ISA-95 (IEC 62264) explained: five levels, level 3 MOM activities, equipment hierarchy, ERP–MES integration

What ISA-95 is

ANSI/ISA-95, published internationally as IEC 62264, is a series of standards for integrating a company's business and logistics systems with its manufacturing control systems. ISA started the work in the mid-1990s, and Part 1 was approved in May 2000, developed by the ISA SP95 committee. Yokogawa's technical report gives the committee's goal as reducing the risk, cost and errors of building interfaces between business and manufacturing functions; Wikipedia adds consistent terminology as a basis for communication between suppliers and manufacturers.

Yokogawa calls ISA-95 a high-level, logical model for organising the exchange of data. It is abstract: OPC Foundation's OPC 10030 notes that it provides no implementation, so it does not say which software must do the work, and B2MML from MESA is one implementation of it.

Where the levels come from: the Purdue model

Part 1 cites the Purdue Reference Model for CIM (computer-integrated manufacturing); Wikipedia describes the same levels under the Purdue Enterprise Reference Architecture (PERA), developed in the 1990s by Theodore J. Williams and the Industry-Purdue University Consortium. OPC 10030 describes ISA-95 as five levels of activity, from the physical process at level 0 to business planning at level 4. The same levels are the starting point for most plant network and security drawings; the IEC 62443 article shows how security architectures group them into zones and conduits.

The five levels and their time frames

Each level also has its own meaning of "real time". OPC 10030 describes level 3 information as available a few seconds after an event on the shop floor, while level 4 needs logistics and material information daily or within a few hours after a shift ends. ISA-95 mainly addresses the exchange between levels 3 and 4 and between systems inside level 3.

Documents place equipment differently. OPC 10030 puts PLCs and DCSs at level 2 and lists SCADA among the systems supporting level 3, while the Purdue model description on Wikipedia puts DCS, HMI and SCADA at level 2, and a figure in Rockwell's security design guide (CPwE) labels the controller as level 1. The level 3 time frame also varies slightly between sources (Wikipedia gives seconds to shifts). For integration work, what matters is which activity a system performs, not the exact number written next to it.

The parts of the standard

ISA lists eight parts. The edition years are those shown on isa.org; Wikipedia still lists the 2010 edition of Part 1.

Parts 1, 2 and 3 are IEC adoptions (IEC 62264-1, -2 and -3, each marked "Mod"). In Yokogawa's description, Parts 1, 2 and 5 define the exchange of production data between business and plant systems, and Parts 3 and 4 cover typical MES functions and add the exchange of maintenance, inventory and quality data.

Level 3: manufacturing operations management

Part 3 models the activities between level 4 planning and level 2 control, and its introduction says they apply regardless of how automated the plant is. It divides them into four categories, each of which coordinates, directs and tracks its own work:

• Production operations: making the product with the available resources, including procedural control such as recipe execution.
• Maintenance operations: upkeep of equipment, tools and related assets.
• Quality operations: testing, measuring and reporting quality, including laboratory work.
• Inventory operations: material transfers within and between work centers, and the location and status of material.

The contents of the 2013 edition describe each category with the same set of activities: definition management, resource management, detailed scheduling, dispatching, execution management, data collection, tracking and performance analysis. Charlie Gifford describes the flow: schedules from the business side drive detailed scheduling, which produces work orders; these are dispatched to work centers and work units, executed and tracked, and the data feeds performance analysis. Definition management keeps recipes, procedures, bills of material and routings; resource management reports current and future capability back to business scheduling.

In practice the matrix is a checklist: for each cell, which system — or which person with which spreadsheet — does the work today?

The equipment hierarchy

ISA-95 names equipment by the role it plays in production. OPC 10030 lists the levels:

• Enterprise, the top, made up of sites;
• Site, a physical, geographical or logical grouping set by the enterprise;
• Area, a grouping set by the site, containing work centers;
• Work centers: process cell (batch), production unit (continuous), production line (discrete), storage zone;
• Work units, any element under a work center and the lowest level usually scheduled by level 3 functions: for example a unit, a work cell or a storage unit;
• below these, equipment modules and control modules as defined in ISA-88 / IEC 61512-1.

OPC 10030 defines the process cell and unit as in IEC 61512-1, the same terms used in batch control projects. It also separates equipment, which describes a role, from a physical asset, identified by serial number with its make and model. The pump position in the hierarchy and the pump with a serial number fitted there today are two different objects.

ERP and MES: what crosses the boundary

The Yokogawa technical report groups the information exchanged between business and manufacturing systems into these categories:

• Resource information: personnel, equipment, material and process segments;
• Product definition: what must be defined to make a product;
• Production capability: which resources are available;
• Production schedule: what to make and what to use;
• Production performance: what was actually made and used (the report's figure calls it production response).

The same report calls two flows the most widely used: production schedules downloaded from ERP to MES, and production performance uploaded from MES to ERP. Gifford adds the reverse flow of capability: resource management reports what level 3 can produce to business scheduling.

B2MML is an XML implementation of ISA-95, with a JSON schema since version 7. It was first created by the World Batch Forum and is now maintained by MESA International's XML Committee. Version 7, announced in November 2020, supports the 2018/19 editions of ISA-95 and IEC 62264, includes the BatchML schemas, and may be used royalty-free with credit to MESA. OPC 10030 maps ISA-95 into OPC UA and follows the B2MML mapping where it can.

Applying it on a plant project

• Draw the level 3 / level 4 boundary first. Write an interface list: each message, its ISA-95 object, direction, trigger, frequency and the system that holds the master copy.
• Assign the activities. Fill the Part 3 matrix with real systems, and decide early where detailed scheduling is done: in ERP, a separate scheduling tool or MES. The 2013 edition of Part 3 has an annex on technical and responsibility boundaries.
• Match the time frames. Send ERP summarised performance per order or shift, not raw tag values.
• Fix the equipment model early. Use one naming scheme for PLC tags, historian points and MES objects. Sepasoft's documentation for its production model (including enterprise, site, area, process cell, unit, line, cell, storage zone and storage unit) recommends naming items correctly from the start, because renaming creates a new item and earlier data is not linked to it.
• Place the historian and MES at level 3. OPC 10030 lists historians, batch control, recipe management and SCADA among the level 3 systems. Sepasoft's user manual has its own ISA-95 overview and describes its MES objects in ISA-95 terms such as segments, definitions, requests and responses. MES on Ignition with Sepasoft modules is covered on the MES page; historians are covered on the SCADA and historian page.
• Mind the network. The Yokogawa interface server was designed to run in a firewalled DMZ between the two domains; see the IEC 62443 article for the IDMZ.
• Clarify "ISA-95 compliant". Ask which parts and objects, which B2MML version, and which transactions.