Numeric displays and indicators in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this PLC ladder logic snippet for a numeric display?

Given a PLC program that reads an analog input value of 1234 and scales it to a 0-100 range for display, what numeric value will the display show?

PLC Programming

AnalogInput = 1234
ScaledValue = (AnalogInput / 4095) * 100
DisplayValue = INT(ScaledValue)

A 32

B 31

C 29

D 30

💡 Hint

Remember to convert the scaled float value to an integer by truncation.

The analog input 1234 divided by max 4095 gives approximately 0.301, multiplied by 100 is 30.1. Truncating to integer gives 30, so the display shows 30.

📝 Syntax

intermediate

Which option correctly formats a numeric value for a 7-segment display in PLC structured text?

You want to convert an integer variable 'Value' to a string for a 7-segment display. Which code snippet is syntactically correct?

A DisplayString := TOSTRING(Value);

B DisplayString = TO_STRING(Value)

C DisplayString := TO_STRING(Value);

D DisplayString := TO_STRING(Value)

💡 Hint

Check the assignment operator and function name spelling.

In PLC structured text, ':=' is the assignment operator. The function to convert to string is TO_STRING. Option C uses correct syntax and function name.

🔧 Debug

advanced

Identify the error causing incorrect numeric display update in this PLC code

This code is intended to update a numeric display with the current temperature reading every cycle, but the display shows stale data. What is the error?

PLC Programming

Temperature := ReadSensor();
IF Temperature > 100 THEN
  DisplayValue := 100;
ELSE
  DisplayValue := Temperature;
END_IF
// Missing update command for display

A The display update command is missing after setting DisplayValue

B DisplayValue is assigned outside the IF block

C The IF condition syntax is incorrect

D Temperature variable is not declared

💡 Hint

Check if the display is refreshed after value assignment.

Assigning DisplayValue alone does not update the physical display. A command or function call to refresh the display is needed after setting the value.

🚀 Application

advanced

How to implement a blinking numeric indicator for alarm in PLC code?

You want a numeric display to blink the value '999' when an alarm is active. Which approach correctly achieves this?

A Use a timer to toggle between displaying '999' and blank every 500ms

B Continuously write '999' to the display without delay

C Set the display value to '999' only once when alarm triggers

D Use a counter to increment display value from 0 to 999 repeatedly

💡 Hint

Blinking means alternating visible and invisible states over time.

Using a timer to toggle the display between '999' and blank creates a blinking effect. Other options do not produce blinking.

🧠 Conceptual

expert

What is the main advantage of using BCD (Binary-Coded Decimal) for numeric displays in PLCs?

Why do many PLC numeric displays use BCD encoding instead of pure binary for numeric values?

A BCD is compatible with all analog sensors directly

B BCD simplifies conversion to decimal digits for display, reducing processing complexity

C BCD uses less memory than binary representation

D BCD allows faster arithmetic operations than binary

💡 Hint

Think about how numbers are shown on screens digit by digit.

BCD encodes each decimal digit separately, making it easier to convert and display decimal numbers without complex binary to decimal conversion.

Practice

1. What is the main purpose of numeric displays in PLC programming?

easy

A. To control the power supply

B. To store data in memory

C. To show numbers clearly on control panels

D. To connect different devices

Solution

1. Step 1: Understand numeric displays

   Numeric displays are designed to show numbers clearly so operators can read values easily.

2. Step 2: Identify the correct purpose

   Among the options, only showing numbers clearly matches the function of numeric displays.

3. Final Answer:

   To show numbers clearly on control panels -> Option C

4. Quick Check:

   Numeric displays = show numbers clearly [OK]

Hint: Numeric displays always show numbers, not control or storage [OK]

Common Mistakes:

• Confusing numeric displays with data storage
• Thinking numeric displays control power
• Mixing numeric displays with device connections

2. Which of the following is the correct syntax to display a numeric value on a PLC numeric display using a function called DisplayNumber with input value?

easy

A. DisplayNumber(value);

B. DisplayNumber = value;

C. DisplayNumber:value;

D. DisplayNumber->value;

Solution

1. Step 1: Identify function call syntax

   In PLC programming, calling a function with an argument uses parentheses, like DisplayNumber(value);

2. Step 2: Check other options

   Options A, B, and C use incorrect assignment or symbol syntax not valid for function calls.

3. Final Answer:

   DisplayNumber(value); -> Option A

4. Quick Check:

   Function call syntax uses parentheses [OK]

Hint: Function calls use parentheses with arguments [OK]

Common Mistakes:

• Using assignment instead of function call
• Using colon or arrow symbols incorrectly
• Missing parentheses for function arguments

3. Given the following PLC code snippet, what will be the displayed output on the numeric display?

int temperature = 25;
DisplayNumber(temperature + 5);

medium

A. Error

B. 25

C. 5

D. 30

Solution

1. Step 1: Calculate the expression inside DisplayNumber

   The code adds 5 to the variable temperature (25 + 5), resulting in 30.

2. Step 2: Understand DisplayNumber output

   The function displays the calculated value, so the numeric display shows 30.

3. Final Answer:

   30 -> Option D

4. Quick Check:

   25 + 5 = 30 displayed [OK]

Hint: Add values before display function call [OK]

Common Mistakes:

• Ignoring the addition operation
• Assuming the display shows the variable only
• Thinking the code causes an error

4. Identify the error in this PLC code snippet intended to display a numeric value:

int speed = 100
DisplayNumber(speed);

medium

A. Incorrect function name

B. Missing semicolon after variable declaration

C. Variable type is wrong

D. DisplayNumber cannot take variables

Solution

1. Step 1: Check syntax of variable declaration

   The line int speed = 100 is missing a semicolon at the end, which is required.

2. Step 2: Verify function call correctness

   The function call DisplayNumber(speed); is correct and uses the variable properly.

3. Final Answer:

   Missing semicolon after variable declaration -> Option B

4. Quick Check:

   Semicolon missing after int declaration [OK]

Hint: Always end statements with semicolon [OK]

Common Mistakes:

• Forgetting semicolon after declarations
• Assuming function name is wrong
• Thinking variables can't be passed to functions

5. You want to display a temperature value with one decimal place on a numeric display in a PLC program. Which approach correctly formats the number before display?

float temp = 23.456;
// Choose the correct formatting method
DisplayNumber(____);

hard

A. round(temp, 1)

B. int(temp)

C. temp + 0.1

D. floor(temp)

Solution

1. Step 1: Understand formatting for decimals

   To show one decimal place, rounding the float to 1 decimal is needed.

2. Step 2: Evaluate options

   Only round(temp, 1) correctly rounds to one decimal place. temp + 0.1 (temp + 0.1) changes the value, B (int(temp)) removes decimals, D (floor(temp)) truncates down.

3. Final Answer:

   round(temp, 1) -> Option A

4. Quick Check:

   Rounding to 1 decimal = round(temp, 1) [OK]

Hint: Use round(value, decimals) to format numbers [OK]

Common Mistakes:

• Using int() which removes decimals
• Adding small values instead of rounding
• Using floor() which truncates without rounding