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Modbus RTU over Ethernet
RTU over Ethernet Problem

I have a controller that uses modbus RTU Over Ethernet. How can I get a touch panel that has modbus TCP/IP to communicate with this controller? Am I going to need a third party device or do I need a new driver to be written?

By Jerry Miille on 2 March, 2009 - 9:09 pm

Well-you might need a new driver and-you might need some additional
hardware-it "depends".

The devil is always in the details so you need to define what you mean by Modbus RTU over Ethernet. Is this a "serial port tunneling" application where Modbus RTU packets are delivered inside a TCP or UDP connection? If so is it TCP or UDP packets? Is your touch panel a Modbus/TCP client or a server?

Also, you need to consider the performance issues like how fast do you need this to work, what happens if there is a communication failure, could your plant "blow up" if there is a problem?

Modbus/TCP is almost a standard and is well supported by many companies. There are several "bridges" that can make the Modbus/TCP to Modbus RTU conversion for you.

They differ in what functions they can perform and of course in what they cost.

Lots of questions for you and the best starting point might be for a direct conversation with potential vendors of this type of equipment. You can contact me directly if you choose. My name and phone number are on our web site

Jerry Miille

By Manikandan on 2 March, 2009 - 10:07 pm

Hi, You can use a MODBUS Gateway to convert Modbus RTU to Modbus TCP. Don't use MODBUS RTU running on Ethernet they will loss of Data. Since i face the same problem. Many third party gateway like Moxa, ADAM, Etc are ready available and it can reduce your development time.

By Dale Agonis on 3 March, 2009 - 9:58 am

Our Ethernet enable HMIs provide ModBus TCP/IP. The H-T60t-Ne at is a 5.7" color HMI with Ethernet.

By Fred Loveless on 3 March, 2009 - 1:59 pm

Modbus over Ethernet is usually Modbus RTU encapsulation in an Ethernet Packet. Modbus TCP is Modbus with a special header and it does not use the checksume that Modbus RTU does. You could easily talk to the device using a serial to ethernet converter useing a serial driver to go serial to the converter and then out to the device.

You could also use Kepware's Modbus serial driver with Ethernet Encapsualtion to connect directly to the device using Ethernet Encapsulation.

Fred Loveless
Senior Application Engineer
Kepware Technologies

So your saying if I use a serial RTU driver and go through a RTU to TCP convertor I should be good?

By Fred Loveless on 10 March, 2009 - 5:28 pm

Yes as long as it is not adding a Modbus Ethernet header and footer to the packet. You want a raw TCP/IP header.

That won't work, Craig. If you want to encapsulate Modbus RTU serial in a TCP or UDP packet you need a serial device server, not a Modbus RTU to Modbus/TCP converter. Most serial device servers will come with software that will let you create a virtual COM port on a PC. With that you can use most any Modbus RTU driver. But Kepware's OPC driver, for example, lets you directly create a channel to the serial device server using either TCP or UDP encapsulation. If your master is not a PC you can use two serial device servers, back to back.

Basically what I have is a touch panel (HMI) that has a Modbus RTU driver in it. If I run this through a standard serial to Ethernet converter would I get RTU over Ethernet? If so has anyone done this before and if so do you have a recommendations on a converter to use?

By Fred Loveless on 18 March, 2009 - 1:25 pm

Yes that should work. The serial to Ethernet converter will be configured to connect the IP of the device that is Modbus RTU over Ethernet.

By Fred Loveless on 18 March, 2009 - 1:32 pm

Forgot to mention that his is what our Modbus RTU Serial driver does when you enable Ethernet connection. It wraps the RTU packet in a simple Ethernet header and footer. When it gets to the serial to ethernet converter the header and footer are striped off and the RTU message goes to the RTU. The RTU responds and the message goes to the serial side. A header and footer are placed around it and sent back to the driver.

We also have customers that have done this in reverse, i.e the master is nonth seral side sends the RTU request, it is wrapped by a header and footer. The RTU recievse the request, responds and so on. In this process there is nothing special done in the communications driver.

Fred Loveless
Kepware Technologies.

Hello, here is an article about Modbus data over ethernet -
that I've found

Have anybody tested this software?