Can or should I not do this with a dmx cable.

Discussion in 'Lighting and Electrics' started by Crisp image, Aug 10, 2019.

  1. Crisp image

    Crisp image Well-Known Member

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    I know I can but should or could I do this with a dmx cable.
    The cable in question runs from the bio box to side stage. It is a 5 core shielded cable that currently delivers dmx to the stage.
    Seeing as dmx only uses 3 cores (d- d+ gnd) will using a common ground and the other 2 cores to send a second universe to side stage be a problem. I seem to recall that the 5 pin dmx was designed to do just this but never took off.
    This will give a little more flexibility at a low price point.
    Regards
    Geoff
     
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  2. RonHebbard

    RonHebbard Well-Known Member Premium Member

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    @Crisp image Let's summon: @STEVETERRY and / or @jfleenor for a definitive answer. . .
    Toodleoo!
    Ron Hebbard
     
  3. JD

    JD Well-Known Member

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    If the cable truly has two independent twisted pairs, then yes, it would work. I do that myself. HOWEVER, I have seen a lot of DMX cable that has four wires twisted together in a "twisted quad", so to speak. That would not work.
    Example of "Twisted Quad":
    SOM540-0051.jpg
     
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  4. Lyle Williams

    Lyle Williams Active Member

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    So we are talking about maybe a 20m run?

    Everything I have experienced about RS485 says this will be easily accomplished. In the real world most RS485 just gets carried on telephone wiring.

    (Everything I read on lighting forums says the DMX gods will smite you for even asking.)
     
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  5. techieman33

    techieman33 Well-Known Member

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    It should work fine. Avolites wired up their consoles that way back in the day. You would have a "Y" cable and universe 1 and 3 would plug into one connector on the back of the console, same for universe 2 and 4. If you look closely you can see it in this picture I found.
    [​IMG]
     
  6. Crisp image

    Crisp image Well-Known Member

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    I will have a close look and decide what it is.

    Rough guess 35 to 40m if you measure the drop to the connection points
     
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  7. RickR

    RickR Well-Known Member

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    I understand the standard has-been updated to officially allow dual universes.

    There are many cautions about all the incompatible other uses for the second pair. But those are pretty rare these days.
     
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  8. JonCarter

    JonCarter Well-Known Member

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    Sure, a piece of ZIP cord and a couple of chuncks of hookup wire will probably carry your two universes--until they don't. A few hundred feet of the proper spec cable will cost FAR less than proper installation of same. Follow the proper spec and do it once and stop worrying about it.
     
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  9. Crisp image

    Crisp image Well-Known Member

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    This particular cable is already in the wall so hence my question. If I was doing a new install it would on separate cables for sure.
     
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  10. JD

    JD Well-Known Member

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    Depends on the cable. Goldmine would be if it were Cat5/Cat6 in conduit, which actually can handle 3 universes even though it is the cheapest.
    It all hinges on how the wires are paired.
     
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  11. Chris Pflieger

    Chris Pflieger Well-Known Member

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    40m, two pairs in the cable?

    No problem. If it's something like Belden 8723, then you'd be good even for max length.
     
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  12. FMEng

    FMEng Well-Known Member Fight Leukemia

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    Belden 8723 is not even close to the correct impedance for DMX, which may cause data errors. There is nothing wrong with using two pair DMX cable, and having a few inches of twisted pair un-shielded in tight shrink tubing. Maintaining uniform pair twist accomplishes most of the noise immunity.
     
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  13. JD

    JD Well-Known Member

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    Belden 8723 is 52 ohms. You want something that is 110 to 120 ohms. Twisted pairs is the other important characteristic. That's why (ironically) cheap Cat6 is great (110 ohm) although not for portable applications as most is solid conductor.
     
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  14. Chris Pflieger

    Chris Pflieger Well-Known Member

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    Looks like I got the wrong part number... it's really hard trying to remember all those numbers.
     
  15. Robert F Jarvis

    Robert F Jarvis Active Member

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    Our U1 and U2 to go onto a 5 wire cable and down the auditorium. U1 on 2 and 3 and U2 on 4, 5. Both tap 1 of course. Downstage we put 1, 2 and 3 (U1) onto a 3 pin female socket and 1, 4 and 5 onto another (U2) 3 pin female socket. If all your doing is separating the wires that's fine. If you want to 'break out' (sniff) U2 on its way to say a dimmer rack then you'll have to run 4 and 5 through an opto isolator
     
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