Purdue ModelPERA
The Purdue Model is a reference architecture that organizes an industrial network into levels, from the physical process at Level 0 up to enterprise IT at Level 5. It is the common map for deciding what may talk to what, and it underlies ISA/IEC 62443 zoning and NIST SP 800-82 guidance.
The levels are: Level 0, sensors and actuators; Level 1, controllers such as PLCs and RTUs; Level 2, supervisory systems such as HMIs and SCADA servers; Level 3, site operations such as historians and engineering workstations; Level 3.5, the industrial DMZ; Levels 4 and 5, enterprise business systems and the corporate network.
The model dates from the Purdue Enterprise Reference Architecture work of the early 1990s and was adopted by ISA-95 and ISA-99. It is a way of thinking about boundaries rather than a product, and modern flat Ethernet plants rarely match it exactly. Its value is in naming the boundary that must exist between the control network and everything above it.
Why it matters in cement, mining, and aggregates
Cement, mining, and aggregates plants that grew by acquisition and integrator projects usually have no explicit levels at all: the batch controller, the historian, and the office file server share one network. Drawing the plant onto Purdue levels is the first step of a topology assessment and the frame in which segmentation is designed.