HMI Screen Hierarchy: Level 1 to Level 4 Displays
Most HMI projects fail at the same point, and it is not the symbols. It is the moment someone decides to put the whole plant on one screen because "the operator wants to see everything". Everything on one screen means nothing is emphasised, and an operator under pressure has to read instead of recognise.
The ISA-101 answer is a display hierarchy: a small number of screen levels, each with a defined job, connected by navigation that survives a bad day. This guide covers what belongs on each level, how to design the navigation between them, and how to size everything for the distance the screen is actually viewed from.
1. Why a hierarchy at all
An operator does three distinct things, and they need three different kinds of screen:
- Monitor. Most of the shift. The question is "is anything drifting?" — answered by a wide, low-detail view.
- Control. Start a unit, change a setpoint, respond to a deviation. The question is "what is this unit doing and what can I change?"
- Diagnose. Something is wrong. The question is "which signal, which interlock, which device?"
A single screen cannot serve all three without being cluttered for the first task and insufficient for the third. The hierarchy just gives each task its own home, and — critically — makes it obvious where you are and how to get to the next one.
2. The four levels
| Level | Scope | Answers | Typical count |
|---|---|---|---|
| 1 — Overview | Whole plant or area | Is anything abnormal, anywhere? | 1–3 |
| 2 — Unit control | One process unit | What is this unit doing? What do I change? | One per unit |
| 3 — Unit detail | Sub-unit or equipment group | Why is the unit behaving like this? | As needed |
| 4 — Diagnostic / support | A device, loop or interlock | Which signal or component is at fault? | Many, rarely used |
Level 1 — the overview
This is the screen that lives on the wall or on the left monitor all shift. It is the hardest to design and the one most often skipped.
A Level 1 display is not a small P&ID. It shows a handful of key performance and health indicators for the whole area, arranged so that "normal" has a recognisable visual shape. Trends, deviation bars and grouped state summaries work well; individual valves do not. The operator should be able to glance at it from across the room and know whether to walk over.
The strongest technique here is deviation-from-normal rather than absolute value: a small horizontal bar with the normal operating band marked, and a pointer showing where the process sits inside it. Ten of those, side by side, let an operator spot a drift long before it becomes an alarm — which is what "situation awareness" actually means in practice.
Level 2 — unit control
One screen per process unit, and the workhorse of the system. It carries the process schematic for that unit, the equipment symbols with their states, setpoints and controls, and the alarms belonging to that unit.
Level 2 is where the nine standard equipment states earn their keep: the operator must be able to tell running from starting from no-feedback without clicking anything.
Rule of thumb: if a Level 2 screen needs scrolling or zooming, it is covering more than one unit and should be split.
Level 3 — unit detail
Opened deliberately, when Level 2 is not enough: the full loop detail, the sub-unit schematic, secondary measurements, the interlock matrix. Detail here is fine — this screen is read, not scanned.
Level 4 — diagnostics and support
Device faceplates, raw I/O values, drive parameters, maintenance data, help pages. Used rarely and usually by a technician rather than an operator. It is legitimate for these to look denser and more technical than everything above.
3. Navigation an operator can use under pressure
Navigation is the part that decides whether the hierarchy helps or hinders. Four rules cover most of it:
Two clicks to anywhere that matters
From any screen, the operator should reach any Level 2 display in at most two actions. Deep menu trees fail in exactly the situation they most need to work — a five-level menu is unusable when something is going wrong.
A fixed navigation area, always in the same place
A persistent bar (top or bottom) with the alarm banner, area/unit buttons, and the current location. It never moves, never reorders, and never changes size between screens. Muscle memory is a legitimate design goal: an experienced operator should be able to hit a unit button without looking at it.
Always show where you are
A breadcrumb or a highlighted button. Operators navigating quickly lose track of context, and the fastest way to a wrong action is to change a setpoint on the unit you did not think you were looking at.
No dead ends, and no navigation inside popups
Every screen offers a way up and a way back. Popups are for a single focused interaction — a faceplate, a confirmation — and should never become a place you navigate from. A popup that hides the alarm banner is worse than a full screen.
4. Consistency: the cheapest quality you can buy
Two screens showing the same kind of thing should be the same screen with different data. That means:
- The same object always looks the same. One motor symbol, one valve symbol, one set of state colours — across every unit, every screen, every language of the project.
- The same thing is always in the same place. Alarm banner, navigation, title, mode indicator, timestamp — fixed positions, fixed sizes.
- The same interaction always works the same way. If a single click opens a faceplate on one screen, it opens a faceplate everywhere. Never mix single-click-acts and single-click-selects.
- Build from templates, not copies. A screen template plus a tag prefix is maintainable; twelve hand-edited copies are not. Every vendor supports some form of this — Weintek group objects, WinCC faceplates, FactoryTalk global objects, Ignition embedded views with parameters.
This is also the argument for a shared symbol library rather than per-screen drawings: consistency stops being a discipline you have to maintain and becomes a property of the source files.
5. Sizing for the distance the screen is actually viewed from
Screens get designed on a desk at 60 cm and used on a panel at 2 m. Legibility scales with distance, so the numbers must too.
A practical rule: character height should be at least 1/200 of the viewing distance for comfortable reading, and closer to 1/150 for anything read under stress. That gives:
| Viewing distance | Minimum character height | Typical use |
|---|---|---|
| 0.5 m | ~3 mm | Desk-mounted HMI, engineering station |
| 1 m | ~5–7 mm | Panel-mounted operator terminal |
| 2 m | ~10–13 mm | Standing operator, machine-side panel |
| 4 m | ~20–27 mm | Wall display, control room overview |
Convert to pixels using the panel's real pixel pitch rather than assuming — a 7" 800×480 panel and a 15" 1024×768 panel have very different physical pixel sizes. In practice, on typical operator panels this puts the floor at around 14 px for body text and 18–20 px for anything the operator must read quickly, which is why the ISA-101 guidance and this site's own symbol conventions both refuse to go below 14 px.
Touch targets follow the same logic: 10 mm is a reasonable minimum for a gloved finger, and controls that cause an action should never sit adjacent to controls that cause a different action without a gap between them.
6. Anti-patterns
- The one-screen plant. The entire P&ID on a single display, requiring zoom. Nothing can be emphasised because everything is present.
- The decorative overview. A Level 1 screen that is a photograph or a 3D rendering of the plant. It looks impressive in the acceptance meeting and tells the operator nothing.
- Navigation by P&ID. Clicking pipes and vessels to travel. Discoverable only to whoever built it.
- Popup stacking. Three faceplates open at once, covering the mimic and the alarm banner.
- Per-unit dialects. Unit 1 uses green for running, Unit 2 uses green for available. This is the single most dangerous inconsistency there is.
- Level 4 pretending to be Level 2. Raw I/O and drive parameters on the main control screen "because the technicians asked for it". Give them their own screen.
7. A checklist before you draw anything
- List the process units. Each one gets exactly one Level 2 screen.
- Decide the Level 1 indicators — no more than about a dozen, each with a defined normal band.
- Fix the navigation area: position, size, contents. It will not change again.
- Fix the screen template: title, breadcrumb, alarm banner, mode indicator, timestamp.
- Fix the symbol set and the state colours once, for the whole project.
- Measure the real viewing distance and derive minimum text and target sizes from it.
- Check the two-click rule against the finished navigation map.
- Walk one abnormal scenario end to end: alarm appears → operator locates it → opens the unit → acts. Count the clicks.
Consistency starts with the symbol set. Draw each piece of equipment once in the Symbol Builder with the ISA-101 palette built in, export the nine states, and reuse the same files across every screen and every unit of the project.
Open the Symbol Builder → Explore community symbols →Frequently asked
Do I really need four levels on a small machine?
No. A single skid with one operator panel often needs two: a main screen and a diagnostics screen. The hierarchy is a way of thinking about screen jobs, not a quota. What matters is that each screen has one job and that navigation is obvious.
Where do trends belong?
Level 1 for the few plant-level indicators, and Level 2 or 3 for unit-level values. A dedicated trend screen is useful as a Level 3 support display, but a trend the operator needs during an upset should be embedded where they already are, not one navigation away.
How many screens is too many?
There is no fixed number, but if operators use a small subset and ignore the rest, the hierarchy is wrong rather than the count. Instrument the HMI if you can: screen-visit statistics are the fastest way to find displays nobody needs.
Should Level 1 be a wall display or a workstation screen?
Either. What matters is that it is visible without being navigated to. If your operators must click to see the overview, they will not see the overview.