The Future of Modbus Gateways

Why Modbus will live forever

Everyone has their fifteen minutes of fame. I’m hardly famous but a few people know me in the industrial automation industry. They know me mostly for the RTA newsletter we publish every other month but also for some of the extraordinary ideas I’ve put forth. One of them is my belief that Modbus will live forever.

I’ve said time and time again that Modbus will never die. It’s too simple to die. People love and will use simple things because it makes our life easier. Life is hard, it’s always been hard – think of the settlers moving west – and it will always be hard. Even today, with smart refrigerators, life certainly isn’t as dangerous but it is complex and that makes it hard. Simple things make life just a little less complex and less hard so people will always gravitate to simple things like Modbus RTU and Modbus TCP.

So, if that’s true, how will Modbus gateways be incorporated into future control systems – the control systems of the rest of the 21st century and even the 22nd century. That’s a fair question to ask and I think I have an answer.

I see the future of Modbus as being a minor, but important part of industrial control systems. We are always going to have simple, unintelligent devices that need a connection to other more intelligent devices. For lots of reasons, many of these devices will still be built with Modbus technology. What they will use are intelligent gateways that will hide the underlying technologies.

We see this already with MQTT and the Sparkplug specification. With Sparkplug, a subscriber to a data stream that originates with a Modbus gateway to some set of Modbus RTU slave or Modbus TCP server devices will know if that device is online or offline. That’s a huge advancement and fixes one of the real problems we faced with implementing MQTT gateways. Without Sparkplug, a subscriber was always in the dark and never knew how old the data was in the latest message published by the MQTT broker.

We also see this in OPC UA. Modbus devices are hidden behind OPC UA interfaces (gateways). Modbus RTU and Modbus server data can be delivered to more distant and complex devices more easily with an OPC UA gateway in front of it. OPC UA much more easily interfaces IT devices than a Modbus device does.

A third example would be CIP, the Common Industrial Protocol. Just like OPC UA, CIP has an encapsulation mechanism that can carry other kinds of data from other industrial protocols. There is no reason that it can’t carry Modbus RTU and Modbus TCP data.

All these encapsulation mechanisms can work in two ways. They can either just encapsulate the data or encapsulate the entire protocol message. When they encapsulate the entire protocol message, a distant device, which can be in another part of the factory or even another part of the world, can be the Modbus TCP client device or the Modbus RTU master. The problem with that architecture is, of course, that those devices have to be conversant in the bits and bytes of building and deconstructing Modbus messages.

When they just encapsulate the data, the data is simply a set of Modbus registers or Modbus coils. In that approach, the distant systems are connected to a Modbus master, which is usually going to be the gateway. They don’t need to know anything about how that data is moved from slave to master (RTU) or server to the client (Modbus TCP). The gateway device handles all that.

I may be wrong, but I still believe that Modbus will live forever, and RTA gateways will be one of the reasons why!

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