Alarm system with latching in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the alarm when the sensor is triggered.

PLC Programming

IF sensor_triggered THEN alarm [1] TRUE;

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

A:=

B==

C=

D->

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' instead of ':=' causes syntax errors.

Using '==' is not valid in PLC assignment.

In PLC programming, ':=' is used to assign a value to a variable or output.

2fill in blank

medium

Complete the code to latch the alarm output when triggered.

PLC Programming

alarm_latched := alarm_latched [1] alarm;

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

AAND

BXOR

COR

DNOT

Attempts:

3 left

💡 Hint

Common Mistakes

Using AND will reset the latch when alarm is FALSE.

Using XOR or NOT will not latch correctly.

Using OR keeps the alarm latched once it is set, even if the alarm input resets.

3fill in blank

hard

Fix the error in the reset condition to clear the latched alarm.

PLC Programming

IF reset_button [1] TRUE THEN alarm_latched := FALSE;

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

A=>

B:=

C==

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using ':=' in IF condition causes syntax errors.

Using '==' or '=>' is invalid in this context.

In conditions, '=' is used to compare values, not ':=' which is for assignment.

4fill in blank

hard

Fill both blanks to complete the alarm latching logic with reset.

PLC Programming

IF sensor_triggered THEN alarm := TRUE; ELSE IF reset_button [1] TRUE THEN alarm_latched := FALSE; END_IF; alarm_latched := alarm_latched [2] sensor_triggered;

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

A=

B:=

COR

DAND

Attempts:

3 left

💡 Hint

Common Mistakes

Using ':=' in IF condition.

Using 'AND' instead of 'OR' for latching.

The reset_button is compared with '='. The alarm_latched uses 'OR' to latch the alarm.

5fill in blank

hard

Fill all three blanks to implement a complete alarm system with latching and reset.

PLC Programming

IF sensor_triggered [1] TRUE THEN alarm := TRUE; ELSIF reset_button [2] TRUE THEN alarm_latched := FALSE; END_IF; alarm_latched := alarm_latched [3] sensor_triggered;

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

A=

B!=

COR

DAND

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' instead of '=' for input checks.

Using 'AND' instead of 'OR' for latching.

Both sensor_triggered and reset_button are compared with '='. The alarm_latched uses 'OR' to latch the alarm.

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