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Author Topic: Y cord at DMX board  (Read 679 times)

Patrick Campbell

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Y cord at DMX board
« 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
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TJ (Tom) Cornish

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Re: Y cord at DMX board
« Reply #1 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
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Erik Jerde

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Re: Y cord at DMX board
« Reply #2 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.
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Jerome Malsack

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Re: Y cord at DMX board
« Reply #3 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.
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Dave Garoutte

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Re: Y cord at DMX board
« Reply #4 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.
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Steve Garris

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Re: Y cord at DMX board
« Reply #5 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.
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duane massey

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Re: Y cord at DMX board
« Reply #6 on: July 03, 2018, 10:19:05 pm »

Quick answer: no, don't do it.
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Duane Massey
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Lyle Williams

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Re: Y cord at DMX board
« Reply #7 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.

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Mark Cadwallader

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Re: Y cord at DMX board
« Reply #8 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.
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Steve M Smith

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Re: Y cord at DMX board
« Reply #9 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.
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TJ (Tom) Cornish

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Re: Y cord at DMX board
« Reply #10 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. 
« Last Edit: July 05, 2018, 07:27:15 am by TJ (Tom) Cornish »
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John Fruits

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Re: Y cord at DMX board
« Reply #11 on: July 05, 2018, 07:31:45 am »

From Dr. DMX himself,
http://www.dfd.com/primer.html
Scroll down to DMX tips.
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Steve M Smith

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Re: Y cord at DMX board
« Reply #12 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.
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TJ (Tom) Cornish

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Re: Y cord at DMX board
« Reply #13 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.
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Steve M Smith

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Re: Y cord at DMX board
« Reply #14 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.




Steve.
« Last Edit: July 05, 2018, 09:20:22 am by Steve M Smith »
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TJ (Tom) Cornish

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Re: Y cord at DMX board
« Reply #15 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.




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.
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Dave Garoutte

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Re: Y cord at DMX board
« Reply #16 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.




Steve.

Put the dead output fixture at the end of the chain.
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duane massey

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Re: Y cord at DMX board
« Reply #17 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
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Duane Massey
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Steve M Smith

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Re: Y cord at DMX board
« Reply #18 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.
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Dave Garoutte

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Re: Y cord at DMX board
« Reply #19 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.
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Lyle Williams

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Re: Y cord at DMX board
« Reply #20 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.

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TJ (Tom) Cornish

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Re: Y cord at DMX board
« Reply #21 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.
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Lyle Williams

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Re: Y cord at DMX board
« Reply #22 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.
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