PLC and HMI Communication Explained for Beginners

Easy Electra September 26, 2026

PLC and HMI Communication Explained for Beginners



Introduction

Human Machine Interfaces, commonly known as HMIs, are widely used in industrial automation to allow operators to monitor and control machines.

A PLC performs the control logic, while an HMI provides a graphical interface through which an operator can view machine status, enter commands, monitor values, and respond to alarms.

For example, an operator can use an HMI screen to start a motor, stop a conveyor, view tank level, monitor temperature, or acknowledge an alarm.

To perform these functions, the HMI needs to communicate with the PLC.

In this article, we will understand PLC-HMI communication, communication methods, HMI tags, practical examples, alarms, trends, and troubleshooting.

What Is an HMI?

HMI stands for Human Machine Interface.

An HMI is a device or software interface that allows an operator to interact with an industrial machine or process.

A typical HMI screen may contain:

  • Start and Stop buttons
  • Motor status indicators
  • Tank level displays
  • Temperature values
  • Pressure values
  • Alarm messages
  • Trends
  • Setpoint fields
  • Manual and automatic controls
  • Machine status information

The HMI does not normally replace the PLC control program. Instead, it provides an interface for monitoring and operator commands.

What Is PLC-HMI Communication?

PLC-HMI communication is the exchange of data between the PLC and HMI.

The HMI can read information from the PLC and can also write commands or values to PLC tags, depending on the system design.

A simple communication structure is:

Operator → HMI → Communication Network → PLC → Machine

For monitoring:

Machine → PLC → Communication Network → HMI → Operator

This allows the operator to see what is happening in the machine and interact with the control system.

Why Is HMI Used in Industrial Automation?

HMI systems make machine operation and monitoring easier for operators and maintenance personnel.

Common HMI functions include:

  • Starting and stopping equipment
  • Displaying machine status
  • Displaying process values
  • Changing setpoints
  • Showing alarms
  • Viewing trends
  • Monitoring production information
  • Selecting Auto or Manual mode
  • Displaying diagnostic information

For example, instead of using several physical meters to monitor a process, an HMI can display pressure, temperature, flow, and level on one screen.

Common PLC-HMI Communication Methods

Different PLC manufacturers and HMI systems support different communication protocols.

Common industrial communication methods include:

  • PROFINET
  • EtherNet/IP
  • Modbus TCP
  • Modbus RTU
  • Ethernet-based proprietary protocols
  • OPC UA

The correct protocol depends on the PLC, HMI, communication hardware, software, and system requirements.

PLC and HMI Communication Using Ethernet

Ethernet is widely used for communication between PLCs, HMIs, computers, drives, and other industrial devices.

A basic network can contain:

PLC → Ethernet Switch → HMI

Each device normally needs an appropriate network configuration.

For example, the PLC and HMI may be assigned IP addresses within the same network.

The exact addressing and network configuration depend on the equipment and plant network design.

PROFINET Communication

PROFINET is an industrial Ethernet communication technology commonly used with Siemens automation systems.

It can be used for communication between PLCs, HMIs, remote I/O, drives, and other automation devices.

A typical Siemens-based system may include:

Siemens PLC → PROFINET Network → HMI

In a TIA Portal project, the PLC and HMI can be configured within the same automation project, depending on the system architecture.

Device names, IP addresses, connections, and tag configuration must be correctly configured for communication to work.

EtherNet/IP Communication

EtherNet/IP is an industrial Ethernet protocol commonly used with Rockwell Automation systems and other compatible devices.

It can be used to exchange data between PLCs, HMIs, drives, remote I/O, and other industrial equipment.

The communication setup depends on the PLC and HMI software being used.

Modbus Communication

Modbus is another widely used industrial communication protocol.

Common versions include Modbus RTU and Modbus TCP.

Modbus RTU generally uses serial communication, while Modbus TCP operates over Ethernet networks.

Modbus communication uses defined data areas such as coils, inputs, registers, and holding registers depending on the implementation.

The exact addressing must be configured according to the PLC and device documentation.

What Are HMI Tags?

An HMI tag is a variable used by the HMI to access a value or status from the control system.

For example, a PLC may have a motor-running status variable.

The HMI can connect an indicator object to that variable.

When the PLC variable becomes ON, the HMI indicator can change its displayed state.

Typical HMI tags can represent:

  • Motor status
  • Push button commands
  • Tank level
  • Temperature
  • Pressure
  • Flow
  • Alarm status
  • Setpoints
  • Auto or Manual mode

Reading and Writing PLC Data

An HMI can generally perform two basic types of data operations.

Reading Data

The HMI reads information from the PLC.

For example:

PLC Motor Status → HMI Motor Indicator

PLC Tank Level → HMI Numeric Display

Writing Data

The HMI sends a command or value to the PLC.

For example:

HMI Start Button → PLC Start Command

HMI Temperature Setpoint → PLC Setpoint

The PLC program should determine whether the received command is permitted and how it affects the machine.

Practical Motor Control Example

Consider a motor controlled through an HMI.

The HMI contains a Start button, Stop button, and motor status indicator.

When the operator presses Start, the HMI sends a command to the PLC.

The PLC checks its control conditions and interlocks.

If the required conditions are satisfied, the PLC activates the motor output.

The PLC then sends the motor status back to the HMI.

The sequence is:

HMI Start → PLC Command → PLC Logic → Motor Output → Motor Running Status → HMI

This is a common structure for industrial HMI applications.

Analog Value Display Example

Suppose a PLC receives a 4–20 mA signal from a tank-level transmitter.

The PLC converts the signal into a raw value and scales it into an engineering value.

The HMI can then read the scaled value and display it to the operator.

The process is:

Level Transmitter → PLC Analog Input → Scaling → PLC Value → HMI Display

For example, the HMI could display:

Tank Level = 65%

The exact value depends on the transmitter measurement and PLC scaling configuration.

HMI Alarms

Alarms are used to inform the operator when an important process condition occurs.

Examples include:

  • High temperature
  • Low tank level
  • Motor overload
  • Emergency stop status
  • Communication failure
  • High pressure
  • Sensor fault

An alarm system can display the alarm condition, time, status, and other information depending on the HMI software.

Alarm acknowledgment and reset behavior should be designed according to the process requirements.

HMI trends allow operators to view how process values change over time.

For example, a trend can display:

  • Temperature over time
  • Tank level over time
  • Pressure over time
  • Flow over time
  • Motor speed over time

Trends can help operators and maintenance personnel understand process behavior and investigate certain operating problems.

Common Communication Problems

Several problems can prevent an HMI from communicating correctly with a PLC.

Common causes include:

  • Incorrect IP address
  • Incorrect subnet configuration
  • Incorrect PLC or HMI device configuration
  • Incorrect communication driver
  • Incorrect tag address
  • Wrong PLC connection
  • Network cable problems
  • Switch or network configuration problems
  • PLC not running
  • HMI communication settings not configured correctly

How to Troubleshoot PLC-HMI Communication

If the HMI is not communicating with the PLC, troubleshoot the system step by step.

  • Check whether the PLC is powered.
  • Check whether the PLC is in the expected operating mode.
  • Check the HMI power supply.
  • Check Ethernet or communication cables.
  • Check the PLC IP address.
  • Check the HMI IP address.
  • Check the subnet configuration.
  • Check the communication driver.
  • Check the PLC-HMI connection configuration.
  • Check the HMI tag addresses.
  • Check PLC and HMI diagnostic information.

It is usually easier to troubleshoot communication first and then investigate individual tag problems.

How to Practice PLC-HMI Communication

You can practice PLC-HMI communication using a small automation project.

For example, create an HMI screen containing:

  • Motor Start button
  • Motor Stop button
  • Motor ON indicator
  • Motor OFF indicator
  • Tank level display
  • Temperature display
  • Auto/Manual selector
  • Alarm display
  • Trend display

Connect these objects to PLC tags and test the communication.

This type of project provides practical experience with PLC programming, HMI design, tags, communication, alarms, and process visualization.

Important Safety Considerations

An HMI command should not be treated as a safety function simply because it controls a machine.

Safety functions such as emergency stops, guard monitoring, and hazardous-motion protection may require dedicated safety-rated hardware and control systems depending on the application.

Always follow the machine risk assessment, electrical drawings, manufacturer's documentation, and applicable safety requirements.

Conclusion

PLC-HMI communication is an important part of industrial automation.

The PLC performs the control logic, while the HMI provides a graphical interface for monitoring and operator interaction.

Understanding communication protocols, IP addressing, HMI tags, data reading and writing, alarms, trends, and troubleshooting will help you build practical industrial automation projects.

In the next article, we can learn about VFD control using PLC and HMI, including Start/Stop commands, speed reference, frequency control, and fault monitoring.

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Learn PLC and HMI communication for beginners. Understand HMI tags, PROFINET, EtherNet/IP, Modbus, Ethernet communication, alarms, trends, troubleshooting, and practical PLC-HMI examples.

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