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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: Patrick Campbell on July 03, 2018, 08:35:27 am

Title: Y cord at DMX board
Post by: Patrick Campbell on July 03, 2018, 08:35:27 am
Has anyone used a y cord to split each side of stage for communication ?

Then us e a terminator on each sides end ??

A DMX Y cord ?

Thanks,

Patrick
Title: Re: Y cord at DMX board
Post by: TJ (Tom) Cornish on July 03, 2018, 08:59:02 am
Has anyone used a y cord to split each side of stage for communication ?

Then us e a terminator on each sides end ??

A DMX Y cord ?

Thanks,

Patrick
I'm sure someone has used that, but it's the wrong idea.  You need an optically isolated DMX splitter like this:

https://www.sweetwater.com/store/detail/DataStream4--chauvet-dj-data-stream-4-4-port-dmx-splitter
Title: Re: Y cord at DMX board
Post by: Erik Jerde on July 03, 2018, 10:10:07 am
Has anyone used a y cord to split each side of stage for communication ?

Then us e a terminator on each sides end ??

A DMX Y cord ?

Thanks,

Patrick

It might work.  It also might produce intermittent gremlins.  I wouldn't risk anything important to it.  Get a proper DMX splitter.
Title: Re: Y cord at DMX board
Post by: Jerome Malsack on July 03, 2018, 10:46:00 am
The optical isolation will also help in trouble shooting when a segment does not work correctly.  It prevents problems on the other segments.
Title: Re: Y cord at DMX board
Post by: Dave Garoutte on July 03, 2018, 12:23:46 pm
You can also split with a wireless Dfi type device.
If I'm doing two lighting tripods, I'll often put a receiver on each and I won't have to run cables to them.
Title: Re: Y cord at DMX board
Post by: Steve Garris on July 03, 2018, 01:38:41 pm
You can also split with a wireless Dfi type device.
If I'm doing two lighting tripods, I'll often put a receiver on each and I won't have to run cables to them.

This is what I use. Those Donner units work flawlessly.
Title: Re: Y cord at DMX board
Post by: duane massey on July 03, 2018, 10:19:05 pm
Quick answer: no, don't do it.
Title: Re: Y cord at DMX board
Post by: Lyle Williams on July 04, 2018, 05:25:33 pm
Don't plan on doing it.  If for some reason outside your control your doing-things-properly plan is ruined and the show is going to fail, sure, go for it.

Title: Re: Y cord at DMX board
Post by: Mark Cadwallader on July 04, 2018, 06:07:33 pm
With a data (DMX) cable that's long enough to run around the upstage side of the stage, you can link the SL and SR upstage lights and not need to split the signal. (Assuming you don't otherwise link too many lights on a single run. 32 instruments is the max per DMX protocol, IIRC.)

Optical splitters can be had for about 100 USD.
Title: Re: Y cord at DMX board
Post by: Steve M Smith on July 05, 2018, 03:04:30 am
I don't see why a Y lead would be a problem.  The in and out sockets are parallel connected. Using a Y cable isn't any different.


Steve.
Title: Re: Y cord at DMX board
Post by: TJ (Tom) Cornish on July 05, 2018, 07:25:13 am
I don't see why a Y lead would be a problem.  The in and out sockets are parallel connected. Using a Y cable isn't any different.


Steve.
The reason DMX signal requires termination is due to the reflective nature of transmission lines.  Every time there is an impedance change (like going from the 120 ohm cable impedance to infinite ohms for an un-terminated cable end), the signal splits and while most of it still continues forward, some of it reflects back the other direction.  The more times this happens, the more the signal is degraded.  This has MUCH greater impact than simple cable distance, as itís not signal amplitude loss thatís the problem, but echo.

Fixtures in a line do indeed degrade the signal to some degree, which is why there is a fixture limit for a DMX branch, but itís not a big deal for the number of fixtures in spec if you use a terminator, which greatly reduces the impedance difference at the end of the line; thereby reducing the end reflection significantly.

If you put the DMX source in the middle of your chain using a Y-cable, now you have signal traveling in two directions, with two endpoints for the signal to reflect back from.  This gets noisy very quickly.  If both branches are terminated and the number of fixtures is modest it may indeed work, but itís going to be worse than if all fixtures are in a line in one branch in pretty much every case.

Optical DMX splitters are SO cheap.  The OP needs to get one and put it in an on-stage rack.  Mine lives in my digital stage box rack, and the DMX line is run with the audio snake (digital in my case).  Itís very convenient to split the signal from this point - upstage/downstage, left/right, etc.  Anything less than this is asking for trouble with a rig of 20+ fixtures. 
Title: Re: Y cord at DMX board
Post by: John Fruits on July 05, 2018, 07:31:45 am
From Dr. DMX himself,
http://www.dfd.com/primer.html
Scroll down to DMX tips.
Title: Re: Y cord at DMX board
Post by: Steve M Smith on July 05, 2018, 07:43:47 am
The reason DMX signal requires termination is due to the reflective nature of transmission lines.  Every time there is an impedance change (like going from the 120 ohm cable impedance to infinite ohms for an un-terminated cable end), the signal splits and while most of it still continues forward, some of it reflects back the other direction.  The more times this happens, the more the signal is degraded.  This has MUCH greater impact than simple cable distance, as itís not signal amplitude loss thatís the problem, but echo.

So would a Y cable with a terminator on each branch be bad?

The only time I have used a Y connection was on a light with an intermittent connection on one of the connectors.  All this did though was duplicate the DMX out rather than send it out to two branches.

I'm amazed at what we have got away with though.  I have a 45 metre length of CAT 5 cable with an XLR at each end which we use for one event per year.  We have a wireless connection now though - so I don't ever need to coil that cable again!


Steve.
Title: Re: Y cord at DMX board
Post by: TJ (Tom) Cornish on July 05, 2018, 09:01:32 am
So would a Y cable with a terminator on each branch be bad?
Do the DMX receiver chips have some ability to deal with reflected noise?  Yes.  Is it wrong?  Yes.  Is it better than two unterminated branches?  Yes.  Is the lighting for your show worth a $120 optical splitter to make sure it works?  Yes.
The only time I have used a Y connection was on a light with an intermittent connection on one of the connectors.  All this did though was duplicate the DMX out rather than send it out to two branches.
I don't understand your last sentence.  Only an active device like an optical splitter can "duplicate the DMX out".  Any other scenario does exactly "send it out to two branches".
I'm amazed at what we have got away with though.  I have a 45 metre length of CAT 5 cable with an XLR at each end which we use for one event per year.  We have a wireless connection now though - so I don't ever need to coil that cable again!


Steve.
Cat 5 cable works very well for DMX and AES digital connections.  I'm not at all surprised it works at 45 meters/150 feet.  Many newer light fixtures actually have RJ45 ins and outs for exactly this purpose.
Title: Re: Y cord at DMX board
Post by: Steve M Smith on July 05, 2018, 09:07:40 am
I don't understand your last sentence.  Only an active device like an optical splitter can "duplicate the DMX out".

I meant duplicate the physical connector rather than the output signal.

DMX out connector was intermittent so I ran into a Y adaptor with one branch going into the light and the other continuing to the next light.

(http://stevesmithphoto.webs.com/DMX.jpg)


Steve.
Title: Re: Y cord at DMX board
Post by: TJ (Tom) Cornish on July 05, 2018, 09:55:11 am
I meant duplicate the physical connector rather than the output signal.

DMX out connector was intermittent so I ran into a Y adaptor with one branch going into the light and the other continuing to the next light.

(http://stevesmithphoto.webs.com/DMX.jpg)


Steve.
Thanks for the picture - that clarifies things. 

In the grand scheme of crimes against humanity this is pretty minor, but technically you did create two branches - one of which is just one fixture long, but it's still an unterminated branch that causes signal reflections.  Once again it's less about the length of the run and more about the number of impedance changes that cause signal degradation.

Fun fact - the transmission lines course I took in college almost killed me, though I did pass the class.  Antennas are hard. High frequency is hard.
Title: Re: Y cord at DMX board
Post by: Dave Garoutte on July 05, 2018, 12:30:07 pm
I meant duplicate the physical connector rather than the output signal.

DMX out connector was intermittent so I ran into a Y adaptor with one branch going into the light and the other continuing to the next light.

(http://stevesmithphoto.webs.com/DMX.jpg)


Steve.

Put the dead output fixture at the end of the chain.
Title: Re: Y cord at DMX board
Post by: duane massey on July 05, 2018, 05:42:05 pm
I have installed over 50 of these over the past 10 years with "zero" failures:
http://www.usledsupply.com/shop/dmx-splitter-1-8.html
Title: Re: Y cord at DMX board
Post by: Steve M Smith on July 06, 2018, 03:44:48 am
Put the dead output fixture at the end of the chain.

Would have been a great idea if I knew which one it was before they were all wired and in place!


Steve.
Title: Re: Y cord at DMX board
Post by: Dave Garoutte on July 06, 2018, 12:16:27 pm
Would have been a great idea if I knew which one it was before they were all wired and in place!


Steve.

You could have skipped the bad fixture and then run a cable back from the end so it was last in the chain, even if it wasn't physically last in line.
Title: Re: Y cord at DMX board
Post by: Lyle Williams on July 06, 2018, 05:02:35 pm
The data rates in DMX are very low.  The physical length of bits on a wire are huge.

Sadly people run unterminated DMX all the time.  That is so much worse than a Y-split in the middle of a line terminated at both ends.  Put a cro on your DMX and have a look at those waveforms.

Dodgey is dodgey.  Don't plan on being dodgey.  Plan on being solid and robust, and if some technical gaffer tape needs to be applied, you may be ok.  Don't plan on operating a huge beachball of technical gaffer-tape on gaffer-tape on gaffer-tape.

But when crisis strikes (opto-splitter left in last town?) don't use learned helplessness masquerading as professionalism to fail to produce a solution that a noob would have got working.

A y-split is 1% less dodgey than letting the show fail.

Title: Re: Y cord at DMX board
Post by: TJ (Tom) Cornish on July 06, 2018, 06:27:27 pm
The data rates in DMX are very low.  The physical length of bits on a wire are huge.

Sadly people run unterminated DMX all the time.  That is so much worse than a Y-split in the middle of a line terminated at both ends.  Put a cro on your DMX and have a look at those waveforms.

Dodgey is dodgey.  Don't plan on being dodgey.  Plan on being solid and robust, and if some technical gaffer tape needs to be applied, you may be ok.  Don't plan on operating a huge beachball of technical gaffer-tape on gaffer-tape on gaffer-tape.

But when crisis strikes (opto-splitter left in last town?) don't use learned helplessness masquerading as professionalism to fail to produce a solution that a noob would have got working.

A y-split is 1% less dodgey than letting the show fail.
A Y-cable with two unterminated ends is significantly worse than an unterminated single-ended transmission.

Not exactly sure what your point is - hopefully a successful show is worth a $100 opto-splitter to do it right the first time.
Title: Re: Y cord at DMX board
Post by: Lyle Williams on July 07, 2018, 04:28:44 pm
Y-in-the-middle to two unterminated ends is worse.

My point was that "linear unterminated" was worse than "y-in-the-middle but terminated at bith ends".

Actually, that wasn't my key point.  My key point was don't plan on being dodgey, but understand that this whole endeavour is one big improv job.  The show must go on, and this issue isn't safety-related.
Title: Re: Y cord at DMX board
Post by: Patrick Campbell on July 16, 2018, 12:37:39 pm
Ordered all new DMX cables and this along with a couple terminators so I will have four "Stations" all terminated at the end

https://www.ebay.com/itm/ENTTEC-D-Split-Optical-Isolator-Splitter-for-DMX512-Distribution-3-5-Pin/122688083323?epid=2035093135&hash=item1c90c7957b%3Ag%3AL8gAAOSwgKpZrupj&_sacat=0&_nkw=Enttec+D-Split+4+port+isolated+DMX+SPLITTER+DMXIS+3+pin&_from=R40&rt=nc&_trksid=m570.l1313&LH_TitleDesc=0



Title: Re: Y cord at DMX board
Post by: Patrick Campbell on July 23, 2018, 10:25:33 am
With the new splitter and all DMX cables and a terminator at each "station"

The 4 way splitter/booster goes to four stations  Upstage Left/Upstage Right/ Downstage Right/Downstage Left

Each station has four slim par 64's and mover and then a terminator

Having no trouble at and that you everyone for the advice

Thanks again

Patrick
Title: Re: Y cord at DMX board
Post by: Debbie Dunkley on July 23, 2018, 10:40:41 am
With the new splitter and all DMX cables and a terminator at each "station"

The 4 way splitter/booster goes to four stations  Upstage Left/Upstage Right/ Downstage Right/Downstage Left

Each station has four slim par 64's and mover and then a terminator

Having no trouble at and that you everyone for the advice

Thanks again

Patrick

I went to a DMX splitter a few years ago and it was a great move. The only lights I DMX daisy chain are the 4 on each lighting tree using 2ft-3ft cords. However each tripod has its own feed  and so does any other lighting I use on stage. I also only use good quality DMX cable and named brand XLR connectors. 
I like the fact that if a fixture or cable goes down on one branch of the system, nothing else is affected. This has saved me a couple of times.
Recently, as I posted here, I had some flickering occur on one or two of the lights on the 4 par bar whenever I had the lights too close to TV's -something I'd never heard of before - but it seemed to happen in 1 particular local venue and happened each time we played there.
I upgraded my cabling on the 2 front lighting sets and next visit to that venue -VOILA! No more issues.
So I think with lighting, a lot like sound, the system is dependent on its weakest link - the difference being that DMX will be far more obvious in letting you know there is something wrong.
Title: Re: Y cord at DMX board
Post by: Steve Garris on July 23, 2018, 01:35:59 pm
I went to a DMX splitter a few years ago and it was a great move. The only lights I DMX daisy chain are the 4 on each lighting tree using 2ft-3ft cords. However each tripod has its own feed  and so does any other lighting I use on stage. I also only use good quality DMX cable and named brand XLR connectors. 
I like the fact that if a fixture or cable goes down on one branch of the system, nothing else is affected. This has saved me a couple of times.
Recently, as I posted here, I had some flickering occur on one or two of the lights on the 4 par bar whenever I had the lights too close to TV's -something I'd never heard of before - but it seemed to happen in 1 particular local venue and happened each time we played there.
I upgraded my cabling on the 2 front lighting sets and next visit to that venue -VOILA! No more issues.
So I think with lighting, a lot like sound, the system is dependent on its weakest link - the difference being that DMX will be far more obvious in letting you know there is something wrong.

Just curious, do you buy those DMX cables or make them? If purchased, where?
Title: Re: Y cord at DMX board
Post by: Debbie Dunkley on July 23, 2018, 02:05:29 pm
Just curious, do you buy those DMX cables or make them? If purchased, where?
I made all the upgraded ones Steve. It seems the ones I purchased pre-made are the ones that let me down - I dont think they were great connectors. I did try some pre-made ones by CBGS first which work fine but in the end I made my own and I keep the CBGS as spares.
I already had the cable from way back when I first stated using DMX but never got around to making the cables myself so the upgrade was easy for me as I already had everything - I just needed to put the soldering time in. I don't remember spending a lot on the cable but I used all Switchcraft and Neutrik connectors.
Title: Re: Y cord at DMX board
Post by: Patrick Campbell on July 23, 2018, 02:15:42 pm
ANY IDEA WHERE TO FIND  18 INCH DMX CABLES ?
Title: Re: Y cord at DMX board
Post by: Jeff Lelko on July 23, 2018, 04:02:20 pm
ANY IDEA WHERE TO FIND  18 INCH DMX CABLES ?

Why not make one?
Title: Re: Y cord at DMX board
Post by: John Fruits on July 23, 2018, 07:53:34 pm
ANY IDEA WHERE TO FIND  18 INCH DMX CABLES ?

Checking around Blizzard Lighting has a 3 pin one for $ 5.99. HERE (https://www.blizzardpro.com/collections/cables/products/dmx-cool-cables-1-5ft-3-pin)
Audiopile has them, 3 pin for $ 17.05 and 5 pin for $ 23.75. HERE (https://www.blizzardpro.com/collections/cables/products/dmx-cool-cables-1-5ft-3-pin)
You could likely also get them from CBI and Lex Products or the other usual companies. 
I do have to wonder about the quality of the Blizzard connectors, their price seems too cheap.