1. New to Christmas lighting? Get started with the AusChristmasLighting 101 Manual:
    auschristmaslighting.com/wiki/AusChristmasLighting-101

Making a crossover cable

Discussion in 'Computers, Cabling & Other Miscellaneous Hardware' started by BundyRoy, Sep 1, 2014.

  1. algerdes

    algerdes Al Gerdes

    Joined:
    Nov 29, 2012
    Messages:
    132
    Likes Received:
    24
    Location:
    Lebanon, Illinois

    You are correct. There is a minimum of .5 meters to help prevent electrical bounce between Ethernet devices. I have been unable to find any such minimum for RS485/DMX. There is a formula in the Ti sheet "RS-485 Design Guide", but no specified constant minimum length. (See http://www.ti.com/lit/an/slla272b/slla272b.pdf)
     
  2. AAH

    AAH I love blinky lights :) Community Project Designer

    Joined:
    Dec 27, 2010
    Messages:
    3,237
    Likes Received:
    344
    Location:
    Eaglehawk
    Find Me On:
    There's no need to a minimum length for the prevention of bounce/reflections as the rs485/dmx line is designed to be terminated into a 120 ohm resistor which is there for that exact purpose.
     
  3. algerdes

    algerdes Al Gerdes

    Joined:
    Nov 29, 2012
    Messages:
    132
    Likes Received:
    24
    Location:
    Lebanon, Illinois
    Agreed. In practice I've had occasion to not have a terminating resistor on a DMX line, yet everything seemed to work very well. Not that I would advise taking the chance, but the show went on and no one (tech crew or audience) knew the difference. And some of those lines were quite long. (Several city blocks.) RS-485 is one heck of a spec, for sure.
     
  4. OP
    OP
    BundyRoy

    BundyRoy Dedicated Elf

    Joined:
    Apr 9, 2014
    Messages:
    1,026
    Likes Received:
    9
    Just realised, even if it was Ethernet I could have got away with the short crossover length as it goes into a 10m length of straight cat5e cable so the total length would be well over 0.5m anyway.
     

Share This Page