Unlike the tech sector, which has essentially converged on the Internet protocol suite, industrial systems continue to use a variety of industry data protocols for communication between the components of control systems. This article gives an overview of some common legacy protocols still in wide use as well as one IIoT-oriented protocol that is expected to see greater adoption in the future.
Open Platform Communications (OPC) is an industrial data protocol that defines communication interfaces for industrial automation devices and systems. The original OPC, now known as OPC Classic, consists of many specifications that create a framework for data access and exchange in an industrial automation network, most notably OPC Data Access (DA) for exchanging process data, OPC Alarms & Events (AE) for communicating alarms and managing state, and OPC Historical Data Access (HDA) for querying and analyzing historical data.
OPC Classic is restricted to Microsoft Windows systems and the COM architecture, and these restrictions impacted the usability, scalability, and security of the protocol over time. In response, the OPC Foundation released the OPC Unified Architecture (UA) as a functionally equivalent replacement protocol without the drawbacks of the original version. It should be noted that while OPC Classic and UA have functional equivalence, they are not compatible.
OPC UA does provide a unified framework that is scalable, secure, and platform-independent while enabling the features of OPC Classic. However, due to the complex nature of UA, many implementations do not cover all features, and development of client applications can be difficult. Still, OPC Classic and UA have both been successful in industrial scenarios and seen wide adoption. At present, OPC remains a key protocol, and IIoT solutions will likely need to implement OPC interfaces on the edge to collect data.
Modbus is a fieldbus protocol commonly used for communication on the lower levels of industrial systems, such as PLCs. As a simple, open, and time-tested protocol, it is widely used in industrial applications, and customized variations of Modbus have been created that run on a wide variety of transport protocols and transmit many kinds of data. Modbus implementations work on a client/server model in which the server must request needed information and the clients then provide it.
Due to the number of variations and the limitations of its 1970s-era design, it can be difficult for new applications to support Modbus. That said, considering the ubiquity of Modbus among data collectors in industrial environments, edge gateways must support Modbus at least for the near future. IIoT solutions that reach the edge will need to consider transmitting and receiving data in Modbus form if they want to communicate with the most essential components – the sensors and actuators providing data – of an industrial data system.
PROFINET is an Industrial Ethernet-based protocol for data communication. Unlike Modbus described above and its predecessor PROFIBUS, PROFINET is built on a provider/consumer model instead of a client/server model. A network consists of devices (sensors, actuators, and so on) and controllers, such as industrial PCs or control systems.
Because it uses Ethernet and provides more modern interfaces, integrating PROFINET into IIoT solutions will be easier than older protocols like Modbus. As adoption of PROFINET grows in the industrial sector, IIoT providers need to consider how their solutions can interface with the protocol to obtain data.
IIoT Protocol: MQTT
MQTT is a publish-subscribe (pub/sub) messaging protocol that, while it has been more closely associated with the tech industry in recent years, was originally designed for industrial applications. In MQTT, a message broker receives messages from source clients (publishers) and forwards them to destination clients (subscribers).
For IIoT scenarios, Sparkplug provides an open-source specification that defines how best to use MQTT in industrial infrastructures. MQTT with Sparkplug gives IIoT setups a lightweight, scalable protocol that can be used to exchange data among all components in consistent formats. Making use of a popular and commonly used protocol also enables faster adoption of IIoT technology.