Connect with us

485 multi-drop, multi-protocol advice

Discussion in 'Electronic Design' started by David Fussell, Oct 7, 2003.

Scroll to continue with content
  1. Hi,

    I'm looking at an application that requires a few different devices to
    share an RS485 connection. The problem (as I see it) is that each
    device has it's own protocol, which I would regard as something of a
    showstopper, since if it's going to work, I have to analyse each
    devices protocols to ensure that (in spite of each device being
    addressable) no inadvertent addressing of the wrong device can occur.

    I'm tempted to say it can't be done without a common protocol, but I'd
    be interested in any comments, either confirming contradicting my
    reluctance.

    Thanks!

    David
     
  2. Tilmann Reh

    Tilmann Reh Guest

    Without more details about the protocols in question, it just
    can't be said.

    Maybe several different protocol can "coexist" on the same bus,
    especially if they use different start characters and do not
    reply with NAK or the like when they receive something unexpected.
    It all depends...

    --
    Dipl.-Ing. Tilmann Reh
    Autometer GmbH Siegen - Elektronik nach Maß.
    http://www.autometer.de

    ==================================================================
    In a world without walls and fences, who needs Windows and Gates ?
    (Sun Microsystems)
     
  3. CFoley1064

    CFoley1064 Guest

    I'm looking at an application that requires a few different devices to
    That's the main problem with using RS-485. I once did an instrumentation
    project of this type (multiple intruments of two different types all using
    RS-485), and couldn't get them talking together on the same port. I ended up
    purchasing another RS-485 card to have separate ports for the two different
    types of instruments. Inelegant, but not too expensive, and it worked.

    With this type of problem, it's sometimes better to remember the value of
    engineering time. An inelegant, but workable solution is usually better than
    blowing a deadline.

    Good luck.
    Chris
     
  4. That's the main problem with using RS-485. I once did an instrumentation
    Actually it is a problem of any multi-drop network. RS-485 as such has
    nothing to do with it, except that it allows multi-drop.


    Wouter van Ooijen

    -- ------------------------------------
    http://www.voti.nl
    PICmicro chips, programmers, consulting
     
  5. Since RS-485 is a single pair, it has to be half-duplex, and only one
    device can talk at a time, so this generally means it's a single master,
    many slaves. You have your PC poll each device, and talk so that only
    it answers back to the PC.

    --
    ----------------(from OED Mini-Dictionary)-----------------
    PUNCTUATION - Apostrophe
    Incorrect uses: (i) the apostrophe must not be used with a plural
    where there is no possessive sense, as in ~tea's are served here~;
    (ii) there is no such word as ~her's, our's, their's, your's~.

    Confusions: it's = it is or it has (not 'belonging to it'); correct
    uses are ~it's here~ (= it is here); ~it's gone~ (= it has gone);
    but ~the dog wagged its tail~ (no apostrophe).
    ----------------(For the Apostrophe challenged)----------------
    From a fully deputized officer of the Apostrophe Police!

    <<Spammers use Weapons of Mass Distraction!>>

    I bought some batteries, but they weren't included,
    so I had to buy them again.
    -- Steven Wright

    FOR SALE: Nice parachute: never opened - used once.

    (Problem) Evidence of leak on right main landing gear
    (Solution) Evidence removed

    F
    o
    d
    d
    e
    r

    f
    o
    r

    s
    t
    u
    p
    i
    d
    "
    n
    o
    t

    e
    n
    o
    u
    g
    h

    i
    n
    c
    l
    d
    u
    d
    e
    d

    t
    e
    x
    t
    "
    e
    r
    r
    o
    r

    m
    s
    g
    ..
     
  6. Tim Shoppa

    Tim Shoppa Guest

    No, it's not a problem of any multi-drop network. Ethernet over thin-net
    doesn't have this problem, I2C doesn't have this problem, HPIB doesn't
    have this problem, etc. That's because the definitions of those
    interfaces specify the protocol and collision dectection/backoff mechanisms.

    The "problem" with RS-485 is that it is only an electrical interface
    standard. I put "problem" in quotes because it was never intended to
    define a protocol.

    Tim.
     
  7. Actually it is a problem of any multi-drop network. RS-485 as such has
    I can follow your reasoning but I think it makes little sense. It is
    not a problem of RS-485, it is a problem of what RS-485 is not (it is
    not a higher layer protocol).


    Wouter van Ooijen

    -- ------------------------------------
    http://www.voti.nl
    PICmicro chips, programmers, consulting
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-