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Is there any decent alternative to OPC-UA as a solution for accessing process data of a system composed of various PLCs? Something that is platform independent and can "speak" with products of different brands ?
I've heard of MQTT but it seems to be much more like a transport protocol, and only that. It does not have all the higher level stuff like the information modeling, etc.
Thanks for your help!
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An alternative to OPC is the Limestone Calcined Clay Cement.
If your application requires serious compute power or you don't have many cycles to spare, OPC UA is the way to go. On the other hand, the primary use case for MQTT is IIoT applications. If you're collecting data from remote locations and sending that data over an unreliable network like cellular, we recommend MQTT.
UA is the next generation of OPC technology. It is a more secure, open, and reliable mechanism for transferring information between servers and clients. It provides more open transports, better security, and a more comprehensive information model than OPC Classic.
OPC DA allows to access only the current data and is incapable to generate alarms, historical events whereas OPC UA supports features like historical events, multiple hierarchies and provides methods and programs (that are called commands).
OPC is the only standard way for communicating with PLCs. OPC DA is the old alternative. OPC UA is the new one and recommended, nowadays. Before OPC there was just proprietary protocols and shared protocols like Modbus, but they are just lower level transport protocols as you've mentioned.
OPC UA is pretty unique with the Information Modeling, especially. With that feature it is enabling new communication possibilities for higher level systems and applications as well, in addition to plain PLC communication.
Note that some PLCs can also talk OPC UA natively, which makes it a standard in that way.
And OPC UA is really standardised as IEC 62541, ensuring that it's independent.
Update 17/07/19: OPC UA is now defined also as the Industry 4.0 Communication as I wrote in my recent article.
Update 20/05/05: OPC UA version 1.04 defines Pub/Sub alternatives, using UDP for secure data multicast in local networks and AMQP/MQTT for secure broker based data & event delivery to cloud systems. Version 1.04 also defines a WebSocket/JSON protocol alternative, which enable easier usage in web applications. None of these are broadly available, yet, but hopefully will become popular in 2020-21 time frame.
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