HMI Basics: What Is a Human Machine Interface and How Does It Work?

Quick overview: This guide explains hmi and the practical checks that help you understand and plan an installation. Use the contents below to go directly to the relevant section.

In an automated machine or process, the PLC may be making the control decisions, but operators still need a practical way to see what the system is doing and interact with it. That is where the Human Machine Interface, or HMI, fits into the automation architecture.

An HMI can provide operators with a visual representation of equipment status, process values, alarms and controls. Depending on the application, it can be anything from a compact panel mounted beside a machine to a larger touchscreen interface providing multiple process screens, trends, recipes and diagnostic information.

Choosing an HMI is therefore not simply a matter of selecting a screen size. The PLC or controller, communications protocol, number of variables, environmental conditions, mounting arrangement, software platform and required operator functions all influence the correct choice.

This guide explains the fundamentals and the information purchasing and technical teams should establish when selecting an HMI for an industrial automation system.

What Is an HMI?

HMI stands for Human Machine Interface.

In industrial automation, an HMI is an operator interface that allows a person to view information from and, where permitted, send commands to a machine or process control system.

An HMI can display information such as:

• Motor running or stopped status
• Tank levels
• Pressure
• Temperature
• Flow
• Speed
• Valve position
• Drive frequency
• Production counts
• Equipment alarms
• Operating modes

It can also provide controls such as start and stop commands, setpoint entry, mode selection, alarm acknowledgement and navigation between process screens, depending on how the automation system is designed.

HMI vs PLC

An HMI and a PLC perform different functions.

The PLC typically executes the control logic. It reads inputs, processes the programmed logic and controls outputs.

The HMI provides the visual and interactive layer through which an operator can view selected PLC data and issue authorised commands.

In a typical system:

Sensors and field devices → PLC → HMI

The HMI does not normally replace the PLC's core control logic. If an HMI fails, a correctly designed PLC system may continue executing its control program, although the operator may lose visibility or manual interaction until the interface is restored.

HMI vs SCADA

HMI and SCADA are related but not identical terms.

A local HMI is commonly associated with one machine, panel or process area. A SCADA system can supervise a much larger process and may collect data from multiple PLCs, RTUs or sites.

SCADA platforms can provide centralised monitoring, historical data, reporting, alarm management and supervisory control across distributed infrastructure.

Some modern HMI products include functions traditionally associated with SCADA, so the boundary can overlap. The actual capabilities of the selected hardware and software should therefore be checked rather than relying only on the product category.

How Does an HMI Communicate With a PLC?

The HMI and PLC exchange information using a supported communications interface and protocol.

Depending on the equipment, this can include:

• Industrial Ethernet
• Modbus TCP
• Modbus RTU
• PROFINET
• EtherNet/IP
• Manufacturer-specific PLC protocols
• RS-232
• RS-485
• Other supported industrial communications

Both devices need to support the required communications method, and the HMI software must have a suitable driver or protocol implementation for the PLC.

Two devices having Ethernet ports does not automatically mean they can communicate with each other.

What Is an HMI Tag?

An HMI tag is a software reference to a value used by the HMI project.

A tag can represent a PLC variable such as:

• Motor_Run
• Tank_Level
• Pressure_Setpoint
• Drive_Frequency
• High_Level_Alarm

The HMI reads or writes the corresponding PLC address or symbolic variable according to the communication configuration.

The number of supported tags can be an important product limitation on some HMI platforms or software licences.

What Can an HMI Display?

An HMI project can include many different visual objects and screens depending on the platform.

Common examples include:

• Pushbuttons
• Status indicators
• Numeric values
• Bar graphs
• Tank graphics
• Motor and pump symbols
• Alarm lists
• Trend graphs
• Setpoint entry fields
• Navigation buttons
• Production counters
• Maintenance information

The HMI should present information clearly enough that operators can understand the state of the process without unnecessary visual clutter.

Touchscreen vs Keypad HMI

Most modern operator panels use touchscreens, but physical-key or hybrid interfaces are still used in particular applications.

Touchscreens allow flexible screen layouts and can reduce the number of separate panel-mounted pushbuttons required for non-safety functions.

Physical buttons can still be useful where operators wear gloves, tactile feedback is important or particular functions need dedicated controls.

Safety-critical functions such as emergency stopping should not be moved onto an ordinary touchscreen merely for convenience. Safety functions require an appropriately designed safety system.

Resistive vs Capacitive Touchscreens

Industrial HMI touchscreens can use different touch technologies.

Resistive touchscreens respond to pressure and are commonly used in industrial panels. Depending on the design, they can often be operated with gloves or a suitable stylus.

Capacitive touchscreens can provide modern multi-touch interaction and high visual clarity, but glove and environmental performance depends on the specific product.

The best choice depends on the operating environment and how personnel will interact with the screen.

Choosing the Right Screen Size

HMI screen sizes range from compact displays to large industrial panels.

A larger screen can provide more space for process information, but bigger is not automatically better.

Consider:

• Amount of information displayed at once
• Viewing distance
• Operator interaction
• Available panel space
• Required button size
• Number of screens
• Resolution
• Existing cut-out dimensions

A well-designed smaller HMI can be easier to use than a large screen overloaded with information.

Resolution Matters

Two HMIs with the same physical screen size can have different display resolutions.

Resolution affects how much information can be displayed and how graphics and text appear.

When replacing an HMI, a different resolution can require changes to the HMI project even when the physical screen size appears similar.

Aspect Ratio

Older HMIs frequently use more square display formats, while newer models often use widescreen displays.

A migration from one aspect ratio to another can require screen layouts to be redesigned or adjusted.

For replacement projects, the original project file and engineering software compatibility are therefore important.

Panel Cut-Out Dimensions

An HMI is normally installed through a cut-out in the front of an electrical or control panel.

Physical screen size does not determine the exact cut-out size.

When replacing an existing HMI, confirm:

• Overall dimensions
• Panel cut-out dimensions
• Mounting depth
• Mounting clamps
• Required clearances

A replacement that is slightly larger or smaller can require mechanical panel modifications.

Power Supply Requirements

Industrial HMIs commonly use DC power supplies, often 24 V DC, although the actual permitted voltage range depends on the model.

Confirm:

• Nominal voltage
• Permitted voltage range
• Power consumption
• Terminal type
• Earthing requirements

The control-panel power supply must have adequate capacity for the HMI and other connected control equipment.

Ethernet Ports

Ethernet is common on modern HMIs and can be used for PLC communications, programming, remote access or other supported functions.

Some HMIs provide more than one Ethernet port, which can support different network arrangements depending on the product.

Do not assume that multiple Ethernet ports operate as an unmanaged network switch. Their intended function must be checked in the manufacturer's documentation.

Serial Communications

HMIs can include RS-232 or RS-485 serial ports for communicating with PLCs, VSDs, instruments and other devices.

When specifying a serial connection, confirm:

• RS-232 or RS-485
• Connector or terminal arrangement
• Protocol
• Baud rate
• Parity
• Stop bits
• Device addressing

The presence of an RS-485 port does not guarantee support for every RS-485-based protocol.

Modbus RTU and Modbus TCP on HMIs

Many HMI platforms support Modbus communications.

Modbus RTU typically operates over serial networks such as RS-485, while Modbus TCP operates over Ethernet.

The HMI can act in different Modbus roles depending on the product and application.

Register addresses, data types, byte order and scaling must be configured correctly for the connected devices.

Connecting an HMI to More Than One Device

Some HMI platforms can communicate with multiple PLCs, drives, instruments or other devices.

This can be useful where one operator station supervises several pieces of equipment.

However, the number of supported devices, simultaneous connections, protocols and performance limitations vary by platform.

Where multi-device communication is required, it should be specified before the HMI is selected.

Alarms

Alarm handling is one of the most valuable HMI functions.

An HMI can display active alarms such as:

• Motor overload
• High pressure
• Low tank level
• Communication failure
• Drive fault
• Emergency-stop circuit status
• Instrument fault

Depending on the platform, alarms can include timestamps, acknowledgement, priority, history and operator messages.

The HMI displays the alarm information, but the underlying alarm logic and safety response should be designed appropriately in the control system.

Alarm History

Alarm history can help maintenance personnel understand the sequence of events leading to a process interruption.

Some HMIs can store alarm records internally or on removable media. Others can transfer data to external systems.

Storage capacity and retention behaviour should be confirmed where historical records are important.

Trends

Trend screens display process values over time.

For example, an operator can view:

• Tank level over several hours
• Pressure changes
• Temperature
• Motor speed
• Flow rate

Real-time trends and historical trends are not necessarily the same capability. Historical trending requires data to be stored somewhere.

Data Logging

Some HMIs can log selected process values to internal memory, SD cards, USB storage or other supported destinations.

Data logging can support troubleshooting, production records and operational analysis.

Before relying on an HMI for important records, confirm storage capacity, file format, sampling rate, retention, time synchronisation and what happens when storage becomes full.

Recipes

A recipe is a stored group of process parameters that can be loaded together.

For example, a machine producing several product types might use different speed, temperature and timing setpoints for each product.

Rather than entering each value manually, the operator can select the appropriate recipe.

Recipe functionality varies by HMI platform and should be checked where it forms part of the machine operation.

Setpoint Entry

HMIs allow authorised operators to enter setpoints such as speed, temperature, pressure or timing values.

Good HMI design should apply appropriate limits so operators cannot enter values outside the intended operating range.

Critical limits should also be enforced in the control logic where necessary rather than relying only on the HMI input field.

User Access and Passwords

Many HMI platforms support different user levels.

For example:

• Operator
• Supervisor
• Maintenance
• Engineer
• Administrator

This can prevent unauthorised personnel from changing sensitive setpoints or configuration.

Access-control capabilities vary, and HMI user accounts should form part of the wider industrial cybersecurity approach.

Remote HMI Access

Some HMIs support remote viewing or control through web interfaces, VNC-type functions, manufacturer platforms or other remote-access technologies.

Remote access can assist maintenance and monitoring, but it also creates cybersecurity considerations.

An HMI should not simply be exposed directly to the public internet because remote access is convenient.

VPNs, firewalls, segmentation, authentication and organisational access procedures should be considered as part of the complete network design.

HMI and Industrial Ethernet

Ethernet-connected HMIs form part of the industrial network and should be considered alongside switches, routers, gateways, fibre links and network segmentation.

Where the HMI communicates with critical controllers, network architecture should provide appropriate reliability and security.

Softcore Group Industrial Solutions's broader industrial networking range can support the physical connectivity around HMI and PLC systems, including industrial Ethernet switching, fibre, media conversion and related connectivity equipment.

HMI and VSD Integration

An HMI can display and control selected VSD parameters where the drive, communications network and HMI platform support the required integration.

Examples can include:

• Run status
• Frequency
• Speed reference
• Current
• Fault code
• Start and stop commands

The exact parameters available depend on the VSD and protocol.

For some systems, the HMI communicates directly with the VSD. In others, the PLC communicates with the drive and the HMI exchanges data with the PLC.

HMI and Remote I/O

Remote I/O extends field signals to a controller over a communications network.

The HMI can then display those values through the PLC or automation architecture.

The HMI itself should not be confused with remote I/O. One provides the operator interface while the other handles physical input and output signals.

HMI Software

An HMI normally requires manufacturer-specific or compatible engineering software to create and download the operator-interface project.

The project can include:

• Screen layouts
• PLC communications
• Tags
• Alarms
• Trends
• Recipes
• User accounts
• Scripts where supported

Software licensing differs between manufacturers. Some platforms provide free configuration software, while others require licences or specific engineering packages.

Why the Original HMI Project File Matters

When replacing a failed HMI, the hardware is only part of the problem.

The replacement HMI needs the correct application project.

If the original editable project file is available, migration can be significantly easier. If only the running HMI exists and the project cannot be uploaded from it, recreating the application can require substantial engineering work.

Backing up HMI project files should therefore form part of automation-system documentation.

Can You Upload the Program From an Existing HMI?

Sometimes, but not always.

Upload capability depends on the HMI platform, how the original project was downloaded, security settings and whether the manufacturer supports retrieval of an editable project.

Some devices may allow a backup to be restored without providing the original editable source project.

Never assume that a working HMI guarantees access to its engineering file.

Replacing a Discontinued HMI

Industrial HMIs often remain in service for many years, and older models can eventually be discontinued.

A replacement may require:

• New HMI hardware
• Project conversion
• Screen resizing
• Protocol-driver changes
• Panel cut-out modification
• Different power wiring
• Different communication cables
• Updated engineering software

A current-generation replacement should therefore be assessed for both physical and software compatibility.

Do Not Choose a Replacement by Screen Size Alone

Two 7-inch HMIs can be completely incompatible.

Important differences can include:

• Resolution
• PLC drivers
• Ethernet protocols
• Serial ports
• Memory
• Software platform
• Cut-out dimensions
• Power requirements
• Environmental ratings

The existing model number and project requirements are far more useful than the diagonal screen measurement alone.

Operating Temperature

Industrial HMIs installed in control panels can experience high internal temperatures.

Confirm the rated operating temperature and consider heat from VSDs, power supplies, transformers and other panel components.

The expected internal panel temperature can be more important than the room temperature.

Ingress Protection

HMI specifications often provide an ingress-protection rating for the front face when correctly installed in a suitable enclosure.

This can be important in dusty, wet or washdown environments.

The rating may apply only to the front of the HMI and only when mounted according to the manufacturer's instructions.

The rear of the device remains inside the enclosure and requires the enclosure to provide suitable environmental protection.

Sunlight and Outdoor Visibility

Standard industrial displays can be difficult to read in direct sunlight.

For outdoor kiosks or exposed installations, brightness, anti-glare characteristics, operating temperature and enclosure design should be considered.

A normal indoor HMI should not automatically be specified for an outdoor installation merely because it fits the panel cut-out.

Gloves and Operator Conditions

Operators in industrial environments may use gloves, work with wet hands or interact with the HMI in dusty conditions.

Touch technology and screen design should suit the real operating environment.

Buttons should also be large enough for reliable operation rather than being designed only for appearance.

USB Ports and Removable Storage

HMIs can provide USB ports or memory-card slots for project transfer, data logging, backups or other supported functions.

Removable media can create cybersecurity and configuration-control risks if unmanaged.

Site procedures should define how USB devices and memory cards are used on industrial control equipment.

Time and Date Synchronisation

Accurate timestamps matter for alarms, trends and event logs.

HMIs can support manual time setting, PLC synchronisation, network time services or other methods depending on the platform.

For systems where event sequence matters, all relevant devices should use a coordinated time strategy.

HMI Cybersecurity

An HMI is a computerised endpoint within the automation system and should be treated accordingly.

Depending on the platform and network, appropriate controls can include:

• Changing default passwords
• Restricting engineering access
• Role-based user permissions
• Disabling unused services
• Network segmentation
• Controlled remote access
• Firmware management
• Project backups
• Physical access control

The exact cybersecurity requirements depend on the site and risk profile.

HMI Availability and Product Lifecycle

Automation equipment can have long service lives, but individual HMI models are eventually replaced by newer generations.

For new projects, consider:

• Manufacturer lifecycle
• Engineering software availability
• Replacement strategy
• Protocol support
• Project backup procedures
• Availability of compatible accessories

For replacement projects, the most appropriate option may be an equivalent current model rather than an obsolete unit sourced purely for physical similarity.

Common HMI Selection Mistakes

• Choosing by screen size alone
• Not checking PLC compatibility
• Assuming Ethernet means protocol compatibility
• Ignoring screen resolution
• Not checking panel cut-out dimensions
• Forgetting mounting depth
• Ordering the wrong supply voltage
• Ignoring serial-interface requirements
• Not checking tag or device limits
• Assuming all HMIs support data logging
• Assuming remote access is included
• Ignoring operating temperature
• Not considering front-face environmental protection
• Replacing hardware without confirming the original project file is available
• Assuming a discontinued HMI can be replaced without project migration

Replacing an Existing HMI: Information to Collect

Before requesting a replacement HMI, record as much of the following as possible:

• Manufacturer
• Exact model number
• Serial number if relevant
• Screen size
• Panel cut-out dimensions
• Supply voltage
• PLC manufacturer and model
• Communication protocol
• Ethernet or serial connection
• Existing communication cable details
• Engineering software
• Whether the editable project file is available
• Required alarms, trends, recipes or data logging

Clear photographs of the HMI label, front panel, rear connections and PLC can be extremely useful when the exact specification is uncertain.

A Practical Planning Checklist

For a new HMI requirement, provide as much of the following as possible:

• Application: ___
• PLC/controller manufacturer: ___
• PLC/controller model: ___
• Required protocol: ___
• Ethernet required: yes or no
• Serial interface required: RS-232, RS-485 or other
• Screen size: ___
• Resolution if specified: ___
• Touchscreen type if specified: ___
• Supply voltage: ___
• Panel cut-out: ___
• Alarms required: yes or no
• Historical alarm storage: yes or no
• Trends required: yes or no
• Data logging required: yes or no
• Recipes required: yes or no
• User access levels required: yes or no
• Remote access required: yes or no
• Operating environment: ___
• Required front-face IP rating: ___
• Existing HMI model if replacing: ___
• Original project file available: yes, no or unknown
• Quantity: ___

Where an HMI is being replaced, the existing model number, PLC model and project-file status should ideally be supplied before an alternative is selected.

How HMI Products Fit Into a Complete Automation System

An HMI is only one layer of an automation solution. It can operate alongside PLCs, remote I/O, VSDs, servo systems, instrumentation, industrial Ethernet switches, routers, gateways and fibre infrastructure.

Understanding these relationships is useful during purchasing because compatibility between components can matter as much as the specification of each individual device.

Softcore Group Industrial Solutions's Knowledge Centre covers these technologies individually so customers and purchasing teams can better identify what information is needed before products are selected or quoted.

Products listed in our active online catalogue can be purchased or quoted through the Softcore Group Industrial Solutions store. Other brands, models and project-specific requirements can be quoted on request, with pricing, availability and lead time confirmed separately.

For HMI enquiries, provide the existing HMI model where applicable, the PLC manufacturer and model, communications protocol, screen requirements and project-file status. These details help narrow the requirement and reduce the risk of specifying incompatible hardware.

Technical Note

This article provides general industrial automation information. HMI selection, programming, PLC communications, machine controls, network architecture, cybersecurity and safety functions must be assessed for the specific application. Manufacturer documentation, applicable standards, machine risk assessments and site requirements take precedence. Safety-related control functions must use appropriately designed and validated safety systems and should not rely on an ordinary HMI alone.

Related catalogue examples: HMIs & Operator Interfaces. For an equipment enquiry, contact Softcore Group Industrial Solutions at info@softcoregroup.co.za.