Modbus and EtherNet/IP

I have an uncle that doesn’t just like to drink, he loves to drink. Most of the time he’ll stick to just one or two but will occasionally overindulge and become a little loose with his tongue, you might say. But we still love him and invite him over for the next family get-together. And that’s exactly how I think we should view Modbus.

It’s the fashion now to say that everything must be Ethernet. The only factory floor architecture considered for Layer 1 communications should be Ethernet over wired or wireless media. People with that opinion do have a point. There is a cost to every media type, link-layer, or application protocol you bring into the plant. You have parts, training costs, documentation, and spare parts to worry about. And it’s not as if the Gen Z gal you hired last week is going to have ever heard of Modbus or daisy-chained RS-485. That’s all understandable.

But there is another side to this coin as there is to most coins. Sometimes Modbus RTU is just the right technology. For one, there are devices that only come with a Modbus RTU connection. There are three reasons for that. One, the designers of the device are not network guys and really don’t know what communications they should consider. If, say, you’re very good at building glue controllers, it’s likely you’ve never heard of EtherNet/IP, PROFINET IO, or even Modbus TCP. Modbus RTU is easy and fast, so they use it. A second reason is cost. There is nothing cheaper and easier to implement than Modbus. No complicated physical Ethernet chip to add, just use the UART that’s already on your microprocessor and you’re done. Third, the device is so simple that Modbus just makes sense. Maybe it is a temperature sensor that only has one value to report or a relay that just wants to report a status bit. There are all kinds of really simple devices in the world, and it can make sense to enable them for Modbus RTU communications.

Now, what you’ll need to really use them on the plant floor is a connection to an EtherNet/IP or PROFINET IO system. You do want to integrate all your plant floor data in one place, to collect it so that it can be manipulated, moved, organized, and archived together. To do that, you need a way to move your Modbus RTU data into an EtherNet/IP controller, which in EtherNet/IP terms is a scanner.

That’s where RTA comes in. We have a large line of gateway products that can use the Modbus protocol in different ways. Here’s only a partial list of what we have.

Modbus RTU Slave to EtherNet/IP Scanner (i.e. Allen-Bradley ControlLogix^®)

This is where you have one or more Modbus RTU slave devices that you want to control from your PLC. For this, you’ll need a device that is a Modbus RTU master and an EtherNet/IP adapter. That part number is the 460ESMM, EtherNet/IP to Modbus master.

Modbus RTU Master to EtherNet/IP Scanner (i.e. Allen-Bradley ControlLogix)

This is an architecture where you already have a Modbus RTU network with a Modbus RTU master and some Modbus slaves, and you’d like your PLC to look like another slave so that the Modbus master can read and write to it. For this, you’ll need a device that is a Modbus RTU slave one side and an EtherNet/IP adapter on the other. That device is the 460ESMRS, EtherNet/IP to Modbus slave.

Modbus RTU Master to BACnet Building Automation

In this architecture, you are connecting one or more Modbus RTU slave devices to a BACnet building automation system to make it look like those Modbus slaves are BACnet end devices. For this, you’ll need a device that is both a BACnet end device and a Modbus master. That device is our 460MMBMS, the Modbus RTU master to BACnet.

So, just like the crazy, lovable uncle, Modbus is still a great tool you want around your factory floor, and RTA is the place to find what you need to use Modbus.