Numeric displays and indicators in PLC Programming - Mini Project: Build & Apply

Numeric displays and indicators

📖 Scenario: You are programming a simple PLC (Programmable Logic Controller) to monitor a temperature sensor and display the temperature on a numeric display. The display should show the temperature value only if it is above a certain threshold. This is common in industrial automation where operators need to see critical values clearly.

🎯 Goal: Build a PLC program that reads a temperature value, sets a threshold, checks if the temperature is above the threshold, and then displays the temperature on a numeric display only if the condition is met.

📋 What You'll Learn

Create a variable called temperature with the exact value 75.

Create a variable called threshold with the exact value 70.

Use a conditional statement to check if temperature is greater than threshold.

If the condition is true, assign temperature to display_value.

Print the display_value to simulate the numeric display output.

💡 Why This Matters

🌍 Real World

In factories and plants, PLCs monitor sensors and show important numbers on displays so operators can quickly see if conditions are safe or need attention.

💼 Career

Understanding how to program numeric displays and indicators is essential for automation technicians and engineers who maintain and program industrial control systems.

1

Set up the temperature variable

Create a variable called temperature and set it to the exact value 75.

PLC Programming

# Create a variable called temperature and set it to 75
# Your code here

Hint

Think of temperature as the sensor reading you want to display.

2

Set the threshold value

Create a variable called threshold and set it to the exact value 70.

PLC Programming

temperature = 75
# Create a variable called threshold and set it to 70
# Your code here

Hint

The threshold is the minimum temperature to show on the display.

3

Check temperature against threshold

Use an if statement to check if temperature is greater than threshold. If true, assign temperature to a new variable called display_value. Otherwise, assign 0 to display_value.

PLC Programming

temperature = 75
threshold = 70
# Use if statement to set display_value
# Your code here

Hint

This controls when the display shows the temperature or stays blank (0).

4

Display the temperature value

Print the variable display_value to simulate the numeric display output.

PLC Programming

temperature = 75
threshold = 70
if temperature > threshold:
    display_value = temperature
else:
    display_value = 0
# Print display_value
# Your code here

Hint

The output should show the temperature value if above threshold.

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