What is an HMI in PLC Programming - Visual Explanation

Concept Flow - What is an HMI

Start

↓

User interacts with HMI

↓

HMI sends commands to PLC

↓

PLC processes commands

↓

PLC sends data back to HMI

↓

HMI displays updated info

↓

User sees results and can interact again

↓

End or Repeat

This flow shows how a Human-Machine Interface (HMI) lets a user control and see data from a PLC by sending commands and receiving updates.

Execution Sample

PLC Programming

User presses button on HMI
HMI sends command to PLC
PLC updates machine state
PLC sends status to HMI
HMI shows new status

This sequence shows how pressing a button on the HMI leads to the PLC updating and the HMI showing the new machine status.

Execution Table

Step	Action	Source	Destination	Result
1	User presses button	User	HMI	HMI receives input
2	HMI sends command	HMI	PLC	PLC receives command
3	PLC processes command	PLC	Internal	Machine state updated
4	PLC sends status	PLC	HMI	HMI receives updated data
5	HMI displays status	HMI	User	User sees updated info
6	User decides next action	User	HMI	Cycle repeats or ends

💡 Process ends when user stops interacting or machine is turned off

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5
User Input	None	Button Pressed	Button Pressed	Button Pressed	Button Pressed
PLC Command	None	Received	Processed	Processed	Processed
Machine State	Idle	Idle	Running	Running	Running
HMI Display	Idle Screen	Idle Screen	Idle Screen	Running Screen	Running Screen

Key Moments - 3 Insights

Why does the HMI need to send commands to the PLC?

Because the HMI is just the interface; it cannot control the machine directly. It sends commands to the PLC, which controls the machine. See execution_table step 2.

How does the user see the machine status update?

The PLC sends updated data back to the HMI after processing commands. The HMI then displays this data to the user. See execution_table steps 4 and 5.

Can the user interact with the machine without the HMI?

Usually no. The HMI is the main way for the user to send commands and see status. The PLC controls the machine but needs input from the HMI or other devices.

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 3?

A User presses a button on the HMI

B PLC processes the command and updates machine state

C HMI sends command to PLC

D HMI displays updated status

💡 Hint

Check the 'Action' and 'Result' columns at step 3 in the execution_table

At which step does the user see the updated machine status?

A Step 5

B Step 4

C Step 2

D Step 1

💡 Hint

Look for when the HMI displays status to the user in the execution_table

If the PLC does not send data back to the HMI, what will happen?

A User will see updated machine status

B PLC will not process commands

C HMI will not update display with machine status

D User cannot press buttons on HMI

💡 Hint

Refer to the flow between PLC and HMI in execution_table steps 4 and 5

Concept Snapshot

HMI (Human-Machine Interface) is the screen and controls a user uses to interact with a machine.
It sends commands to the PLC (controller) and shows machine status.
User input -> HMI -> PLC -> Machine
Machine status -> PLC -> HMI -> User
HMI does not control machine directly; PLC does.
This cycle repeats as user interacts.

Full Transcript

An HMI is a device or screen that lets a person control and watch a machine. When the user presses a button on the HMI, it sends a command to the PLC, which controls the machine. The PLC processes the command and updates the machine's state. Then, the PLC sends the new status back to the HMI, which shows it to the user. This way, the user can see what the machine is doing and send new commands. The HMI and PLC work together in a cycle to let the user control the machine safely and easily.

Practice

1. What does HMI stand for in PLC programming?

easy

A. Hardware Machine Integration

B. High-speed Machine Input

C. Human-Machine Interface

D. Hybrid Motor Indicator

Solution

1. Step 1: Understand the abbreviation HMI

   HMI is commonly used to describe the interface between humans and machines in automation.

2. Step 2: Match the correct full form

   Human-Machine Interface correctly describes a screen or device that allows people to interact with machines.

3. Final Answer:

   Human-Machine Interface -> Option C

4. Quick Check:

   HMI = Human-Machine Interface [OK]

Hint: Remember HMI means interface for humans and machines [OK]

Common Mistakes:

• Confusing HMI with hardware terms
• Thinking HMI is a type of motor or sensor
• Mixing up abbreviations in automation

2. Which of the following is the correct way to describe an HMI in PLC systems?

easy

A. A screen interface to control and monitor machines

B. A programming language for PLCs

C. A type of sensor connected to the PLC

D. A PLC communication protocol

Solution

1. Step 1: Identify the role of HMI

   HMI is not a programming language, sensor, or protocol but a user interface device.

2. Step 2: Match the description to HMI

   It is a screen interface used to control and monitor machines connected to PLCs.

3. Final Answer:

   A screen interface to control and monitor machines -> Option A

4. Quick Check:

   HMI = Screen interface for control and monitoring [OK]

Hint: HMI is always about user screens, not code or sensors [OK]

Common Mistakes:

• Thinking HMI is a programming language
• Confusing HMI with sensors or protocols
• Assuming HMI is hardware only

3. What output would you expect from an HMI connected to a PLC controlling a conveyor belt?

medium

A. A log of network errors

B. A list of PLC programming codes

C. A sensor reading temperature only

D. A screen showing conveyor speed and start/stop buttons

Solution

1. Step 1: Understand HMI function with PLC

   HMI displays machine data and controls, not raw code or unrelated logs.

2. Step 2: Identify expected HMI output for conveyor control

   The HMI would show conveyor speed and provide buttons to start or stop the belt.

3. Final Answer:

   A screen showing conveyor speed and start/stop buttons -> Option D

4. Quick Check:

   HMI output = machine data and controls [OK]

Hint: HMI shows controls and data, not code or unrelated info [OK]

Common Mistakes:

• Expecting code or logs on HMI screen
• Confusing sensor data with HMI output
• Thinking HMI only shows errors

4. You designed an HMI screen but the start button does not send commands to the PLC. What is the most likely cause?

medium

A. The PLC program is missing the start command code

B. The button is not linked to the correct PLC tag or address

C. The HMI screen resolution is too low

D. The PLC is powered off

Solution

1. Step 1: Check HMI to PLC communication setup

   If the button does not send commands, it is often because it is not linked to the correct PLC tag or address.

2. Step 2: Rule out other causes

   While PLC power or program issues can cause problems, the question focuses on the HMI button not sending commands, so linkage is key.

3. Final Answer:

   The button is not linked to the correct PLC tag or address -> Option B

4. Quick Check:

   Button linkage to PLC tag = key for command sending [OK]

Hint: Check button linkage to PLC tag first when commands fail [OK]

Common Mistakes:

• Assuming screen resolution affects command sending
• Ignoring PLC program or power status
• Not verifying HMI tag connections

5. You want to create an HMI screen that shows machine temperature and allows the operator to reset alarms. Which two features must you include?

hard

A. A display area for temperature data and a reset button linked to PLC alarm reset tag

B. A PLC programming editor and a sensor calibration tool

C. A network monitor and a data logger

D. A motor speed controller and a temperature sensor

Solution

1. Step 1: Identify required HMI features for display and control

   The HMI must show temperature data and provide a button to reset alarms by sending commands to the PLC.

2. Step 2: Match features to options

   Only A display area for temperature data and a reset button linked to PLC alarm reset tag includes a display area for temperature and a reset button linked to the PLC alarm reset tag.

3. Final Answer:

   A display area for temperature data and a reset button linked to PLC alarm reset tag -> Option A

4. Quick Check:

   HMI needs display + control linked to PLC tags [OK]

Hint: Include display and control linked to PLC tags for HMI [OK]

Common Mistakes:

• Confusing HMI features with PLC programming tools
• Including unrelated network or motor controls
• Forgetting to link controls to PLC tags