A key element of any automated production system is the asset model. This is the conceptual model of the physical or logical assets that a company uses to perform its business. It typically lives in the MES system and is organized hierarchically according to the ISA-95/ISA-88 standard. The ISA-95/ISA-88 standard provides a common model for production processes and resources, and a standard interface to expose the model to higher-level ERP systems and lower-level control systems. The ISA-95 standard was originally developed to bridge the gap between low-level machine control and high-level ERP systems that are responsible for planning and scheduling. Currently, ISA-95 is the international standard for the integration of enterprise and control systems, and consists of both models and terminology. These can be used to determine what information has to be exchanged between systems for sales, finance, and logistics and systems for production, maintenance, and quality. This information is structured in UML models, which are the basis for the development of standard interfaces between ERP and MES systems. ISA-95 is based on the hierarchical structuring of ISA-88 for the physical assets of the industrial company. While ISA-88 focuses on the process cell, ISA-95 focuses on a higher level. For ISA-95, the levels of the process cell and lower are only of interest if it is necessary to exchange information between the enterprise and control systems regarding these levels.

The ISA-95 equipment model is a hierarchy of various logical entities that are used to model aspects of production. The general structure of the ISA-95 equipment model is shown in the following diagram:

This model has a number of important characteristics. These include the following:

• There are similarities between the process cell, the production line, and the production unit. These are essentially the same levels of entity that are applied to batch, continuous, or discrete production.
• Storage zones and storage units are independent of the type of production.
• Equipment modules can contain other equipment modules, and control modules can contain other control modules. This allows you to build a complex hierarchical model of a piece of equipment.
• Equipment modules and control modules are not a part of the base ISA-95 standard. They are added as extensions of the model to unify the model with the ISA-88 standard, which includes these objects.