Ethernet/IP Communication

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>Modbus/TCP seems to be everyones first
>or second choice or option but
>Modbus/TCP's only advantage over
>Ethernet/IP is simplicity but this
>comes at a great cost.
>
>1. Modbus/TCP has a very resistricted
>addressing space. 64K words is not
>enough for many applications. <

I've never found this to be a restriction in any of my applications. However, there are ways around this by using the "Unit Identifier" in the MBAP header.

>2. Modbus/TCP doesn't have data types.
>Floats, DWORDS, and DINTS are transfered
>as words. Ethernet/IP supports data
>types. <

Not true. See page 24 of the Open Modbus/TCP Specification Release 1.0 29 March 1999. There in B.2 is a chapter on data types.

>3. There should be a Modbus/UDP as well
>as a Modbus/TCP. TCP is not the best
>way to send I/O. UDP is much better.
>Ethernet/IP got this right. TCP should
>never be used for I/O. If something goes
>wrong the TCP stack will retry trying to
>send or receive old data. UDP will just
>send or receive new data the next time a
>transfer is made. This has been covered
>many times and at many forums. <

Again not true. Modbus/UDP is a well known and used by many. Kepware's KepServerEx supports it in their Modbus Suite.

>4. Modbus/TCP packets/messages are very
>small compared to Ethernet/IP messages.
>One can send a lot of data with just one
>MSG block. We have transfer 32767 words
>with one MSG block. MSTR blocks are
>limited to just 100-125 words. This
>means that a PLC program must download
>1000 words in many transfers. Many
>programmers are not capable of writing
>code that does that. <

Are you stating Ethernet/IP is simpler to code compared to Modbus?

>We have 3 Ethernet/IP certified
>products. They can communicate using
>any one of many Ethernet application
>layers including Ethernet/IP and
>Modbus/TCP. We have a pretty good idea
>of how our products being used and what
>protocol are used. Most of our customers
>use Ethernet/IP because most use
>Rockwell PLCs. Duh. Rockwell makes
>Ethernet/IP so easy and fast. Unless
>Schneider and others that use TCP have
>a bigger market share than Rockwell I
>don't see how Modbus/TCP can be
>'kicking Ethernet/IP butt'. <

Industry analysts have reported over 7 million Modbus nodes in North America and Europe alone. Why would ODVA be putting support for Modbus into Ethernet/IP if there wasn't something to gain?

Remember Modbus was created back in 1979 while it might have some limitations in 2008 it success is because its truly open, unlike Ethernet/IP.
 
Mark,

The reason you see many products talking to Rockwell controllers via Ethernet/IP even though those products and companies are not ODVA members is this:

Those guys bought ready made EIP stacks from someone else. The producer of the stack is no doubt a member of ODVA.

I used to work for one, and that's how we generally handled it.
 
So where can I get The EtherNet/IP Specification by ODVA for evaluation?

Can anyone send me a link or a copy?
 
> If anyone has any other ideas, I would very much like to hear them. <

ODVA oversees product compliance with the CIP Network specifications using the following processes:

Each vendor is required to sign a Terms of Usage Agreement for each ODVA technology for which they intend to make, have made, sell or have sold products. In signing this agreement, the vendor agrees to comply with the network technology specification and meet a set of user responsibilities.

ODVA's conformance testing provides general industry with the vendor-independent assurance that products built to the CIP Network specifications comply with those specifications.
 
Modbus is about as "focused" on I/O as EtherCAT is -- that is, not much, unless you only look at the surface.

EtherCAT standards are free as long as you join their user organization (for free, too), and agree to some terms. Those terms are fairly reasonable (much more reasonable than ODVA's).

EtherCAT can be used entirely for acyclic messaging and then there are no real-time requirements of any sort.

There are two protocol variants in EtherCAT: device protocol and automation protocol (EAP). The former is used to do real-time data exchange with I/O points, drives, sensors, etc. The latter is used across the factory floor to link PLCs, factory supervision, etc.
 
The specification is $1000 for non members, I wanted a copy to allow me to include it as part of a training session on Industrial networking and include E/IP. I will show Modbus/TCP only as I can get and explain that spec.
 
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