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DMX help needed please, wiring questions

Discussion in 'DMX, E1.31 & Networking' started by robt, Aug 10, 2014.

  1. robt

    robt Apprentice Elf

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    Hi, i've been building some lights over the past six months or so, and some of it i probably would have had one heck of a time finding help for if it weren't for these forums, and a lot of really knowledgeable people here. I almost have my lighting system worked out, but as i was wiring everything up, i ran into what i would think is the simplest kind of problem, and it's blown up into some kind of monster nightmare wiring kind of thing.

    I always thought of dmx as serial- i plug dmx to an input of a device, and then the dmx out to the in for the next device etc, daisy chained, it's always seemed serial to me. Well i won't bore the people here that know better...

    I'm using some very inexpensive led drivers made in china. They don't even come with instructions, but i had to use these because i need so many of them it's just not workable budgetwise using more expensive (and more well documented) ones.

    The controllers have solder pads, one for d-, one for d+. The next solder pads are for a different kind of control signal that's unrelated, also marked + and -. The ones next to them are labled v+ and gnd. So, there isn't a pad for dmx ground.

    I'm not sure if i need to use ground or not? I think it's in the dmx specs, so it must need to be used, can i use the pad labled GND next to v+? I know that's for power in, but i don't know where else i would connect it. I would hate to just try it and blow it up, it would take a few weeks to get more from china.

    The other question i have, like i was saying, i just found out that dmx is parallel, not serial. Well the data is serial, but it's wired up in parallel. A different version of this driver has d+ and d-, one set for in, one for out, and the signal is regenerated before it reaches the out pads. But this one only has one set of d+ and d-. Can i just hook two wires to each (including GND if that's what i need to do) and use one set as dmx in and one set as dmx out?

    I need to hook several dozen of these on the same universe, so i need to be sure i'm doing it properly, i know what dmx troubleshooting for wiring problems is like!

    Since it's only one set of pads, no signal is being regenerated, so how many units can i connect before i should start to think about using a dmx amp/regenerator? I've read that 32 is a good number, assuming the device conforms to dmx512 (input impedance, and other things that go about 12 feet over my head).

    I am using cat6 cable to get the signal from the dmx transmitter to the first unit, and more cat6 to get from one unit to the other. The first unit's cat6 won't likely be more than 50', the other units distance will vary between one and 15 feet. There will be dozens at some point, but for now only 10-12 units.

    I'm using mini-t connectors for now, if you don't know about them, maybe look into them, i think they're really great! They're designed top be used to connect batteries to other elements in R/C hobbyist equipment. Anyway, i just wondered if they'll cause any problems with the data. There are 2,3,4 and i think 5 pin versions.

    Also, if you don't know about traxxas connectors, those are also great for higher current connections (main power etc). I used to use terminal blocks or just straight solder to connect things, but suddenly it's a reliefe to be able to disconnect and reconnect things quickly.

    I know that dmx over cat6 works because of the twisted pairs, i'm using a twisted pair inside the cat6 for the other two pins, but if you think i need the ground also, does it matter if that one isn't twisted? This seems like a silly question now that i type it out, but i'm not sure. The other question i had, when i strip the outer insulation off
    the outside of the cat6, how much of the end of a twisted pair can i safely untwist to solder the connector without worrying about signal problems?

    Sorry to make a book to read! But i've been doing searches for a day or so, a lot of stuff i did find, and found out a lot of things i didn't know that i didn't know! I found semi-answers to most of these, but mostly it either went over my head (again) or wasn't complete info.

    The mini-t and traxxas connectors are something i found out about when searching, so even if i had found no answers to any of my q's at all (i did find a lot), those two were well worth all the time spent, look into them if you don't know about them.

    i did see the recent thread about not using ground and fasteddy said that it's usually ok not to use it, but with so many devices being used i still wanted to be sure. These will be used on a stage with a band, along with all sorts of gear, wireless devices for guitar, cables running everywhere, so RFI is a big question mark.

    Thanks!
     
  2. David_AVD

    David_AVD Bite my shiny metal ass!

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    Yes, DMX requires a ground. In your case it's the same potential as the V- pad.

    Yes, DMX is normally daisy chained and each device is effectively in parallel.

    You sure can.

    The limit is 32 devices assuming that are normal "1 unit load" receivers. You can get "1/8 unit load" receivers but they are more expensive, hence they are almost never used on cheaper equipment.

    DMX can go a long long way. If you use the correct wiring methods you won't run into that limit in practice.

    Normally the DMX common reference wire (ground) is assigned to a pair when used with CAT5/6 cable. That's more for convenience than the fact that's it's a twisted pair.

    A few inches of untwisted data wiring is not going to make a significant impact for a DMX signal. Even professional lighting fixtures use 3 plain wires from the connectors to the RS485 chip most of the time. See the ACL wiki for more information.

    If all of the DMX devices are sharing the same power supply, the ground for the DMX connection will already be there as the V- wire. If each device is floating (mains powered by itself) then you should be using an explicit common reference (ground) wire between them.
     
  3. OP
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    robt

    robt Apprentice Elf

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    <blockquote>I'm not sure if i need to use ground or not?</blockquote>Yes, DMX requires a ground. In your case it's the same potential as the V- pad.

    Ok, i just see that sometimes people say it's no problem without, but some think it's absolutely necessary, even the wiki page that you linked to suggests it's needed.
    <blockquote>I did see the recent thread about not using ground and fasteddy said that it's usually ok not to use it, but with so many devices being used i still wanted to be sure.</blockquote>If all of the DMX devices are sharing the same power supply, the ground for the DMX connection will already be there as the V- wire. If each device is floating (mains powered by itself) then you should be using an explicit common reference (ground) wire between them.

    That makes sense. I think i can power 8 or so lights from one power supply, so there will be several. The power supplies hook to ac and supply 12vdc to 8 controllers, each controller is wired to it's LED (so one controller per led). So there will be multiple power supplies as soon as i've built enough of the lights. In that case, i can't use a ground then?

    Thanks for all of that info, exactly what i needed to know.
     
  4. David_AVD

    David_AVD Bite my shiny metal ass!

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    DMX requires a common reference wire to work reliably.

    In the case of all devices sharing the same DC power supply output, that function is usually performed by the V- connection.

    In the case where each device has it's own supply, that V- connection doesn't span all devices, hence the need for an explicit connection.
     
  5. OP
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    robt

    robt Apprentice Elf

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    Since they're being used on a stage with all sorts of equipment, some high power, some wireless, all sorts of gear, i worry that might cause problems that i wouldn't know about until after countless hours of building, and a live show is under way. But since there are multiple power supplies, there's no option for it so that solves that, thank you!
     

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