HMI and SCADA

Why it matters

Operators rarely look at ladder logic. They run machines from touch screens at the machine and from control rooms that watch whole plants, water systems or pipelines. Technicians need to know how those screens get their data, how alarms should behave, and why these systems are a target for cyber attacks.

HMI: human-machine interface

An HMI is the operator's screen for a machine or a process area, typically an industrial touch panel or a PC.

• It reads PLC tags to show status: motor running, tank level, temperatures, counts, alarms.
• It writes tags when the operator acts: start and stop buttons, setpoints, recipe selection, mode changes.
• It shows alarms and their history.
• It often has user levels: an operator can start a line, a technician can change a setpoint, an engineer can change configuration.

An HMI button is a request, not a command. Pressing START on the screen sets a bit; the PLC logic still decides whether the motor may run, using all its interlocks. That is why the PLC, not the HMI, is the right place for permissives and interlocks, and why an HMI button must never be the only emergency stop: hard-wired E-stops are required.

SCADA: supervisory control and data acquisition

SCADA systems supervise many controllers spread over a large area: a water utility with pump stations miles apart, an electric grid, a pipeline, or a large factory.

Typical parts:

• Field controllers: PLCs and RTUs (remote terminal units) at each site.
• Communications: plant Ethernet, fiber, radio, cellular or leased lines, using protocols such as Modbus, DNP3, EtherNet/IP, PROFINET or OPC UA.
• SCADA servers that poll the field devices, store data and raise alarms.
• A historian that stores time-stamped values for trends and reports.
• Operator workstations in a control room.

The rule of thumb: the HMI controls one machine or area up close; SCADA supervises many, collects data and lets a small team manage a large system. A related term, DCS (distributed control system), is used in continuous process plants such as refineries, where control and supervision come from one vendor's integrated system.

The automation pyramid

Plants often describe their systems in levels (the ISA-95 model):

Level 4  Business systems (ERP: orders, inventory)
Level 3  Manufacturing operations (MES: scheduling, quality, genealogy)
Level 2  Supervision (HMI, SCADA, historian)
Level 1  Control (PLCs, drives, RTUs)
Level 0  Field devices (sensors, valves, motors)

Data flows up for reporting; orders and setpoints flow down.

Good screen design

Modern guidance (ISA-101) favors high-performance HMI design:

• Mostly gray screens, so color is reserved for abnormal conditions and alarms stand out.
• Trends and analog indicators with normal ranges, not just numbers.
• A clear hierarchy: plant overview, area, unit, detail.
• Consistent navigation and button placement.

Alarm management

An alarm should demand an operator action. Bad alarm systems flood operators with hundreds of nuisance alarms, so real ones get missed, a factor in several major industrial accidents. The ISA-18.2 standard describes the alarm life cycle. Good practice:

• Each alarm has a defined cause, consequence and response.
• Priorities (for example, high, medium, low) used sparingly.
• Deadbands and delays to stop chattering alarms (a short TON in the PLC often does this).
• Regular review of the most frequent alarms.

Worked example: following a value

A tank level shows 72 percent on the HMI. Where does it come from? A level transmitter (Level 0) sends 15.5 mA to an analog input (Level 1). The PLC scales it: (15.5 − 4) / 16 × 100 ≈ 72 percent and stores it in a tag. The HMI (Level 2) polls that tag over Ethernet and displays it; the historian logs it every few seconds for trends. If the HMI shows a stale value with a communication error, the problem is the network or HMI, not the transmitter.

OT security

Industrial control systems were designed for isolated networks and many old protocols have no authentication. Today they are connected for remote access and data, so security matters:

• Segment networks: keep control networks separate from office IT, with firewalls and a demilitarized zone between them.
• Control remote access tightly, with multi-factor authentication and no shared accounts.
• Patch carefully and keep backups of PLC programs and HMI projects.
• Never plug unknown USB drives into HMIs or engineering laptops.

The ISA/IEC 62443 series is the main OT security standard family (https://www.isa.org/standards-and-publications/isa-standards/isa-iec-62443-series-of-standards), and CISA publishes free ICS guidance and advisories (https://www.cisa.gov/topics/industrial-control-systems). Checked on 2026-10-23.

Common mistakes

• Putting interlocks only in the HMI.
• Using an HMI button as the emergency stop.
• Bright, colorful screens where alarms do not stand out.
• Leaving default passwords on HMIs and PLCs.

Recap

• HMI: one machine or area, reads and writes PLC tags; the PLC still decides.
• SCADA: supervises many sites, with RTUs, communications, servers and a historian.
• Good HMIs are calm; good alarms demand an action.
• Segment, control access, back up: OT security is part of the job.

Try it

Open the Do tab and take the quiz on HMI and SCADA roles.