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RS485 Qustions

Discussion in 'Electronic Design' started by Anti-Spam, Mar 27, 2007.

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  1. Anti-Spam

    Anti-Spam Guest

    Not used RS485 before, so I need some advice please.

    I am designing a control system where there is one master and 80
    slaves on a RS485 system. The area I need to cover is some 200+ metres
    square. The slaves will be scattered over the location in no
    particular order. I was going to use standard twisted pair Cat5 cable.
    My questions are

    I have chosen the MAX487 chip for the driver/receivers, is this a good

    I cannot daisy-chain the wiring because of the way the conduits run
    under the roads etc. My only option is to run 4 "backbone" runs of 200
    metres and take taps off these backbones, off to the slave modules in
    a tree like fashion. Is this going to work?

    Where do I place the terminating resistor?

    Is it better to use shielded twisted pair cable or unshielded?

    If shielded is the better option, do I connect the shield to mains
    earth or to my 0 Volt supply?

    Thanking you in anticipation.

    (remove the X to reply)
  2. Paul Burke

    Paul Burke Guest

    You can get almost anything to work if it runs slow enough. I'd drive
    the 4 runs using 4 drivers, and keep the branches as short as possible.
    A lot depends on the way you want to work the system. If you need a fast
    fixed- time poll rate it's a lot harder than if you are checking every
    few minutes.
    Useful article:

    How are yoou getting power to the remote units? How much do they take?
    Is this the caravan site it sounds like?

    Paul Burke
  3. Anti-Spam

    Anti-Spam Guest

    I go pretty much go as slow as I like, refresh interval time for each
    slave, can be as much as five minutes or so.

    Can't do 4 drivers, not possible with the cabling routes available to
    Decided to power each slave individually, as mains power is available
    at each point.
    You have a good memory.
  4. mpm

    mpm Guest

    Definitely take a look at:

    I haven't worked with 485 since a "nightmare" remote camera
    installation project many years ago (pan/tilt/zoom, etc..)

    But the above link has lots and lots of ideas, hardware, and the like.
  5. James Beck

    James Beck Guest

    Is there a reason why you are going half duplex?
    With a single master arrangement there is no need for everyone to listen
    to everything.
    If you are going to use off the shelf CAT5 cables you have plenty of
    conductors and if you go full duplex you can make hubs along the
    backbone to run off to different areas. Just take advantage of the
    failsafe of the Maxim receivers and it is pretty easy to make active
    hubs with a pass through for the backbone. Then each "leg" can handle
    its' own termination and so on.

  6. Hawker

    Hawker Guest

    I'm suspecting you will need a few repeaters or drivers since you can't
    daisy chain.

    The best app note I know of for understanding RS-485 wiring is TI's
    SLLA036. Unfortunately TI does not have this anymore. I googled for it
    and found it here
    TI seems to have another document SLLA173 now. I have not read it so
    can't vouch for it. It appears to deal with High Speed issues only and
    not basics.
    also might want to look at

  7. john jardine

    john jardine Guest

    Seems you've nothing untowards there.
    I've done RS485 a few times and the "Art and science of RS-485" PDF at ...
    is a really good write up.
    In a kind, sensible world, we'd be using RS485 instead of that USB
  8. TT_Man

    TT_Man Guest

    Up to about 56K you could probably get away with no terminators and run in a
  9. Anti-Spam

    Anti-Spam Guest

    Thanks All appreciated, now onwards and upwards.
  10. jasen

    jasen Guest

    you could do a folded bus

    (view with fixed pitch font)

    T O
    R | detail 1 |
    U O |
    N | branch |~~~~~~| -----------o +
    K \=-=-=-=-=-=-=-\|DEVICE| -------. /|
    /=-=-=-\/--=-=-/|______| \/ |
    | |O(2) (1) /\ |
    O O| -------' \|
    | |O .... sub branch -----------o -
    O O| |
    | |O
    O .--\/--. detail 2
    | | | ------------------
    O |device| ------------------
    | `------'
    O ------+ +-------
    | ----+ | | +-----
    O | | | |
    .-|--. | | | |
    |[R] | last device has
    | | terminating
    `----' resistor
    | and -=-=- represent a twisted pair
    ---- and | etc in the detail diagrams
    | represent individual conductors

    power wires are not shown, but if you use 4-pair cat5 cable there's
    three unused pairs on the trunk and two on the branches that can be
    used for power.

  11. Mr. C

    Mr. C Guest

    Up to about 56K you could probably get away with no terminators and run in a
    I agree with TT_Man. The only reason to have line termination is to
    eliminate reflections that can distort the waveform. If the bit rate
    is low enough, you can show that reflections will not interfere with
    the signal significantly. A lot of our RS-485 applications ran at
    19200 and 38400 and they never needed termination. Also, without
    terminators, there is less load on the drivers which means you can
    have longer runs or more slave devices. What is your bit rate?

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