# Ethernet or Modbus a comparison

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#### Scott Vaughan

What are the major differences between using Modbus and Ethernet TCP/IP? I know that you can use the Modbus protocol over Ethernet but I was wondering what the real differences between the 2 protocols are? Any information that you could provide would be greatly appreciated. Thank you, Scott Vaughan

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#### BKR

I'm not extremely familiar, but I think this is right: Ethernet is a broad term covering a local area networking method. Ethernet is very commonly used everywhere from corporate office networks to factories to my own living room It covers everything from wiring on up to data link. You can think of it as more or less equivalent to a phone line that lets two devices "talk" to each other. TCP/IP is a higher level protocol that handles things like how hosts find each other, address messages to each other and start and end a session. So to follow the analogy above, it's like the actual language the devices are speaking, similar to English, Italian or Esperanto. Modbus (this is what I'm least familiar with) communicates between various industrial automation devices. It's like TCP/IP in that it defines how the devices talk to each other. In fact it is analogous to TCP/IP in that it's like a language. The difference is that Modbus was designed to run over direct links between two devices (various types of serial cabling) whereas TCP/IP was designed for networks where a large number of devices are all connected to the same wire. Of course, Modbus/TCP allows Modbus devices to talk using the TCP/IP protocol. This is accomplished by (almost) literally encapsulating the Modbus data inside TCP/IP. So, you can basically stack these on top of each other. Modbus runs on top of TCP/IP, which in turn runs on Ethernet. Nifty, ain't it? Keep in mind though that I'm far from an expert, esp. in Modbus.

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#### Peter Nachtwey

Modbus is a protocol used on serial lines. Modbus/TCP is basically the same protocol in a Ethernet TCP/IP packet.

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#### Rich Baker

While there are many differences between the protocols, the best news here is that you can have both Modbus and TCP/IP, by using Modicon's Modbus/TCP protocol. This protocol encapsulates a Modbus message within the TCP/IP protocol. See www.modbus.org for more details. The differences between Modbus RS-232 and Ethernet TCP/IP are as follows: 1. Speed - Modbus RS-232 runs at upto 19200, ,whereas Ethernet runs at 10-100 Mbit (or even 1 Gbit). 2. Addressing - Modbus addressing is limited to 247 different nodes, whereas IP can address an exceptionally large number of nodes. 3. Internetworking - Modbus is primarily designed to operate in a small local network, whereas IP is designed to link to other networks, the Internet, and the rest of the world. 4. Cabling - Modbus RS-232 typically uses an RS-232 cable, whereas Ethernet typically uses Cat-5 twisted pair Ethernet cable. (There are lots of other options for both protocols). 5. Encoding of messages - Modbus has an applications layer protocol that specifies how a message is encoded. Ethernet TCP/IP does not (this is why we need to layer Modbus on top of Ethernet TCP/IP). I am sure there are many other differences, this is just a 5 minute summary of the key points. Rich Baker Schneider Automation Modicon Division

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#### David Corking

Last night I read a really nice short article (by Perry Sink) about the use of Ethernet in industrial networks. Modbus/TCP gets a strong mention - and he also highlights some home truths that our marketing friends are tempted to avoid. It is an ISA conference so I guess this is a plug for ISA membership to A-List readers. Hope this web link to the article works for you ... http://www.chemicalonline.com/content/news/article.asp?DocID={B5761E72-DC0E-11D4-A76E-00D0B7694F32} David Delaware ISA Member in a personal capacity

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#### CK

Modbus is a serial protocol. It's good for 9600 baud or 19200 baud. Ethernet is thousands of times faster. Modbus over TCP/IP is actually a protocol called Modbus/TCP. Other than speed, it's more or less a Modbus message encapsulated in an ethernet packet.

CK

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#### Alex Pavloff

It's a stack of protocols. ModbusTCP sits on top of TCP/IP which can (and is the most common usage nowadays) sits upon Ethernet (which is available in multiple flavors). This is very different from other ways to network things, which usually bundle everything in one big black box all the way from the wiring up to the protocol. Not coincidentally, the companies that bundle everything together have "vertical integration" as part of the business plan.

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#### Darold Woodward

Ethernet TCP/IP is only a physical layer and protocol stack without an application layer data exchange protocol. TCP/IP is connection oriented rather than broadcast message oriented like typical Modbus applications. You can get the Modbus TCP specs for free at Modicon.com. Darold Woodward PE SEL Inc. [email protected]

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#### Bill Sturm

>Modbus is a serial protocol. It's good for 9600 baud or 19200 baud. >Ethernet is thousands of times faster. I would be careful about saying thousands of times faster. The bit rate may be thousands of times faster, but the protocols and packet size limitations often reduce the throughput for small messages. I have heard reports of messages being sent every couple of milliseconds on ethernet TCP/IP. I have an application that sends data every 50 milliseconds or less with RS-232 at 19.2 kb and A-B DF1 protocol. I would have to say that for small messages, that ethernet is approx. 10 to 100 times faster. If you crank your serial rate up to 115 kb, then the difference is smaller. For large packets of data, then I would agree that there is no comparison between ethernet and RS-232. If anyone has any real number that prove or disprove what I just said, then please add in. Bill Sturm

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#### Quim_Sabate

Dear Scott, I think fields in protocol are the same. The difference is that fields to check errors (like CRC) are not included in Modbus/TCP; network layer does it. Regards, Quim Sabate http://www.valen.es

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#### Robert Smyth

The defining difference is that Modbus is a telemetering protocol. That is, it is a protocol with commands to do things like "set point" or "read register". TCP/IP does not have any such commands, it is not a telemetering protocol. It is a protocol for transferring information in frames. These frames could be, for example, Modbus frames. Telemetry is data communications for the control and monitoring of devices. On the other hand, it is really not practical to transfer other protocol frames using modbus Modbus as it just does not have commands to do this (although Modbus has been distorted to do just about everything). I hope this helps. Robert Smyth EngWare Pty Ltd ? Voice: 0411 202 514 ? Fax: 02 9222 1457 ? E-mail: [email protected]

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#### Rich Baker

Schneider Automation has benchmarked the Momentum M1E and the Quantum NOE modules at around 4000 round trip messages (Read Register commands) per second (250 Microseconds per message) over Modbus TCP/IP. Bill Sturm's number of 20 messages per second (50 msec per message) is about the best you could get RS-232 to handle under ideal situations. Thus, Ethernet is at least 200 times faster than RS-232. Realize that there are lots of factors that go into such a number. For instance, the Ethernet cable can handle other nodes working at similar speeds, whereas the RS-232 cable is using all of its bandwidth. Rich Baker Schneider Automation

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#### David Corking

On Friday 19th, I linked to: > http://www.chemicalonline.com/content/news/article.asp?DocID={B5761E72-DC0E-11D4-A76E-00D0B7694F32} > Disclaimer 1 - I am sure you noticed that the article favourably mentions Control.com's own Ken Crater, who spoke. I wasn't intending to crawl. Disclaimer 2 - here follows unashamed attempt to ingratiate: Ken - are transcripts/slides from your (or others') presentation at this conference published for those of us unwise enough not to attend the conference? David, in a personal capacity

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#### William G. Go

Dear CK, if you are going to choose which one will you prefer modbus or ethernet? what is the advantage & disadvantages between the two? yours william

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#### Darold Woodward

I've done some testing with networks with high data transmission speeds (1mb/s and 10mb/s). One was Modbus Plus and other was Ethernet using UCA2 (utility protocol). Because of the increased complexity and communication overhead, the "high speed" networks are not proportionally faster in effective data throughput when you set up a real-world network. Simply increasing speed without a comparably huge increase in processing horsepower will probably result in much smaller increases than the difference in data transmission speeds. So, I would agree with Mr. Sturm's assertions. Darold Woodward PE SEL Inc. [email protected]

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#### Alex Pavloff

One doesn't pick between Modbus and Ethernet if you're trying to talk to devices. One can pick between Modbus/TCP and Modbus/RTU (serial, which is what most people think of when you say "Modbus") if you're trying to talk to a device. Picking between the two, depends on far too many factors, and if you gave us some of the things that you have to consider, I'm sure we'd be happy to give you advice (which will probably conflict, but hey! This is the Internet &lt;g>).

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#### CK

If I had to choose which one I would use, it would be Modbus/TCP (ethernet). Ethernet is a faster network, and although the modules to add TCP/IP ethernet to the PLCs are much more expensive than just using the PLC's built in Modbus port (assuming you are using a Modicon PLC), In the end it can be worth the tradeoff. I have a large number of PLCs in my lab here, and personal preference is always to program them over ethernet as it's faster. Ethernet is a developed protocol, it's been around for years, and so devices for ethernet are getting very cheap. We just put ethernet cabling in our classroom, and got a 24 port 10/100 megabit ethernet intelligent switch (not just auto-sensing hub) for just over $400. 5 years ago that would have been over$1000. It's an open standard, and when you look at the installed base, no other network topology can compare. I used to be a computer network administrator, and even still am Novell certified, so I have a computer background. There ethernet has been king for a long time. Another nice thing with ethernet is the Internet. Programming a PLC across the internet is not a pipe dream. I've done it. It's also important to note that Modicon has some PLC equipment with web server technology. That allows you to actually create fairly robust operator interfaces in a web server module on the PLC. No more per seat SCADA licenses, this stuff works in your ordinary web browser. Now I'll sit down and shut up

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#### Burda, Jason M.

Based on what I've seen with available tech support and information on Modbus....Ethernet would be my choice JB Process Control & Automation Engineer