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'best' control network? Devicenet vs Lonworks vs CAN vs Fieldbus vs Ethernet ???

Discussion in 'General Electronics' started by perfb, Jul 23, 2004.

  1. perfb

    perfb Guest

    no doubt this is a FAQ oft beat to death, but, assuming
    one wanted to control a network of about 10 i/o modules each
    with maybe 100 i/o digital/analog points, at maybe a 100Hz
    maximum rate (e.g. an automatic industrial processing machine of some kind)
    ....

    which control network would be, ahem, for lack of a better word, 'best'?

    (i.e. in terms of price/node, support, ease of development, availability
    of second-sources, availability of developers etc.?)

    Personally, being somewhat naive and ignorant of the field,
    I would go for ethernet, for sheer availability of tools and
    peripherals. But would I be making a gross faux-pas in doing so?

    is ethernet gaining ground as a general-purpose control network
    over e.g. CAN etc.?
     
  2. Jim Stewart

    Jim Stewart Guest

    Ethernet. Development costs might be a
    little higher the first time through, but
    the bandwidth per $ and the infrastructure
    availability can't be beat.
     
  3. j.b. miller

    j.b. miller Guest

    Whatever system you look at, be sure it can handle broken wiring or loss of
    communication.Especially it this is a 'critical' application.
    30 years ago we chose to do an 'in-house' design for our remote energy
    control system.Simple,reliable,lightning
    proof,bidirectional,interlaced,tri-level,hackeproof and best of all ran on a
    true single wire.This gave us a built in backup as we leased wires from Bell
    and they were always in pairs! The system is still in use today.
    Jay
     
  4. Do you have more information regarding this system ?
     
  5. H.-J.Oertel

    H.-J.Oertel Guest

    CAN could be able to handle the required bandwidth, using the max. speed of
    1Mbit/s. You can choose between CANopen or DeviceNet as higher layer
    protocol. In both cases you can easily upgrade to an Ethernet based Data
    Link Layer, e.g. Ethernet-Powerlink or EthernetIP, without changing much
    your application level software.

    Ethernet based field busses will definitely complement CAN based if high
    bandwidth is really needed.

    Regarding you subject line:
    Ethernet ???

    DeviceNet _is_ CAN.
    --

    with best regards / mit freundlichen Grüßen

    Heinz-Jürgen Oertel
    +===================================================================
    | Heinz-Jürgen Oertel port GmbH http://www.port.de
    | mailto:eek:
    | phone +49 345 77755-0 fax +49 345 77755-20
    | Regensburger Str. 7b, D-06132 Halle/Saale, Germany
    | CAN Wiki http://www.CAN-Wiki.info
    | Newsletter: http://www.port.de/engl/company/content/abo_form.html
    +===================================================================
     
  6. Do you have a simple master (multi)slave system or a peer-to-peer
    system ? What is the distribution between the 100 digital+analog
    points ? What kind of distances are between nodes ?

    If there is only a single master polling all the stations, practically
    any system would be usable and still get very predictable latencies.
    CAN based protocols are nice if nodes transmit messages spontaneously,
    since the hardware based arbitration solves most of the problems.
    However, the CAN frame can only contain 64 usable data bits (8 bytes),
    so if there is a lot of analog points, quite a few frames would be
    required.

    However, with your requirements, less than 10 frames could be sent
    from a single module in each cycle at 1 Mbit/s. However, due to the
    arbitration method, the maximum distances become quite short,
    especially if optoisolators are used. For systems distributed over a
    large area 500 kbit/s would be more usable, but this will enable only
    3-5 frames/module/cycle.

    Paul
     

  7. Powerlink and EtherCAT (real-time Ethernet protocols) are already used
    in the field with great success. Both protocols are going to be
    standardized by standardization bodies ... which very important.

    Powerlink and EtherCAT are using Ethernet as transport media together
    with their real-time protocols. Higher protocols can be use in parallel
    (low priority) or encapsulated with EtherCAT.

    IOs are available ... but the number of vendors is still limited.

    Regards

    Armin Steinhoff

    http://www.steinhoff-automation.com
     
  8. James

    James Guest

    Ethernet will be you least expensive and most flexible medium. Any
    technology will provide lots of compensated support folks.

    Ethernet base technology is more likely to be 'truely open' in the
    fact that you do not have to pay to join or use 'open' technology from
    a consortium of vendors.

    You may want to enter these keywords into a Google search:

    "RFC real time" RFC are Request For Comment and they form the basis
    of truely open standards that are recognized for no-cost and not
    patented. Many 'open' standards are economically encumbered by vendor
    groups.

    Can does offer a very useful feature: if one of the 2 wires is cut,
    you can still communicate with remote devices. However, as the prices
    of cat 5 cables and wireless ehternet devices continue to plunge, you
    can also set up redundancy in your ethernet network, quite cost
    effectively.

    If you do not use raw ethernet, use CAN-Open, as devicenet is a
    particularly *BAD* implementation on Can (from an embedded software
    developer's point of view) that is artificially expensive, due to a
    'good ole boys club' of vendors...... There is NOTHING useful that
    Device net can do, that cannot be done on plan old CAN, with a little
    extra programming.

    James
     
  9. Kevin Kramb

    Kevin Kramb Guest

    This is similar to saying, "There is nothing useful that
    TCP/IP can do that cannot be done on plain old Ethernet,
    with a little extra programming."

    DeviceNet defines a complete 7-layer OSI networking model.
    CAN is the Data Link layer but DeviceNet goes well beyond
    that.

    Three useful things that DeviceNet adds to CAN are:
    - DeviceNet's Physical layer includes power for devices
    on the network (plus cable and connector standards),
    - A conformance testing policy and service that helps to
    ensure interoperability of devices from different vendors,
    - Market recognition and acceptance in the industrial
    networking arena.

    I'm not saying that there aren't other application
    layers for CAN that provide similar useful benefits.
    And I'm not saying that these benefits are useful for
    every application. But if these *are* useful to your
    application then it would require more than "a little
    extra programming" to build these sorts of things into
    a home-brewed application layer for CAN.

    -- Kevin
     
  10. To throw in a few more cents (of most likely biased information):

    I would not only look at the "best" technology, but also at the
    slightly bigger picture:


    1.) Do you want to use off-the-shelf components, or develop your own,
    or mix both?

    If you use ONLY off-the-shelf components, I would make sure that
    whatever you pick is supported by at least 2, better 3 big vendors and
    that their components can be exchanged and that they have "reasonable"
    pricing. Which network technology is actually used then becomes less
    relevant.

    If you want to develop some or all of the nodes yourself, you should
    pick a technology "as open" as possible. One of the reasons we focused
    on CANopen is, that it is one of the few protocols that is very
    flexible in regards to the functionality you actually implement.
    Functionality you don't need you simply don't implement. This allows
    for minimal implementations (like www.MicroCANopen.com) greatly
    reducing development time and cost, but also helping in keeping
    per-node costs down, as smaller microcontrollers can be chosen.


    2.) You never mentioned volume and price requirements

    If it is a low volume application and the per-node costs are not
    terribly important, then again the specific technology is less
    important. However, if you have any per-node pricing restraints, you
    need to start looking at how to save money.

    Here CAN and CANopen are still at the lower end of the scale. A
    digital CANopen I/O node can still be built using an 8-bit
    microcontroller. Even with all peripheral chips and glue logic needed,
    the costs of the hardware components can be below $5 (volume
    depending).

    Olaf
    Tutor at ESAcademy
    www.CANopenBook.com
     
  11. Barry S

    Barry S Guest

    which control network would be, ahem, for lack of a better word, 'best'?
    <snip>

    In addition to the good advice Olaf gave you, let me throw in another
    perspective:

    You never mention what your wiring contraints are (ie space and length
    of cable runs). Some of the "serial-style" protocols like CANopen (and
    others) are great for minimizing cable runs and providing you a much
    greater total length of your runs.

    Ethernet, on the other hand, has some limitations regarding cable runs.
    Mostly, ethernet would be run using a switch (or hub, if they still
    exist). This would affect many logistical aspects of your design.
    Namely, can you support many 'home-runs' back to the switch from your
    devices -- and are they within 200 FT or so from the switch.

    Of course, the serial world has its own set of drawbacks concerning
    logistics. Mainly, you need to be concerned with induction. Since each
    device is daisy-chained, and if they are in a high EMF and high
    electrostatic environment, you will need to look at using optoisolators.
    Otherwise, be prepared for a world of pain. Nothing like get zapped and
    having all your devices go off line while you run around like a madman
    with an oscillator / test equipment.

    Moreover, you need to be honest regarding your own skill set. You never
    mentioned what protocol you would be running over Ethernet, but I assume
    it would be TCP/IP. TCP has a wealth of tools available for diagnostics
    (ie. ping, traceroute, etc). Many of the proprietary serial-based
    protocols have very little with regard of diagnostics. It can be hard to
    find why one particular device won't resond. So, you need to
    either "know" that protocol or be prepared for some learning
    curves.

    From my own perspective, I've used many types and I've always wished
    that I had attributes of the "other type" from time to time. Like you
    hinted in your question, there is no *best*, there is only *best for
    the task at hand*.

    Overall, if you choose a serial-type protocol, go with something open.
    Olaf had some very good advice regarding finding numerous suppliers,
    ease of documentation, etc. There is nothing worse then needing to get a
    part shipped *ASAP* because your network is down, but the only supplier
    is closed because its Friday afternoon.

    -Barry
     
  12. perfb

    perfb Guest

    thanks to all for the valuable feedback!

    I notice no one brought up LON/LONWorks as a candidate, is that
    becoming an orphan technology, or just a narrow niche technology?
    Or, is it just too proprietary/unsupported to consider for
    general machine control?

    tia!
     
  13. LON was developed by Echelon Corporation way back when, and is a proprietary
    protocol that is only "open" in the sense that Echelon are quite happy to
    sell the comms chips to OEMs - at the right price (a little like Devicenet
    now I think of it ;-).

    It is designed specifically for Building Automation and is not a machine
    control network. It is also rapidly losing market share to BACNet (short
    for Building Automation Control Network) which actually is an "open"
    ASHRAE-standard network.

    I hope this helps,
    Cameron:)
     
  14. Mike V.

    Mike V. Guest

    In my case, I don't want to tear up ceilings and walls to run cables
    through, so i'm holding off til Zigbee is ready.

    Yeah, i know Zigbee is still a pipe dream, but I have other projects
    i'm working on at the moment after which I hope Zigbee will be ready
    by then.

    The best you can get right now with Zigbee are "Zigbee-ready" boards,
    since i guess they haven't figured out the stack yet.

    By the way, i am making my current projects "CAN-ready", i.e. it has
    the hardware ready, but the firmware won't have any CAN functionality
    until later product releases. It seems to be a cheaper alternative to
    something proprietary like LonWorks. Besides, many of your 8/16/32 bit
    micros (e.g. PICs, AVRs, ARM7s, ST-type of micros, Coldfires, etc.)
    come ready with a CAN interface. Throw in an 80-cent transceiver, and
    you're CAN-ready.

    -Mike
     
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