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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Lighting Forum => Topic started by: Kirby Yarbrough on February 27, 2017, 09:20:02 am

Title: Y cable with DMX?
Post by: Kirby Yarbrough on February 27, 2017, 09:20:02 am
Pardon my elementary question: can I use a 3-pin Y cable to split DMX signal coming from a controller?
Title: Re: Y cable with DMX?
Post by: TJ (Tom) Cornish on February 27, 2017, 09:39:59 am
Pardon my elementary question: can I use a 3-pin Y cable to split DMX signal coming from a controller?
It might work depending on length of your run and if you use terminators (in this case no terminators would be better than multiple terminators), but that's not really the right way to do it.

You want a DMX splitter/opto-isolator such as this: https://www.sweetwater.com/store/detail/DataStream4?adpos=1t1&creative=175866237996&device=c&matchtype=b&network=g&gclid=CO_V_-bAsNICFR65wAodRQIEhw
Title: Re: Y cable with DMX?
Post by: John Fruits on February 27, 2017, 10:56:41 am
Doug Fleenor says no.
http://www.dfd.com/primer.html
It's also sort of like using mic cables, it might work......until it doesn't.
Title: Re: Y cable with DMX?
Post by: Scott Hofmann on February 27, 2017, 05:24:37 pm
Pardon my elementary question: can I use a 3-pin Y cable to split DMX signal coming from a controller?

The answer is NO.
Title: Re: Y cable with DMX?
Post by: duane massey on February 27, 2017, 08:32:34 pm
Uhm, NO.
Title: Re: Y cable with DMX?
Post by: Don T. Williams on February 28, 2017, 12:07:59 am
It really doesn't work.  I've had lots of customers try it and called me in when it didn't work.  I have never heard of it working correctly!  DMX is not at all like audio.  Opto-isolator splitters are not that expensive and solve your problem.

They also work when mixing fixture types (some don't play well with others), and when the strings of fixtures get larger.  I've read that 32 fixtures is the limit, but I know of situations where a lot fewer fixtures didn't work until they were isolated/split.
Title: Re: Y cable with DMX?
Post by: Lyle Williams on February 28, 2017, 01:27:17 am
Doug Fleenor says no.
http://www.dfd.com/primer.html
It's also sort of like using mic cables, it might work......until it doesn't.

It will work better than people are suggesting, but I wouldn't plan to work that way.  If doing this keep your legs short.

For me, this is in my emergency workarounds bag of ideas.

Not the "right thing", but not nearly as dodgey as the mythology suggests.

DMX runs on RS485, and anything you can get away with on RS485 will work for DMX.
Title: Re: Y cable with DMX?
Post by: TJ (Tom) Cornish on February 28, 2017, 09:18:04 am
It will work better than people are suggesting, but I wouldn't plan to work that way.  If doing this keep your legs short.

For me, this is in my emergency workarounds bag of ideas.

Not the "right thing", but not nearly as dodgey as the mythology suggests.

DMX runs on RS485, and anything you can get away with on RS485 will work for DMX.
Agreed.  It probably will work in some situations, but having the right tool is so inexpensive, it seems silly to work very hard to avoid spending $130 to do it the right way.
Title: Re: Y cable with DMX?
Post by: duane massey on February 28, 2017, 09:19:01 pm
I have installed over 30 of these over the past 5 years, and have not had a single issue:
http://www.usledsupply.com/shop/dmx-splitter-1-8.html
Title: Re: Y cable with DMX?
Post by: Kemper Watson on March 03, 2017, 07:07:59 am
I have installed over 30 of these over the past 5 years, and have not had a single issue:
http://www.usledsupply.com/shop/dmx-splitter-1-8.html

I use this as well, going on 4 years now. At $35 it's a no-brainer
Title: Re: Y cable with DMX?
Post by: Tim McCulloch on March 06, 2017, 02:41:05 pm
Doug Fleenor says "people spend thousands of dollars flying me all around the world to tell them: use real DMX cables and don't daisy-chain more than 24 fixtures - use a DMX splitter/isolator."

So to parse Mr. DMX's statement:

NO.  A BIG, FAT NO.
Title: Re: Y cable with DMX?
Post by: Lyle Williams on March 06, 2017, 04:34:13 pm
Doug Fleenor says "people spend thousands of dollars flying me all around the world to tell them: use real DMX cables and don't daisy-chain more than 24 fixtures - use a DMX splitter/isolator."

So to parse Mr. DMX's statement:

NO.  A BIG, FAT NO.

Inside many (most?) fixtures the "DMX daisy chain" wiring is implemented as a y-split.  The two XLR connectors are wired together, and a single set of wires runs to the controller board.

When you don't wire lights to the "rules" the assumptions embodied in the rules (cable type, max distance, number of fixtures, etc) can go out the window.  Many implementations don't need anything like 400m AND 32 fixtures though.

Title: Re: Y cable with DMX?
Post by: Lyle Williams on March 07, 2017, 04:44:31 am
One also has to give consideration to when it is OK and not OK to be dodgey.

The established rules for things like DMX allow a system to be built and maintained by different people over time.  They don't need to be aware of previous anomalies and system-specific constraints.

Any sort of permanent installation should play by the rules.  The next person along shouldn't have to "discover" the Improvised Electronic Devices (IEDs) you have left behind.

If you are throwing something together for an evening and through poor planning (or a lost box of equipment) a y-split or two will save the show, go for it.  You know what you have done, you will troubleshoot it and get it working, nobody else needs to get their head around it, and in six hours it'll all be torn down and packed up never to be spoken of again.  :-)
Title: Re: Y cable with DMX?
Post by: Bob Charest on March 07, 2017, 04:17:03 pm
Inside many (most?) fixtures the "DMX daisy chain" wiring is implemented as a y-split...

Hi Lyle,

I'm certainly not the most experienced guy around, but, of the four brands of LED fixtures I've owned for our band, I've not seen just a y-split. DMW in & DMW out connected to a circuit board. We first started using DMX controlled LED lights in 2006 with Wiedamark, then some Chauvet gear, a company that was in CA that went broke, and now have Blizzard Pucks.

Is it really that common? Yikes, I wouldn't want to have to deal with that.

Best regards,
Bob Charest
Title: Re: Y cable with DMX?
Post by: Tom Bourke on March 07, 2017, 11:53:30 pm
Inside many (most?) fixtures the "DMX daisy chain" wiring is implemented as a y-split.  The two XLR connectors are wired together, and a single set of wires runs to the controller board.
Yes they are wired that way, the difference is that the "Y" inside the instrument has just a short leg to the chip that reads the DMX off the bus.  That short leg to to the chip does not create reflections that are a problem.  That is very different from putting a "Y" cable in a random spot with instruments and long cables.  In that case you end up with reflections that may or may not cause problems.
Title: Re: Y cable with DMX?
Post by: Scott Holtzman on March 08, 2017, 12:26:53 am
Yes they are wired that way, the difference is that the "Y" inside the instrument has just a short leg to the chip that reads the DMX off the bus.  That short leg to to the chip does not create reflections that are a problem.  That is very different from putting a "Y" cable in a random spot with instruments and long cables.  In that case you end up with reflections that may or may not cause problems.


It's more complicated from that.  The chip itself is a high impedance connection but the fixture has a balun in it to properly couple to the ~110 ohm bus impedance.



Title: Re: Y cable with DMX?
Post by: Lyle Williams on March 08, 2017, 08:02:50 am
Remember that wavelengths at 250kbit/s are quite big.  If your legs aren't long, reflections are not going to be significantly out of phase.
Title: Re: Y cable with DMX?
Post by: Marc Paolella on March 12, 2017, 07:50:02 pm
I use this as well, going on 4 years now. At $35 it's a no-brainer

I use this (Chauvet Data Stream) and it is a solid and wonderful product that gives you a lot of flexibility with routing DMX:

https://www.bhphotovideo.com/bnh/controller/home?O=&sku=751524&gclid=Cj0KEQiAgJTGBRDLr5_az_Ouk44BEiQAIxaA4mfYW2O7ZH9icAHg7jx3PuayGF9TCU3xTo9BggYV5B0aAmKV8P8HAQ&is=REG&ap=y&m=Y&c3api=1876%2C92051678762%2C&A=details&Q=

Worth every penny.
Title: Re: Y cable with DMX?
Post by: Janell Fleenor on March 30, 2017, 05:14:12 pm
Doug Fleenor says no.
http://www.dfd.com/primer.html
It's also sort of like using mic cables, it might work......until it doesn't.

I just checked with him, and he still says "NO!".

- Janell Fleenor
(daughter of the Doctor)
Title: Re: Y cable with DMX?
Post by: Debbie Dunkley on March 30, 2017, 07:04:33 pm
I've been using a DMX splitter for a couple of years now and it allows me to not have to daisy chain if I don't want to.

I got this one on sale which works every time - bit cheaper than some of the better known brands. I have mine rack mounted.

http://electronics.mcmelectronics.com/search?cataf=&view=list&w=dmx+splitter&x=0&y=0
Title: Re: Y cable with DMX?
Post by: Rob Spence on March 30, 2017, 07:24:57 pm
I have a splitter and it is great.
It helps with troubleshooting and lets you test each string as soon as it is wired. Saves a ton of long length cables too.


Sent from my iPad using Tapatalk HD
Title: Re: Y cable with DMX?
Post by: Lyle Williams on March 31, 2017, 05:54:40 am
I just checked with him, and he still says "NO!".

- Janell Fleenor
(daughter of the Doctor)

I did not recommend y-splits as a first course of action.

If you end up stuck in a situation, and through lack of equipment/planning/luck are in a situation where a short y-split would fit the necessary topology, I said give it a go.

If you had seen the crud (cable quality and cable topology) that works in RS485 for security and building automation applications you would have a sense of how far from perfect rs485 can be and still work.
Title: Re: Y cable with DMX?
Post by: brian maddox on March 31, 2017, 08:37:10 pm
I did not recommend y-splits as a first course of action.

If you end up stuck in a situation, and through lack of equipment/planning/luck are in a situation where a short y-split would fit the necessary topology, I said give it a go.

If you had seen the crud (cable quality and cable topology) that works in RS485 for security and building automation applications you would have a sense of how far from perfect rs485 can be and still work.

Went to work at a church that had 20 Martin 250's daisy chained together with, you guessed it, microphone cable.  When i pointed out that that was really not the way to do things they responded that it had always worked, so what's the problem?  After about a year the lighting guy decided to add some LED up lighting at the back of the stage and just continue the DaisyChain already in place.  Managed to get one to work but as soon as he tried to add more Chaos ensued.  Turns out the maximum number of fixtures he was gonna get away with using that wiring scheme was only 1 more than he needed.  Which wasn't a problem. 

Until it was.

After installing proper cabling and DMX splits everything of course worked swimmingly....  :)
Title: Re: Y cable with DMX?
Post by: Lyle Williams on April 02, 2017, 03:40:00 am
If you don't use the same cable/topology that underpins the DMX rules-of-thumb on distance and number of fixtures, those rules-of-thumb will no longer apply.

.

What is the correct way to cross a river?

Use the bridge.  I'm ok with that.

But if you can't use the bridge, don't throw your hands up in the air and say the river can't be crossed.  Get a boat or swim or think of something else.  :-)

.

Learned helplessness is when a professional  can't find solutions that a novice can.

.