC Programming

IF SensorInput THEN Alarm := TRUE;
IF ResetButton THEN Alarm := FALSE;

A Alarm remains TRUE after SensorInput is deactivated

B Alarm resets to FALSE immediately after SensorInput is deactivated

C Alarm toggles between TRUE and FALSE continuously

D Alarm never turns TRUE

💡 Hint

Think about what latching means: once set, the alarm stays on until reset.

The alarm latches ON when the sensor input activates it and stays ON even after the sensor input deactivates, until the reset button is pressed.

🧠 Conceptual

intermediate

Purpose of latching in alarm systems

Why is latching used in alarm systems in PLC programming?

A To keep the alarm active even if the triggering condition disappears until manually reset

B To automatically reset the alarm when the triggering condition disappears

C To prevent the alarm from ever turning ON

D To make the alarm blink on and off continuously

💡 Hint

Think about what happens if the alarm turned off immediately when the problem stopped.

Latching keeps the alarm ON so the operator notices the problem even if the triggering condition is momentary.

🔧 Debug

advanced

Identify the error in this latching alarm code

What error will occur when running this PLC code snippet for an alarm with latching? IF SensorInput THEN Alarm := TRUE; IF NOT SensorInput THEN Alarm := FALSE;

PLC Programming

IF SensorInput THEN Alarm := TRUE;
IF NOT SensorInput THEN Alarm := FALSE;

A Alarm output will cause a runtime overflow error

B Syntax error due to missing ELSE statement

C Alarm will latch ON correctly and reset only on manual reset

D Alarm will never latch ON; it resets immediately when SensorInput is off

💡 Hint

Check what happens when SensorInput turns off.

The alarm resets immediately when SensorInput is off, so it does not latch.

🚀 Application

advanced

Implementing manual reset in a latching alarm

Which code snippet correctly implements a latching alarm with a manual reset button in PLC pseudocode?

A IF SensorInput THEN Alarm := TRUE ELSE Alarm := FALSE;

B

IF SensorInput THEN Alarm := TRUE;
IF ResetButton THEN Alarm := FALSE;

C

IF ResetButton THEN Alarm := TRUE;
IF SensorInput THEN Alarm := FALSE;

D Alarm := SensorInput AND NOT ResetButton;

💡 Hint

The alarm should stay ON after SensorInput activates it until ResetButton is pressed.

Option B sets Alarm ON when SensorInput is TRUE and resets Alarm OFF only when ResetButton is pressed, implementing latching.

📝 Syntax

expert

Correct syntax for latching alarm in structured text

Which of the following structured text code snippets correctly implements a latching alarm with manual reset?

A Alarm := SensorInput OR ResetButton;

B

IF SensorInput THEN Alarm = TRUE;
IF ResetButton THEN Alarm = FALSE;

C

IF SensorInput THEN Alarm := TRUE; END_IF;
IF ResetButton THEN Alarm := FALSE; END_IF;

D IF SensorInput THEN Alarm := TRUE ELSE Alarm := FALSE;

💡 Hint

Check assignment operators and block endings in structured text.

Option C uses correct assignment ':=' and properly ends IF blocks with END_IF.

Practice

1.

What does a latching alarm system do in a PLC program?

easy

A. Keeps the alarm ON after the trigger until manually reset

B. Turns the alarm ON only while the trigger is active

C. Automatically resets the alarm after a delay

D. Disables the alarm permanently after one trigger

Solution

1. Step 1: Understand alarm behavior

   A latching alarm stays ON even if the trigger stops, unlike a momentary alarm.

2. Step 2: Identify manual reset role

   The alarm stays latched until a manual reset input clears it.

3. Final Answer:

   Keeps the alarm ON after the trigger until manually reset -> Option A

4. Quick Check:

   Latching alarm = stays ON until reset [OK]

Hint: Latching means alarm stays ON until reset manually [OK]

Common Mistakes:

• Confusing latching with momentary alarms
• Thinking alarm resets automatically
• Assuming alarm disables permanently after trigger

2.

Which of the following is the correct ladder logic expression to latch an alarm ALARM when TRIGGER is true, and reset it when RESET is true?

ALARM = (TRIGGER OR ALARM) AND NOT RESET

easy

A. ALARM = (TRIGGER AND ALARM) OR RESET

B. ALARM = (TRIGGER OR ALARM) AND NOT RESET

C. ALARM = TRIGGER AND NOT ALARM AND RESET

D. ALARM = NOT TRIGGER OR ALARM OR RESET

Solution

1. Step 1: Understand latching logic

   The alarm stays ON if triggered or already latched, unless reset.

2. Step 2: Match expression to logic

   ALARM = (TRIGGER OR ALARM) AND NOT RESET matches: ALARM = (TRIGGER OR ALARM) AND NOT RESET.

3. Final Answer:

   ALARM = (TRIGGER OR ALARM) AND NOT RESET -> Option B

4. Quick Check:

   Latching = OR previous alarm + trigger, AND NOT reset [OK]

Hint: Use OR for latch, AND NOT for reset in logic [OK]

Common Mistakes:

• Using AND instead of OR for latch condition
• Forgetting to invert RESET input
• Mixing AND and OR incorrectly

3.

Given the following PLC code snippet, what is the value of ALARM after these inputs?

ALARM = False
TRIGGER = True
RESET = False

ALARM = (TRIGGER or ALARM) and not RESET

TRIGGER = False
ALARM = (TRIGGER or ALARM) and not RESET

RESET = True
ALARM = (TRIGGER or ALARM) and not RESET

What is the final ALARM value?

medium

A. True

B. Undefined

C. Error

D. False

Solution

1. Step 1: Evaluate first assignment

   ALARM = (True or False) and not False = True and True = True

2. Step 2: Evaluate second assignment

   TRIGGER = False, ALARM = (False or True) and not False = True and True = True

3. Step 3: Evaluate third assignment

   RESET = True, ALARM = (False or True) and not True = True and False = False

4. Final Answer:

   False -> Option D

5. Quick Check:

   Reset clears alarm = False [OK]

Hint: Reset input clears alarm regardless of trigger [OK]

Common Mistakes:

• Ignoring reset input effect
• Assuming alarm stays True after reset
• Confusing OR and AND logic

4.

Identify the error in this PLC latching alarm code snippet:

ALARM = TRIGGER AND NOT RESET
ALARM = ALARM OR TRIGGER

medium

A. The alarm will never reset because reset is ignored in second line

B. The alarm resets correctly but never triggers

C. Syntax error due to missing parentheses

D. No error, code works as intended

Solution

1. Step 1: Analyze first line

   ALARM is set only if TRIGGER is true and RESET is false.

2. Step 2: Analyze second line

   ALARM is overwritten by ALARM OR TRIGGER, ignoring RESET input.

3. Step 3: Effect on reset

   Reset input is ignored after first line, so alarm never clears.

4. Final Answer:

   The alarm will never reset because reset is ignored in second line -> Option A

5. Quick Check:

   Reset ignored after first line = alarm stuck ON [OK]

Hint: Reset must be included in all latch logic lines [OK]

Common Mistakes:

• Overwriting alarm without reset condition
• Assuming OR line resets alarm
• Ignoring order of assignments

5.

You want to design a latching alarm that activates when either TEMP_HIGH or PRESSURE_HIGH is true, and resets only when MANUAL_RESET is true. Which expression correctly implements this?

hard

A. ALARM = (TEMP_HIGH AND PRESSURE_HIGH) AND NOT MANUAL_RESET

B. ALARM = (TEMP_HIGH OR PRESSURE_HIGH) AND MANUAL_RESET

C. ALARM = (TEMP_HIGH OR PRESSURE_HIGH OR ALARM) AND NOT MANUAL_RESET

D. ALARM = (TEMP_HIGH AND PRESSURE_HIGH AND ALARM) OR MANUAL_RESET

Solution

1. Step 1: Understand alarm trigger conditions

   Alarm triggers if TEMP_HIGH or PRESSURE_HIGH is true.

2. Step 2: Include latching behavior

   Alarm stays ON if already latched (ALARM) or triggered.

3. Step 3: Reset condition

   Alarm resets only when MANUAL_RESET is true, so use NOT MANUAL_RESET to keep alarm ON.

4. Final Answer:

   ALARM = (TEMP_HIGH OR PRESSURE_HIGH OR ALARM) AND NOT MANUAL_RESET -> Option C

5. Quick Check:

   Latch = OR triggers + alarm, reset = NOT manual reset [OK]

Hint: Use OR for triggers and latch, AND NOT for reset [OK]

Common Mistakes:

• Using AND instead of OR for triggers
• Reset condition inverted incorrectly
• Missing latch condition in expression