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Ethernet/IP: CIP & Multcast messaging

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    A colleague and I were trying to figure out where the idea for mulicast messaging came from and were wondering weather this was actually required by the CIP (common industrial protocol). We were also thinking that CIP pre-dated Ethernet/IP? If so, multicast must therefore be an implementation of something in the CIP specification. If so, what? Does anyone have any idea of the background and source for this?
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    I ran across this article which I thought was helpful answering my own question:

    http://www.odva.org/Portals/0/Library/Publications_Numbered/PUB00138R2_CIP_Adv_Tech_Series_EtherNetIP.pdf

    Also I scheduled a blog post to be published next week that discusses some of this in more detail. http://HEnnulat.Wordpress.com
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      CommentAuthormelvin foo
    • CommentTimeJun 22nd 2010
     
    You only ever have to worry about multicasting if you are doing I/O (or what is termed Implicit messaging). Saying that, you aren't restricted to using multicasting with I/O now... you can now do CIP with unicast (unicast I/O). It's supported in Logix5000 ver 18. Whether it is an implementation of CIP itself or is product orientated, I am not 100% sure as I don't follow the CIP development as closely as an Ethernet/IP product developer. I'm kind of torn with whether it is specification or product related -- ODVA doesn't really highlight it much (only in their CIP motion section) but Rockwell does... esp. with the fact that the support of Unicast I/O came with the new version of Logix software and controllers. Unless I missed it, I didn't see any ODVA announcement that said, *we tweaked the spec to include Unicast I/O*.
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    I hadn't considered that the CIP would have to be modified to allow unicast messges, but it makes sense.

    I see that UDP (implicit messaging) can be unicast, multicast, or broadcast.... err.... is that right? How can an implicit message (which I thinks means no explicit address) do a unicast message (which I presume has a specific device IP address)?

    That is what is says in the ODVA white paper http://www.odva.org/Portals/0/Library/Publications_Numbered/PUB00035R0_Infrastructure_Guide.pdf on page 5-62, and I quote:
    "Traffic generated during time-critical data exchange is called implicit messaging and consists mostly of User Datagram Protocol/Internet Protocol (UDP/IP) unicast and multicast packets".

    It does seem like the more you get into it the more head scratching that occurs. It may be that the unicast UDP message is the acknowledgement from the PLC with the output status updates. It differs from the TCP unicast message in that it is a response to a multicast message. I'm thinking the UDP unicast message must have an IP address of the Ethernet I/O adaptor and perhaps even the connection ID...

    Fortunately this doesn't have a lot of application importance that I can see, just nice to know...
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    Here is the specific link to my summary about Ethernet I/P

    http://hennulat.wordpress.com/2010/06/28/ethernetip-multicasting-based-on-cip/

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