Five-layer AVEVA PI System architecture implements the ISA-95 Purdue Model for secure OT/IT convergence in industrial environments. Field devices (PLCs, sensors, actuators) at Level 0-1 feed SCADA and HMI systems at Level 2, which connect through PI Interface Nodes and protocol connectors (OPC UA, Modbus, MQTT) to the core PI Data Archive Collective in a DMZ at Level 3.5. Enterprise systems (Power BI, Data Lake, ERP, MES) at Level 4 access historian data through load-balanced PI Vision and PI Web API servers, with dual firewalls enforcing security boundaries between process, OT, and IT networks. This reference architecture demonstrates defense-in-depth for critical infrastructure, ensuring operational data flows securely from shop floor to boardroom while maintaining network segmentation and high availability through collective members and buffer subsystems. Fork this diagram on Diagrams.so to customize for your plant's specific SCADA vendors, protocol mix, or cloud integration requirements (AWS, Azure). Download as .drawio, .svg, or .png for compliance documentation, or embed in your ICS security policies and digital transformation roadmaps.