HMI stands for Human-Machine Interface. It is the screen through which the operator sees the status of a machine or process and gives it commands: start, stop, change a setpoint or acknowledge an alarm.

What Is an HMI? HMI vs SCADA vs PLC Explained

Q: What is the difference between HMI and SCADA?

The HMI is the screen for one machine or cell. SCADA is a system that supervises many machines or sites at once, with history, centralized alarms and several stations. Every SCADA has HMI-type screens, but an HMI on its own is not a SCADA.

Q: What is the difference between HMI and PLC?

The PLC is the controller that runs the logic and moves the machine. The HMI only displays information and sends commands to the PLC. If the HMI shuts down, the PLC keeps controlling; if the PLC stops, the HMI has nothing to show.

Q: What is a high-performance HMI?

It is a design style, described in the ISA-101 standard, that uses neutral backgrounds and reserves color for abnormal conditions, shows trends instead of isolated numbers and organizes screens in levels, from overview to detail.

Q: Can you view the HMI from a phone?

Status can be shown on a phone or in a browser, but those views should be read-only. Commands to the machine should stay at the operator station, with its security and its log.

An HMI (human-machine interface) is the screen through which the operator sees what a machine is doing and gives it commands. It shows speeds, temperatures, states and alarms, and lets the operator start, stop or change a setpoint. It controls nothing on its own: the logic lives in the PLC. And it is not a SCADA: SCADA supervises many machines or sites at once. This guide sorts out the three pieces and explains how to design a screen the operator understands in two seconds.

What is an HMI? #

It is the visible layer of control. It reads variables from the PLC (for example, a tunnel's temperature or a motor's status), draws them on a screen and writes back what the operator commands. Its typical functions are four: displaying the process, operating (start, stop, change setpoints), managing alarms and keeping a short event log.

What types of HMI are there? #

Type	Where it is	When it makes sense
Operator panel	Touchscreen mounted on the panel or on the machine	One machine or cell; environments with dust, water or vibration
HMI on an industrial PC	Computer in the control room or next to the line	Several machines, richer screens, recipes
Web HMI	Browser on a tablet, a TV or a phone	Viewing status from somewhere else; ideally read-only

How do HMI, SCADA and PLC differ? #

	PLC	HMI	SCADA
What it does	Runs the logic and moves the machine	Displays and sends commands to one machine	Supervises many machines or sites
Scope	One machine or process	One machine or cell	One plant or several
History	None, or very little	Short	Yes, long-term
If it shuts down	The machine stops	The machine keeps running, blind	The machines keep running; supervision is lost

Above SCADA there is usually the MES, which organizes production by order and lot; the boundary between the two is covered in MES vs SCADA. Full SCADA, with its components, is in what is SCADA, and how to read a PLC from outside in connecting a PLC to the cloud.

How do you design a good HMI screen? #

The ISA-101 standard describes the so-called high-performance HMI. Its ideas are useful even if you do not apply it in full:

Neutral backgrounds, color for the abnormal. If everything is green and red, nothing stands out. Gray as the base makes an alarm visible immediately.
Trends, not just numbers. "72 °C" does not tell you whether it is rising or falling. A line showing the last hour does.
Screen levels. A plant overview, one per area, one per piece of equipment and one for detail. The operator drills down when needed.
Ranges in view. Showing the value against its normal range saves the operator from having to remember it.
Few alarms, and meaningful ones. The ISA-18.2 standard covers alarm management: every alarm should call for an action. A screen with a hundred active alarms teaches people to ignore them.

What mistakes are common in an HMI? #

3D drawings of the machine that take up the screen and hide the data.
Command buttons next to navigation buttons, with no confirmation.
Missing units, or units that differ between screens.
Alarms nobody has reviewed since commissioning.
Shared user accounts: there is no record of who changed a setpoint.

How do you take HMI data beyond the machine? #

The HMI sees what is happening on one machine right now; it almost never stores the history or cross-references it with others. For that, the same PLC is read, in read-only mode, from an industrial computer in the plant, which logs each variable with its timestamp and publishes it to dashboards, MES or ERP without touching the control logic. That is how the iGromi edge does it: the operator keeps using their HMI and management sees the same data with history. More on this in industrial edge computing, industrial data logger and Node-RED in industry.

Frequently asked questions #

What is an HMI?

The human-machine interface: the screen through which the operator sees a machine's status and gives it commands.

What is the difference between HMI and SCADA?

The HMI is the screen for one machine or cell; SCADA supervises many machines or sites with history and centralized alarms.

What is the difference between HMI and PLC?

The PLC runs the logic and moves the machine; the HMI only displays and sends commands. If the HMI shuts down, the PLC keeps controlling.

What is a high-performance HMI?

A design style described in ISA-101: neutral backgrounds, color only for abnormal conditions, trends and screens organized in levels.

Can you view the HMI from a phone?

Yes, but those views should be read-only. Commands to the machine should stay at the operator station.