OPC UA vs Modbus vs MQTT: which should you use to connect machines?
Use OPC UA to read structured, secure data from modern PLCs and SCADA; Modbus for drives, meters and older controllers that only speak it; and MQTT to publish data from edge gateways to many applications. They are not rivals: a common architecture reads machines over OPC UA or Modbus at the edge and publishes upward over MQTT.
Talk to an engineer about thisWhat each protocol is
OPC UA is an industrial interoperability standard from the OPC Foundation with an information model, data types, discovery and built in security, used by PLCs, SCADA and MES. Modbus is a simple register based protocol from the late 1970s, running over serial lines (RTU) or Ethernet (TCP), still common in drives, energy meters and older controllers. MQTT is a lightweight publish and subscribe messaging protocol: devices publish to a broker and any number of applications subscribe.
Where each fits
OPC UA: reading and writing tags on modern controllers and SCADA servers, with typed data and authenticated sessions. Modbus: reaching simple or old devices; you need the register map, and there are no data types or security in the protocol itself, so keep it inside a protected network segment. MQTT: moving data from many edge gateways to many consumers across lines and sites, with low bandwidth and buffering. The Sparkplug B specification adds a standard payload and state management on top of MQTT for industrial use.
How they work together
A typical plant design: an edge gateway at each line reads PLCs over OPC UA and meters or drives over Modbus, adds context such as plant, line and machine names, and publishes over MQTT to a broker. MES, OEE, maintenance and analytics subscribe to the broker. Writes back to controllers, such as recipes or order data, go over OPC UA with defined handshake tags and change control. This pattern is the basis of what is often called a unified namespace.
Questions to ask before choosing
Which protocols do the controllers support natively, or with a licence option? Do you need to write to the machine, or only read? How many applications need the data? What does your OT security policy allow across the boundary? The answers usually decide the design without a protocol debate.
Related questions
Ajinkya Technologies builds MES, OEE, IIoT, traceability and RFID systems for manufacturers in India and the US from Belagavi and Bengaluru. Questions: bdo@ajinkyatechnologies.in.
References and scope
- OPC Foundation: OPC Unified Architecture
- MQTT: the standard for IoT messaging (mqtt.org)
- ISA: ISA/IEC 62443 series of standards for automation security
External references explain the underlying technology or regulatory context. Ajinkya pricing, schedules and project results are company provided estimates or examples, not independently audited industry benchmarks. Confirm scope and current requirements before making a purchasing decision.
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Reviewed by Amey Kadle, Founder, Ajinkya Technologies. Last reviewed: .