The short answer
HMI Navigation
HMI navigation is the structure and the controls that let an operator reach any display in the system quickly and predictably. ISA-101 practice organizes displays in a hierarchy of overview, area, unit, and detail levels, provides a navigation bar that is identical on every display and shows the alarm state of each area, and adds context links from alarms, devices, and values to the displays that explain them. The goal is that any display is reachable in two clicks and that an operator always knows where they are.
Key points
- Four levels: overview, area, unit, detail. Every display has a place in the hierarchy.
- The navigation bar is identical on every display and carries the area alarm indicators.
- Two clicks to anywhere. Three is a design failure.
- Every alarm, device, and value links to the display that explains it.
- Fewer displays, well organized, beat many displays with everything on them.
The hierarchy
ISA-101 describes displays in levels, and the levels are what make navigation possible. Without them, a system of eighty displays is a maze; with them, it is a tree the operator can hold in their head.
| Level | Purpose | Content | Example |
|---|---|---|---|
| 1, Overview | The state of the whole system at a glance; the display that is up when nothing is happening | Key values with embedded trends, area alarm indicators, high-level status; very little control | The utility: plant, distribution, collection, each as a block with its health |
| 2, Area | The state of one area and the control that area needs | The process in that area as a simplified diagram, the important values, the devices with status | The water plant; the lift stations; the distribution system |
| 3, Unit | One process unit or one site, with everything needed to operate it | The unit diagram, its devices, its loops, its sequence status | Filter 3; lift station 12; the high service pump station |
| 4, Detail | One device, loop, or sequence in full | Faceplates, loop detail, sequence step lists, diagnostics | Pump P-3 faceplate; the backwash sequence display; the chlorine loop detail |
An operator lives on level 2 and 3 displays, glances at level 1, and opens level 4 when acting or investigating. Every display is assigned to exactly one level and one branch of the tree, and the navigation bar reflects that tree.
The navigation bar
A strip in the same place on every display, containing a button for each area and, on area displays, a row for the units in that area. Each button carries the highest priority active alarm in its area as a small indicator, in the alarm color and symbol, so that an operator on any display sees that another area has an alarm. The bar also holds the fixed utilities: the alarm summary, the trend tool, the alarm banner, the user login, and a home button that returns to the overview.
- Same position, same size, same order on every display. A bar that moves or changes teaches operators to look for it instead of at the process.
- The current location highlighted, and the path shown: Overview, Water Plant, Filters, Filter 3.
- One click to any area from any display, two clicks to any unit. Detail displays open as pop-ups over the unit display, so the unit context is not lost.
- The alarm indicator on each area button is the highest priority unacknowledged alarm in the area; it clears when the alarms are acknowledged and follows the same blink rule.
- No more than about eight area buttons; a system with more areas groups them.
Context navigation
The bar gets the operator to a place; context links get them to the thing. Every alarm entry in the banner and the summary links to the display that contains the device, and to the device faceplate. Every device symbol opens its faceplate. Every value with a trend link opens its trend group. A faceplate has a link back to the display it lives on. A sequence status shows the current step and links to the step display. These links are what make two clicks to anywhere true in practice: the operator rarely uses the bar to find a problem, because the problem finds them and links them to itself.
What to leave out
- Duplicate paths
- A device that appears on three displays with three different faceplate bindings is three places for a mistake. A device lives on one unit display; other displays show its status and link to it.
- Deep menus
- A drop-down menu with sub-menus adds clicks and hides the alarm indication. The bar is flat.
- Hidden navigation
- Clicking on an unlabeled region of a diagram to go somewhere. Navigation controls look like navigation controls.
- Popup chains
- A faceplate that opens a detail that opens a diagnostic that opens a trend. Two levels of popup at most, with a clear close on each.
- Per-client differences
- Every client, in the control room, at the plant, and remote, uses the same navigation. An operator who moves between them should not relearn.
- Screens nobody uses
- A display that has not been opened in a year, as the SCADA usage log shows, is either merged into another or removed. Every display costs maintenance.
Multiple monitors and remote clients
A control room with several monitors commonly fixes the overview on one, the alarm summary on another, and uses the rest for area and unit displays. The navigation bar behaves the same on each, and a display opened from an alarm opens on the monitor the operator is working on, not on a fixed one. Remote and mobile clients get the same hierarchy with the layout adapted to the screen; what they do not get is a different structure. An operator who knows the plant HMI knows the phone client.
Frequently asked questions
- How many displays should a utility SCADA system have?
- As few as the hierarchy needs. A mid-size utility often lands at one overview, three to five area displays, a unit display per plant process and one per remote site type with a site selector, and the detail displays that the faceplates and sequences generate. Fifty well-structured displays serve better than two hundred accumulated ones.
- Should the overview have any control on it?
- Very little. The overview is for seeing; control happens on unit displays and faceplates. A start button on the overview is a start button an operator can press while looking at the wrong thing.
- Where does the alarm summary live in the hierarchy?
- Outside it, as a utility display reachable from the bar on every screen. It is a view across all areas and does not belong to one.
- How do I handle a system with many identical remote sites?
- One unit display template with a site selector, so the operator picks a station and the display binds to it. The alarm link from the banner opens the template bound to the station in alarm. This is far better than eighty copies of a display that diverge over time.
Related topics
- ISA-101: Human Machine InterfacesWhat the ISA-101 standard for human machine interfaces covers, its lifecycle from philosophy to maintenance, the display hierarchy, and what adopting it changes on a project.
- High Performance HMI DesignGrey backgrounds, reserved color, and displays designed to make abnormal conditions obvious rather than to look impressive in a demo.
- Alarm Indication on the HMIHow an alarm looks and sounds on the operator screen: the ISA-101 and ISA-18.2 rules for color, shape, and blink, the alarm banner and summary, acknowledgment behavior, navigation to the alarm, and indication that survives colorblindness and a noisy control room.
- HMI FaceplatesThe pop-up that controls one device: what a pump, valve, or loop faceplate must show, the mode and command layout, interlock and permissive display, security by role, and building faceplates from a standard object so every device behaves the same.
- Trends on the HMITrend displays that answer questions: embedded sparklines on process displays, pre-configured trend groups, the axes and time spans that make a trend readable, and the difference between a trend for operating and a trend for investigating.
- Color in HMI DesignHow color is used on a high performance HMI: a gray palette for normal operation so that color means something, alarm colors reserved by priority, equipment state by fill and text rather than red and green, color-blind safe choices, and the palette table.
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