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RS-485 Question (Troubleshooting)

Discussion in 'Electronic Design' started by mpm, Jul 30, 2008.

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  1. mpm

    mpm Guest

    Friday, I have to go take another look at our Church's theatrical
    lighting controller, which died yesterday in an electrical storm. The
    dimmer packs and computer all seem to work, but the communiction
    between them is suspect. (No remote control of lights/dimmers)

    It uses a 3-wire RS-485 link (A-B and Ground?).

    Friday, I want to put a scope on the computer end and just see if it's
    transmitting anything. Been a long time since I worked on RS-485.

    Can I just scope the A or B lead to frame ground, or should I put a
    load from A or B to the Ground (3rd wire), or some other reference and
    measure that?

    In short, what is the easiest way to scope an unknown RS-485 link,
    just to see if there's data being exchanged? I don't reall care what
    the data is: I am going to assume it's correct.

    Oh, I should mention I'm also planning to swap out the LTC485 chips.
    The top was blown off one of them, which put a damper on further
    testing at that time.


  2. Sounds like DMX-512, which is a simple 250kbaud protocol.

    Ideally you'd use two channels in A-B mode and connect the ground on
    the scope ot the ground lead.

    There's a good chance that at least that one is no longer usable. ;-)

    Best regards,
    Spehro Pefhany
  3. mpm

    mpm Guest

    Thanks Mike.
    You and Spehro have the same basic approach.

    It's a Strand controller, by the way.
    48 lights.

  4. mpm

    mpm Guest

    WCNC-TV (6) Charlotte had an ugly, old Kligel system.
    (The kind you had to plug in by hand - patch cables everywhere...)

    Was still in use when they moved to their new studios. Back around
    It looked like a nightmare, starring Lily Tomlin. :)

    I don't think the Strand uses DMX, but it might.?
    I am just going to assume that if it's spitting out any data at all,
    then is OK.
    If it's not, it won't matter because it's all surface mount, and I
    don't have that much time to donate to the Church. It's a really
    nice controller, though. Strand wants a fortune for a service call.
    (Out of Atlanta, daily minimums, etc...)

    I did manage to get their fire alarm, burglar alarm, and PBX back up
    Mostly just blown fuses and an occasional reset here and there.
    (Don't tell anyone.)

    The Strand has about a 200-foot antenna, er, I mean RS-485 link.
    It's also in outside conduit - a clear sign of an afterthought if ever
    there was one...
    The Strand looks to have taken the brunt of the "non-easily-fixed"

    I'm kinda hoping the new 485 transceivers do the trick.
    If it's a burned cable, that's gonna be a bitch to pull...

  5. mpm

    mpm Guest

    Well, here's the update (after several hours working today..)

    It's a Strand model 300, which they don't make anymore. (8 years old)
    It does use DMX512. I scoped the bus and nothing coming out the XLR's
    on the back.
    Programming looks good.

    On the dimmer packs, I shotgunned all the LTC485 transceivers, but (of
    course!) that did not help.
    So, I un-did all the screws and plug-in jacks to peek at the bottom of
    the board for lifted traces, etc..
    No problems there.

    Did a continuity check on the cabeling - everything looked good there
    I was able to control the lights manually (panic mode), and also dim
    them with the analog controls.
    So, it looks like the dimmer pak might actually be repaired now - even
    though I can't test it.
    (I wish I had another DMX transmitter of any sort..!!) Do they have
    test gear for this??

    So, back to the 300 controller. Opened it up, took out the XLR card,
    and starting looking back "earlier" in the circuit.
    Low and behold, just in front of this output card, I can see the data
    every time I move a slider...!!

    So, it looks like the daughter board has a problem. It's a simple
    circuit, but too much time to trace out by hand. Also, the parts are
    surface mount, etc.. You know the drill.

    There are surface mount components labeled D1, D2, etc, and some
    marked Z1. I'm guessing these are diodes.
    The seem to meter out OK in-circuit, but that might not mean too
    much. My guess is the protection is blown on this board, and that
    what's killing the comm-link to the dimmer pak.

    Either that, or I am about to send back the wrong component to the
    repair depot. ;?

  6. JosephKK

    JosephKK Guest

    Actually after reading a few more posts, my tech skills kicked in.
    Measure DCR from A to B Also from each to ground. A to B should be
    about 120 ohms. It is a differential bus. Either to ground may be
    half that (the best case) or very high. Since it is designed to be a
    linear bus, split in method half yields log(2) of n (where n is number
    of nodes) for single faults and finds multiple faults quickly as well.

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