Modbus/TCP vs Modbus RTU on Ethernet

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Thread Starter

John Macdonald

We are planning to use Ethernet to network Power Measurement Ltd. (PML) digital power meters to our Plant Information (PI) System data servers. Since the PML meter Ethernet port will be used for another protocol (ION), we are planning to use a Lantronix Modbus to Ethernet Bridge to convert the PML's serial port Modbus RTU to Modbus/TCP on Ethernet. The PML meter also has the option of assigning the Ethernet port to use the Modbus RTU protocol. We have been told that the PI system's "Modbus Ethernet" interface supports two "flavors" of Modbus over Ethernet:
1) Modbus RTU over Ethernet "wrapped in TCP/IP"
2) Modbus/TCP We have tested and confirmed this.

Is there a difference between Modbus RTU over Ethernet and Modbus/TCP? Can any Modbus/TCP master get data from a Modbus RTU Ethernet slave?.
 
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Greg Goodman

> Is there a difference between Modbus > RTU over Ethernet and Modbus/TCP?

Yes, there is a difference. The Modbus/TCP protocol looks like the RTU protocol, with an additional 6-byte header, and without the CRC.

> Can any Modbus/TCP master get data from > a Modbus RTU Ethernet slave?.

I'm not sure how to read the question. do you mean (a) "is there any Modbus/TCP master that can talk serial RTU over ethernet," or do you mean (b) "if i pick any modbus/TCP master, will it necessarily talk modbus RTU"? i don't know about (a), but the answer to (b) is "no". on the other hand, i've encountered several modbus serial RTU implementations that allowed the user to specify an IP address and port instead of a serial port. this is typically used for talking to modbus devices through a terminal server. Hope this helps, Greg Goodman Chiron Consulting
 
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Darold Woodward

The short answer is that yes, there is a difference and it is very important to how your application works. No, the two (Modbus RTU over Ethernet and Modbus/TCP) will not be interoperable (however you can have these two protocols in operation over the same Ethernet network with different masters). More than likely, the Modbus RTU over Ethernet is a "wrappered" version of the Modbus RTU protocol. This can be accomplished any one of a number of ways. One of the simplest is to use a TCP/IP Telnet session as the Ethernet transport mechanism and "talk" Modbus RTU across the link. This would be similar to using serial tunneling offered by Lantronix, Z-world and other devices/software that send the characters in a typical serial connection between two devices on an Ethernet network using a Telnet session. The Modbus RTU over Ethernet was probably developed by someone (maybe PML) before the Modbus/TCP specs were available and was developed to work with a specific vendor's implementation. You can also insert Modbus RTU messages into UDP/IP frames and essentially broadcast them onto the network (similar to a typical multidrop EIA-485 Modbus network). This is a simple process and works well for networks that are essentially a single "segment." Modbus/TCP on the other hand, is a specification developed by Schneider/Modicon to specify how the Modbus protocol operates and is "layered" on top of the TCP. The spec defines the connection types and basic messaging structure. The application layer function codes, requests, responses, etc. in Modbus/TCP are identical to those used in other Modbus systems making it part of the family. Modbus/TCP also includes conformance classes and in general tightens up the spec to help insure interoperability. One alternative that you may wish to consider would be using a smart device like a communications processor to collect data from several PML meters at the same time and make it available via a single port through a serial tunnel or a direct Ethernet connection. This would make the configuration simpler at the PI master and sharply reduce overall communications overhead on the Ethernet network. The other thing that you should consider is that Ethernet devices are typically capable of operating more than one Application layer data exchange protocol at the same time through the same interface. You may be able to use the ION and Modbus/TCP simultaneously. I suggest checking with PML on this. Darold Woodward PE SEL Inc. [email protected]
 
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Greg Goodman

> The Modbus RTU over Ethernet was > probably developed by someone (maybe PML) before the Modbus/TCP specs were > available and was developed to work with a specific vendor's implementation. I have seen modbus RTU masters that allow the user to configure an ethernet IP address and port (instead of the more typical serial port) in order to support communication between the host computer and serial modbus devices plugged into a terminal server. Greg Goodman Chiron Consulting
 
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Men-Shen Tsai

Dear Darold:

In the following response, you wrote:

> The short answer is that yes, there is a
> difference and it is very important to how your
> application works. No, the two (Modbus RTU over
> Ethernet and Modbus/TCP) will not be
> interoperable (however you can have these two
> protocols in operation over the same Ethernet
> network with different masters).

Is the 'Modbus RTU over Ethernet' the same as 'Modicon Ethernet' which is used by Modicon's bridges (174 CEV 200 30)? Do you have any infomration or specification for 'Modicon Ethernet' or 'Modbus RTU over Ethernet'?

Regards,

Men-Shen Tsai
 
Do you have the rtu implementation source code on how to send data by specifying a ip address and a port or can you list a code snipplet to tell me
how to talk rtu over a tcp/ip network

Would appreciate if you could help.Can email me if you have an answer. Thanks

Thanks and Regards,
Casey
[email protected]
 
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