modbus rs485 4 wires problem ?

Discussion in 'General Electronics Chat' started by nqchanh, Feb 3, 2013.

  1. nqchanh

    Thread Starter New Member

    Dec 5, 2012
    18
    0
    i've got a slide about Modbus RS485 4 wire structure but there is something i haven't understood !
    here is the slide:
    [​IMG]
    hope someone can give me an explaination about "main segment" and "drops: 40 m in total (20 m for one tap link)" ! ^~^
     
  2. SPQR

    Member

    Nov 4, 2011
    379
    48
    Main segment = the main wires running between point A and B
    1000 m(eters) = the typical maximum length of the main segment (depends on data rate)
    drop(s) = a "branch" or "take-off" another "connection" off of the main segment
    I believe the "40 m(eter)" length refers to the total length of the connections taken off of the main trunk.
    So
    40 1 meter drops
    20 2 meter drops
    10 4 meter drops
    etc

    The maximum drop length for a single take-off is 20 m.

    Let's get another opinion here.
     
    nqchanh likes this.
  3. John P

    AAC Fanatic!

    Oct 14, 2008
    1,632
    224
    I would interpret it as main trunk max length 1000m, and the total length of all branches coming off it mustn't exceed 40m, nor must any single one of them exceed 20m.

    It is confusing that they refer to "drops" and "tap link" which I believe mean the same.
     
  4. SPQR

    Member

    Nov 4, 2011
    379
    48
    Excellent!
    We got an expert who's not watching the Super Bowl!:D
     
  5. nqchanh

    Thread Starter New Member

    Dec 5, 2012
    18
    0
    sorry, but could you mind giving me a presentation clearer, you can use my picture! i mean when u refer "point A & B", it's not clear to me because there is no presentation showing me about it!
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,418
    3,355
    RS-485 should be ONE long cable not to exceed 1000m with no branching.
    If you need devices along the cable you can tap on to the line directly with a short connection and no termination.
    This information is suggesting that a "drop", "tap" or "branch" off the main "trunk" must not exceed 20m. I suggest that any "drop" should not exceed 20cm.
     
    nqchanh likes this.
  7. nqchanh

    Thread Starter New Member

    Dec 5, 2012
    18
    0
    as SPQR has mentioned above, is it right that consider element T of Slave 1 for example, it has 2 wires so the totoal length of them must not exceed 40 meters ?
     
  8. MrChips

    Moderator

    Oct 2, 2009
    12,418
    3,355
    No. The statement says a single drop must not be more that 20m away.
    The total of all drops must not exceed 40m.

    When I hook up RS-485 networks, my drops do not exceed 2cm.
     
    nqchanh likes this.
  9. nqchanh

    Thread Starter New Member

    Dec 5, 2012
    18
    0
    you mean, for example considering the above wire of slave pair, the total length of wires dropping from it must not exceed 40 meters ?
     
  10. MrChips

    Moderator

    Oct 2, 2009
    12,418
    3,355
    No. When they say a "drop" they are not counting wires.
    They mean the distance of the device from the connection. But this does not matter.
    Your goal is to make the distance from the connection zero metres.
     
    nqchanh likes this.
  11. John P

    AAC Fanatic!

    Oct 14, 2008
    1,632
    224
    Branches off a balanced line such as RS-485 should be as short as possible, but this spec is telling you what you can do and still keep the system working. It's wasteful to have a fetish about making the system better than it has to be--good engineering isn't entirely about making things perfect, it also includes making things work adequately while conserving time and materials. In this case, running a branch off a main stem may be the most economical way to do the job, and if the system works with that branch in place, it's a good design.

    Re the Super Bowl, I consider it a good year if the thing comes and goes before I find out who's playing.
     
    nqchanh likes this.
  12. nqchanh

    Thread Starter New Member

    Dec 5, 2012
    18
    0
    you mean, consider slave 1, the distance from slave pair pair to it must not exceed 20 m.Then the total distance from slavepair to slave 1...n must not exceed 40 m ?And it's similar to master pair ?
     
  13. MrChips

    Moderator

    Oct 2, 2009
    12,418
    3,355
    No. A "drop" is also called a "stub".
    Keep all your drops or stubs to a minimum.
     
  14. SPQR

    Member

    Nov 4, 2011
    379
    48
    Main line
    |-----------------------------------1 km--------------------------------------|

    xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
    __________x________________x________________x_____x
    ___________________________x________________x
    ___________________________x
    _________drop A___________drop B___________drop C__drop D

    drop = "stub" "connection" "take off" and many other names
     
    nqchanh likes this.
Loading...