HMI screen design basics in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this HMI tag configuration?

Given a tag configured to display a temperature value in Celsius with 1 decimal place, what will the HMI show if the tag value is 23.456?

A 23.4

B 23.45

C 23

D 23.5

💡 Hint

Think about how rounding works when showing one decimal place.

The value 23.456 rounded to one decimal place is 23.5. The tenths digit is 4, the hundredths is 5 followed by a non-zero digit (6), so round up to 23.5.

🧠 Conceptual

intermediate

Which principle is most important for HMI screen readability?

When designing an HMI screen, which principle helps users quickly understand the status of the system?

A Place all controls randomly to fill the screen

B Use as many colors as possible to attract attention

C Use consistent colors to represent specific statuses

D Use very small fonts to fit more information

💡 Hint

Think about how color helps users recognize information fast.

Consistent use of colors for statuses (like green for normal, red for alarm) helps users quickly identify system conditions without confusion.

🔧 Debug

advanced

Why does this HMI button not trigger the PLC action?

An HMI button is configured to write a boolean TRUE to a PLC tag when pressed, but pressing it does nothing. Which code snippet shows the likely cause?

PLC Programming

Button configuration:
Write Tag: Motor_Start
Write Value: "TRUE"

A The tag name Motor_Start is misspelled

B The write value is a string "TRUE" instead of boolean TRUE

C The button is not linked to any tag

D The PLC is offline

💡 Hint

Check the data type of the value being written.

Writing the string "TRUE" instead of the boolean TRUE causes the PLC to ignore the command because the data types do not match.

🚀 Application

advanced

How to design an alarm indicator that changes color based on severity?

You want an alarm indicator on the HMI that shows green when no alarm, yellow for warning, and red for critical alarm. Which approach is best?

A Use a single indicator with conditional color properties linked to alarm severity tags

B Use three separate indicators and show/hide them based on alarm severity

C Use text labels only without color changes

D Use blinking red color all the time

💡 Hint

Think about simplicity and clarity in design.

Using one indicator with conditional colors is efficient and clear, avoiding clutter and confusion.

🧠 Conceptual

expert

What is the main benefit of using navigation buttons with clear labels on HMI screens?

Why should navigation buttons on HMI screens have clear, descriptive labels?

A To help users quickly find and access different screens without confusion

B To make the screen look colorful and busy

C To reduce the number of screens needed

D To prevent users from accessing advanced functions

💡 Hint

Think about user experience and ease of use.

Clear labels on navigation buttons guide users efficiently, reducing errors and improving workflow.

Practice

1. What is the main purpose of an HMI screen in PLC programming?

easy

A. To store machine data permanently

B. To write complex PLC code

C. To replace the PLC hardware

D. To allow users to control and monitor machines easily

Solution

1. Step 1: Understand HMI function

   An HMI (Human-Machine Interface) screen is designed to help users interact with machines.

2. Step 2: Identify main purpose

   The main goal is to make control and monitoring simple and clear for users.

3. Final Answer:

   To allow users to control and monitor machines easily -> Option D

4. Quick Check:

   HMI = control and monitor [OK]

Hint: HMI means user interface for machine control [OK]

Common Mistakes:

• Confusing HMI with PLC programming itself
• Thinking HMI stores data permanently
• Assuming HMI replaces hardware

2. Which of the following is the correct way to label a button on an HMI screen?

easy

A. Use only icons without text

B. Use vague text like 'Click Here'

C. Use clear text like 'Start Motor'

D. Use very long sentences explaining the button

Solution

1. Step 1: Understand button labeling

   Buttons should have clear, simple labels so users know their function immediately.

2. Step 2: Evaluate options

   'Start Motor' clearly tells the user what the button does, unlike vague or overly long labels.

3. Final Answer:

   Use clear text like 'Start Motor' -> Option C

4. Quick Check:

   Clear labels = better usability [OK]

Hint: Label buttons with clear, simple text [OK]

Common Mistakes:

• Using unclear or vague labels
• Relying only on icons without text
• Writing long explanations on buttons

3. Consider this HMI screen design rule: "Use consistent colors for similar controls." What is the likely result if this rule is ignored?

medium

A. Users may get confused about control functions

B. The PLC program will stop working

C. The screen will load faster

D. The machine will run automatically

Solution

1. Step 1: Understand color consistency

   Consistent colors help users recognize similar controls easily.

2. Step 2: Predict effect of ignoring rule

   If colors vary randomly, users may not know which controls do similar things, causing confusion.

3. Final Answer:

   Users may get confused about control functions -> Option A

4. Quick Check:

   Consistent colors = less confusion [OK]

Hint: Consistent colors help users understand controls [OK]

Common Mistakes:

• Thinking color affects PLC operation
• Assuming color changes speed up screen
• Believing color affects machine automation

4. You designed an HMI screen but users report they can't find the navigation buttons easily. What is the most likely design mistake?

medium

A. Navigation buttons are hidden or placed inconsistently

B. Buttons have clear labels and consistent colors

C. Screen title is too large and clear

D. Screen uses standard navigation layout

Solution

1. Step 1: Analyze user feedback

   Users can't find navigation buttons easily means buttons are not visible or confusingly placed.

2. Step 2: Identify design mistake

   Hidden or inconsistent placement of navigation buttons causes difficulty in finding them.

3. Final Answer:

   Navigation buttons are hidden or placed inconsistently -> Option A

4. Quick Check:

   Visible, consistent navigation = easy to find [OK]

Hint: Place navigation buttons clearly and consistently [OK]

Common Mistakes:

• Ignoring button placement importance
• Assuming large titles improve navigation
• Using non-standard layouts without testing

5. You want to design an HMI screen for a machine with multiple steps. Which approach best helps users navigate smoothly between steps?

hard

A. Place all controls on one screen without navigation

B. Use a clear menu with buttons labeled for each step

C. Use random colors for each step's controls

D. Hide navigation buttons to save screen space

Solution

1. Step 1: Understand multi-step navigation needs

   Users need to move easily between steps without confusion.

2. Step 2: Evaluate navigation methods

   A clear menu with labeled buttons helps users know where to go next and jump between steps.

3. Step 3: Reject poor options

   Putting all controls on one screen can overwhelm users; random colors confuse; hiding buttons blocks navigation.

4. Final Answer:

   Use a clear menu with buttons labeled for each step -> Option B

5. Quick Check:

   Clear menus = smooth navigation [OK]

Hint: Label navigation buttons clearly for each step [OK]

Common Mistakes:

• Overloading one screen with all controls
• Using inconsistent colors that confuse users
• Hiding navigation buttons to save space