ISA-95 explained: the levels, the equipment hierarchy, what passes between business and operations, and how it relates to the Purdue model
ISA-95 and IEC 62264 explained: the five functional levels and their time frames, the role based equipment hierarchy from enterprise to work unit, the four kinds of information exchanged between business and operations, manufacturing operations management, the parts of the standard, and ISA-95 against the Purdue model in OT security. With a drawing of the levels and hierarchy.
Key takeaways
- ISA-95, published internationally as IEC 62264, is the standard for how a plant's control and operations systems connect to its business systems. It gives a shared set of levels, an equipment hierarchy, and models of the information that passes between them.
- The functional levels run from 0, the physical process, to 4, business planning. Level 3, manufacturing operations management, is where MES, historians and laboratory systems sit, and it is the layer ISA-95 describes in most detail.
- The role based equipment hierarchy, enterprise, site, area, work centre and work unit, is the part most sites use every day, in the CMMS, the historian, the MES and unified namespace topic trees.
- Four kinds of information cross between level 4 and level 3: product definition and production schedule go down, production capability and production performance come up.
- The Purdue model and ISA-95 share their levels. In OT security the same levels set which systems may talk to which, which is a different use from the integration models.
An ERP project and an MES project start on the same site in the same year. The ERP team calls a crushing circuit a cost centre, the MES team calls it a production unit, the CMMS calls it a functional location with a different number, and the historian has its own tag path. Every interface between them needs a mapping table, and every mapping table is maintained by hand. Nothing in the plant is wrong. The systems were just never given a shared model of it.
This guide explains ISA-95: the functional levels, the role based equipment hierarchy, the activities of manufacturing operations management, the information exchanged between business and operations, the parts of the standard, and how it relates to the Purdue model and to OT security. It sits beside the guides to MES, system integration and the unified namespace. The standard is a reference point, and most sites use parts of it rather than all of it.
Levels and the equipment hierarchy
ISA-95 starts with a functional hierarchy of five levels. The physical process is level 0. Instruments and controllers that sense and manipulate it are level 1. Supervisory control and monitoring is level 2. Manufacturing operations management, the work of running production shift by shift, is level 3. Business planning and logistics is level 4. Each level works on a longer time frame than the one below it, from milliseconds at the bottom to months at the top.
Alongside the levels sits a role based equipment hierarchy: enterprise, site, area, work centre and work unit. It gives every system the same way to name a part of the plant. It is the part of ISA-95 most sites use without realising it, in CMMS functional locations, historian tag paths and unified namespace topic trees.
The levels say what each system is for. The equipment hierarchy says what each system calls the plant. Most integration problems come from the second.
The equipment hierarchy in detail
Below the area, ISA-95 names work centres and work units by the kind of production: process cells and units in batch, production units in continuous processing, production lines and work cells in discrete manufacturing, and storage zones and storage units for material held between steps. A mining or minerals site usually maps its circuits to production units and its stockpiles and bins to storage zones and units.
| Level | What it is | Example |
|---|---|---|
| Enterprise | The organisation as a whole | The operating company |
| Site | A physical location with one or more areas | Plant 1 |
| Area | A part of a site with its own production role | Crushing, processing, load-out |
| Work centre | A process cell, production unit, production line or storage zone, depending on the kind of production | The secondary crushing circuit, a stockpile |
| Work unit | The lowest level ISA-95 names: a unit, work cell or storage unit | Cone crusher 2, stockpile 3 |
What passes between business and operations
Part 1 of the standard describes the information that crosses the boundary between level 4 and level 3 as four kinds of object. Two go down from the business to operations and two come back up. Agreeing these four on a site, with their content and timing, settles most of the arguments between an ERP project and an MES project.
| Information | Direction | What it holds |
|---|---|---|
| Product definition | Down | How to make a product: the recipe, bill of materials and route |
| Production capability | Up | What the plant can do: the people, equipment and material available |
| Production schedule | Down | What to make and when |
| Production performance | Up | What was made, used and lost, against the schedule |
Manufacturing operations management
Part 3, published as IEC 62264-3, describes level 3 as four operations areas: production, maintenance, quality and inventory. Each has the same pattern of activities: definition management, resource management, detailed scheduling, dispatching, execution management, data collection, tracking and performance analysis. Seen this way, maintenance management is an operations activity alongside production, which is why the CMMS sits at level 3 in many architectures, with its financial side at level 4.
The parts of the standard
ISA-95 has grown into several parts, published by ISA as ANSI/ISA-95 and internationally as IEC 62264. Most sites need only Part 1 for the shared vocabulary and the equipment hierarchy, and Part 3 for the activity model. Integration projects that exchange data in a standard format use Parts 2 and 5, often through B2MML.
| Part | Title | What it covers |
|---|---|---|
| Part 1 | Models and terminology | The levels, the equipment hierarchy and the models of information exchanged |
| Part 2 | Objects and attributes | The detail of each object in the exchange, such as personnel, equipment and material |
| Part 3 | Activity models of manufacturing operations management | The four operations areas and the activities in each |
| Part 4 | Objects and attributes for operations management integration | The objects exchanged between activities inside level 3 |
| Part 5 | Business to manufacturing transactions | The verbs for the exchange, such as get, show, process and sync |
| Parts 6 to 8 | Messaging, alias and information exchange profiles | How messages are passed, how names are mapped between systems, and agreed subsets for common exchanges |
ISA-95, Purdue and OT security
The levels in ISA-95 came from the Purdue Enterprise Reference Architecture, and the two names are often used together. In integration, the levels describe what systems do. In OT security, the same levels are used as a network architecture: systems at each level are grouped into zones, an industrial DMZ sits between levels 3 and 4, and traffic is allowed only through defined conduits under IEC 62443.
The two uses can pull against each other. Newer architectures such as a unified namespace let a system at any level reach data directly, which simplifies integration but needs the security zones designed with it. The levels remain useful for security even where data no longer climbs them one step at a time.
Where ISA-95 goes wrong
- Treated as a product to buy rather than a shared model to agree.
- Each system given its own name for the plant, with mapping tables maintained by hand.
- The four exchanges between ERP and MES never written down, so both systems record production.
- Every part of the standard implemented at once, when Part 1 and Part 3 would do.
- The levels used as a network design without the zones and conduits that make it secure.
- The equipment hierarchy designed for the ERP's cost structure rather than the plant's physical layout.
Common questions
What is ISA-95?
ISA-95, Enterprise-Control System Integration, is a standard from the International Society of Automation for integrating a manufacturing or processing plant's operations with its business systems. It defines functional levels from the physical process to business planning, a role based equipment hierarchy, the activities of manufacturing operations management, and models of the information exchanged. IEC and ISO publish it internationally as IEC 62264.
What are the ISA-95 levels?
Level 0 is the physical process. Level 1 is sensing and manipulating it, the instruments and controllers. Level 2 is monitoring and supervisory control, such as SCADA and operator stations. Level 3 is manufacturing operations management, such as MES, historians and laboratory systems, working in shifts, hours and minutes. Level 4 is business planning and logistics, such as the ERP, working in days, weeks and months.
What is the difference between ISA-95 and the Purdue model?
The Purdue Enterprise Reference Architecture, developed at Purdue University in the 1990s, introduced the levels that ISA-95 adopted. ISA-95 builds on them with the equipment hierarchy, activity models and information models for integration. In OT security, the Purdue model usually means a network architecture that uses the same levels, with a DMZ between levels 3 and 4, to decide which systems may communicate.
What is the ISA-95 equipment hierarchy?
It is a role based structure for naming a plant's equipment: enterprise, site, area, work centre and work unit. Work centres and units are named for the kind of production, such as process cells and units for batch, production units for continuous, production lines and work cells for discrete, and storage zones and storage units for material. Many CMMS functional location structures, historian tag paths and unified namespace topic trees follow it.
What is B2MML?
B2MML, the Business to Manufacturing Markup Language, is a set of XML schemas implementing the ISA-95 object models, so systems can exchange schedules, performance, material and equipment information in a standard format. It was developed by MESA International and is available royalty free, with a JSON version alongside the XML.
Key terms
Plain-language definitions from our glossary for the concepts this article leans on.
Standards and further reading
Related case studies and tools
- Real-time production analytics (case study)
Related reading
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