Modbus_TCP vs Modbus RTU on Ethernet
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Transcript of Modbus_TCP vs Modbus RTU on Ethernet
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from the What's the difference? department...
Modbus/TCP vs Modbus RTU on EthernetPosted by John Macdonald on 30 April, 2001 - 12:20 pm
We are planning to use Ethernet to network Power Measurement Ltd. (PML) digital power meters to our PlantInformation (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 toModbus/TCP on Ethernet. The PML meter also has the option of assigning the Ethernet port to use the Modbus RTUprotocol. We have been told that the PI system's "Modbus Ethernet" interface supports two "flavors" of Modbus overEthernet: 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 datafrom a Modbus RTU Ethernet slave?.
Posted by Greg Goodman on 30 April, 2001 - 5:19 pm
> 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 RTUover ethernet," or do you mean (b) "if i pick any modbus/TCP master, will it necessarily talk modbus RTU"? i don'tknow about (a), but the answer to (b) is "no". on the other hand, i've encountered several modbus serial RTUimplementations that allowed the user to specify an IP address and port instead of a serial port. this is typically usedfor talking to modbus devices through a terminal server. Hope this helps, Greg Goodman Chiron Consulting
Posted by Casey on 25 March, 2002 - 3:32 pm
Do you have the rtu implementation source code on how to send data by specifying a ip address and a port or canyou list a code snipplet to tell mehow 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,[email protected]
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Posted by Darold Woodward on 1 May, 2001 - 10:14 am
The short answer is that yes, there is a difference and it is very important to how your application works. No, thetwo (Modbus RTU over Ethernet and Modbus/TCP) will not be interoperable (however you can have these twoprotocols in operation over the same Ethernet network with different masters). More than likely, the Modbus RTUover Ethernet is a "wrappered" version of the Modbus RTU protocol. This can be accomplished any one of anumber 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-worldand other devices/software that send the characters in a typical serial connection between two devices on anEthernet 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'simplementation. You can also insert Modbus RTU messages into UDP/IP frames and essentially broadcast themonto the network (similar to a typical multidrop EIA-485 Modbus network). This is a simple process and works wellfor networks that are essentially a single "segment." Modbus/TCP on the other hand, is a specification developed bySchneider/Modicon to specify how the Modbus protocol operates and is "layered" on top of the TCP. The specdefines 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 tocollect data from several PML meters at the same time and make it available via a single port through a serialtunnel or a direct Ethernet connection. This would make the configuration simpler at the PI master and sharplyreduce overall communications overhead on the Ethernet network. The other thing that you should consider is thatEthernet devices are typically capable of operating more than one Application layer data exchange protocol at thesame time through the same interface. You may be able to use the ION and Modbus/TCP simultaneously. Isuggest checking with PML on this. Darold Woodward PE SEL Inc. [email protected]
Posted by Greg Goodman on 1 May, 2001 - 5:03 pm
> The Modbus RTU over Ethernet was > probably developed by someone (maybe PML) before theModbus/TCP specs were > available and was developed to work with a specific vendor's implementation. I haveseen modbus RTU masters that allow the user to configure an ethernet IP address and port (instead of the moretypical serial port) in order to support communication between the host computer and serial modbus devicesplugged into a terminal server. Greg Goodman Chiron Consulting
Posted by Men-Shen Tsai on 16 May, 2001 - 9:53 am
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
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> 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 (174CEV 200 30)? Do you have any infomration or specification for 'Modicon Ethernet' or 'Modbus RTU overEthernet'?
Regards,
Men-Shen Tsai
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