ISA-101: Human Machine Interfaces

What the standard is

ANSI/ISA-101.01, Human Machine Interfaces for Process Automation Systems, was published in 2015 by the International Society of Automation. It grew out of decades of incident investigations in which the operator interface was a contributing cause: alarms buried in color, critical values indistinguishable from routine ones, screens that differed from unit to unit so that an operator moving between them had to relearn the conventions. The standard is the process industry answer to that history.

It is deliberately not a style guide. It does not say that a pump must be a certain shape or that a running motor must be green. It says that the owner must decide those things, write them down, apply them everywhere, and control changes to them. The design guidance it does give, on hierarchy, on the use of color, on what an overview display is for, is guidance toward writing that document well.

The lifecycle

The standard organizes HMI work into stages, each producing something the next stage uses.

1. 1

   Philosophy.

   The owner states the principles: what the HMI is for, who uses it, how abnormal situations are shown, how alarms are presented, what the display hierarchy is. This is a short document with a long life; it outlasts any one project and any one platform.

2. 2

   Style guide.

   The philosophy made specific: the colors and what each means, the fonts, the symbols, the layout grid, the navigation model, the way a value is shown with its units and limits. Everything a designer needs to build a screen that looks like every other screen.

3. 3

   Toolkit.

   The style guide made reusable: the library of faceplates, symbols, and templates in the actual SCADA platform, built once and used on every display so that consistency is automatic rather than disciplined.

4. 4

   Design.

   The displays for a specific system, built from the toolkit, organized in the hierarchy, and reviewed against the style guide and against the operating tasks they support.

5. 5

   Implementation and testing.

   The displays are built, and they are tested against the design and the style guide, with operators, before they go into service.

6. 6

   Operation and maintenance.

   The HMI is used, and every change to it goes through management of change so that the consistency that was designed in is not eroded one screen at a time.

The display hierarchy

The standard describes displays in levels, each answering a different question for the operator. The levels are the part of ISA-101 most people have heard of, and they are the part most often implemented without the philosophy behind them.

What it changes on a project

Adopting the standard moves decisions earlier and moves them to the owner. On a project run without it, the integrator builds screens in whatever style the platform defaults to and the owner reacts to them at FAT. On a project run with it, the owner has a philosophy and a style guide before the integrator starts, the integrator builds the toolkit or uses the owner’s existing one, and FAT compares the displays to a document. Arguments about color happen once, in the style guide, instead of on every screen.

For the owner

A style guide is an asset that carries across projects and vendors. Every new screen looks like the existing ones, and a new operator learns one set of conventions.

For the integrator

A clear acceptance criterion. A display either matches the guide or it does not. The toolkit also makes screen development faster after the first project.

For the operator

Consistency and a quiet normal state, so that the abnormal stands out and the same thing means the same thing on every display.

Relation to high performance HMI and ISA-18.2

High performance HMI is a design approach, documented by several authors and consultancies, built on the same principles the standard codifies: a gray normal state, color reserved for abnormal, analog indication of values against their ranges, and a strict hierarchy. ISA-101 is the standard that makes those principles auditable through the lifecycle and the documents. A high performance HMI style guide is one legitimate answer to the philosophy the standard asks for.

ISA-18.2 covers alarm management and the two are written to work together. The alarm philosophy under 18.2 and the HMI philosophy under 101 are companion documents, and how alarms are presented on the displays is where they meet.

Starting without a philosophy

Most utilities and plants do not have an HMI philosophy, and the first project that adopts the standard has to write one. It does not need to be long. A few pages that settle the purpose of the HMI, the display hierarchy, the meaning of color, the treatment of alarms, and the rules for change will carry a style guide. Writing it before the screens exist is the whole point; writing it afterward describes what was built rather than what was intended.