Mid-market manufacturers are accelerating the replacement of aging programmable logic controllers (PLCs) and proprietary I/O interfaces with modular, cloud-native controllers built on open communication standards. Pressure is mounting to integrate AI analytics and reduce vendor lock-in. The shift is being codified by major industry bodies and hyperscaler cloud providers, whose combined influence is pushing OPC UA (OPC Unified Architecture) and MQTT from niche pilots to mainstream retrofit strategy.
Background
The industrial controller landscape has long been fragmented. Leading PLC vendors-including Siemens, Rockwell Automation, Mitsubishi Electric, and Schneider Electric-have each maintained proprietary communication stacks, hardware connectors, and software environments, creating integration friction and increasing total cost of ownership in multi-vendor facilities. For mid-market plant operators running mixed fleets of aging equipment, the result has been a persistent "interoperability gap" between operational technology (OT) systems on the shop floor and the IT analytics platforms increasingly demanded by management.
Open standards offer a practical path around that gap. OPC UA and MQTT can be implemented together during digital transformation: OPC UA handles horizontal communication with PLCs, SCADA systems, and edge gateways, while MQTT suits platform- and enterprise-level data transport. OPC UA incorporates a broad feature set covering most requirements for a platform-independent interoperability standard, capable of transmitting data from the factory floor to the enterprise level. It is well positioned for deployment in smart embedded devices targeting Industry 4.0 environments with heterogeneous communication nodes.
A stark market signal underscores the urgency of modernization: the global market for OPC-UA gateways enabling legacy PLC connectivity was valued at USD 268.3 million in 2025 and is projected to reach USD 1.01 billion by 2036, expanding at a compound annual growth rate (CAGR) of 13.0 percent, according to Fact MR analysis. Manufacturing integration is expected to dominate the application category.
For related context on multi-vendor automation cells and cross-vendor modular interoperability frameworks advancing in parallel, see earlier coverage of Multi-Vendor Modular Cells Advance Interoperability in Factory Automation and Cross-Industry Push Strengthens Industry 4.0 Interoperability Standards.
Details
The architecture most widely adopted in current retrofit projects follows a layered model. Edge devices aggregate data from multiple OPC UA servers on the factory floor, perform local analytics such as statistical process control or anomaly detection, and publish summarized results to cloud platforms using OPC UA Pub/Sub over MQTT. This approach reduces bandwidth requirements while preserving the semantic richness needed for enterprise-level analysis.
Legacy controllers lacking native OPC UA support are not excluded. Retrofit IoT sensors and edge gateways convert legacy PLC data from protocols such as Modbus, PROFINET, and EtherNet/IP into OPC UA format; MQTT then transports the data to the cloud. Vendors such as Softing have containerized this bridge capability, enabling connection of up to 20 controllers-including Siemens SIMATIC, Allen-Bradley, Modbus TCP, and FANUC CNC systems-with a single container runtime. This permits retrofit IIoT integration of existing controllers without modifying their running configurations.1Industry Insights: IEC 61131 Standardizes PLC Programming | Motion Control & Motor Association
The most significant industry-level consolidation came in 2024 and 2025. The OPC Foundation's Cloud Initiative is a major collaborative effort to accelerate IT and cloud application interoperability through the OPC UA standard, bringing together leading cloud providers, automation companies, and manufacturers to create a unified approach to industrial data exchange and digital transformation. Supporters include Alibaba Cloud, AWS, Google Cloud, Huawei, Microsoft Azure, and SAP, along with a broad range of end users and automation firms. Leading industrial automation companies-ABB, Beckhoff, Honeywell, Mitsubishi Electric, Rockwell Automation, Schneider Electric, Siemens, and Yokogawa-have also joined.
At HMI 2025, the OPC Foundation's Cloud Initiative Reference Architecture was unveiled as the first reference architecture jointly supported by all major cloud and automation providers, integrating OPC UA, Asset Administration Shell (AAS), and Eclipse Dataspace Components into a cohesive open-source framework. Certification programs for OPC UA FX (Field eXchange) controllers have launched, and the first certified devices are expected in 2026.
On the security side, open standards such as MQTT and OPC UA should be built into device architecture from the outset rather than added as an afterthought. Cybersecurity must keep pace with openness through adherence to the EU Cyber Resilience Act and IEC 62443 standards, as well as segmented networks, multifactor authentication, and role-based access controls. Only 35% of manufacturing facilities currently have PLCs with updated firmware or hardened network configurations, and compliance with IEC 62443 remains patchy, especially among small and mid-sized manufacturers.
Industry observers caution that standards alone are insufficient. "A certified server connected to an uncertified client offers no guarantee of interoperability," one industry analyst noted, adding that the OPC Foundation's two primary initiatives-Field eXchange (FX) and Cloud eXchange (CX)-serve a common purpose: driving available information sources as close to the manufactured product as possible and disseminating that information as widely as possible.
Outlook
The OPC UA for AI Working Group is exploring how artificial intelligence can simplify engineering tasks, enable natural language interaction with industrial data, and support advanced analytics without requiring specialized data science teams. Key PLC trends include AI/ML integration, edge-native computing, native cloud connectivity, and virtualized PLCs. By 2028, 25% of all new PLC deployments are expected to be virtualized and 60% will have built-in AI capabilities, according to industry projections. For plant managers assessing retrofit kits, protocol support for OPC UA and MQTT-alongside vendor responsiveness to interoperability certification tests-is emerging as a primary procurement criterion as deployment cycles shorten and AI-enabled maintenance analytics become standard requirements.
