Design Issues and Solutions DMX- tell me why this is wrong.

Crisp image

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Jun 18, 2017
Location
Eastern Victoria Australia
Hi All,
Exciting times here in Oz where we are getting back in to our venue. This week I am on the maintenance crew (cleaning, testing and taging electrical stuff and sorting out cabling issues.) I have come across a cabling point that is DMX.
Now let me start by saying I know how a DMX run should look and I know what I am going to describe is not conventional but I am trying to understand why it is not right.
If you are to take a DMX run it comes out of the console to fixture 1 then 2 then 3 so on and so on and then terminates with a terminator (best practice).
At every fixture between the in and out is tap off the traces that goes to the control of the fixture. If I was to draw it it would look like this _________I_________I_________I...... where the vertical lines are the fixture.
My cabling issue is at a female socket in the wall there are 3 cables coming in to the back of the socket. One cable is labelled From SWroom another is labeled FOH Bar and the 3rd cable goes out to LX3. From the socket a cable is connected to 2X12 2.4K dimmers.
Essentially it is doing the same thing as having an in and out at a fixture but hard wired behind the socket.
Lately (last year) the DMX fixtures on LX3 are starting to play up (This cable has been like this for 6 or more years I am told)
I am going to investigate it further tomorrow and figure out where exactly cables are coming from and going to so I have abetter understanding of our venue and I know that I really should separate the cables and put an opto splitter in there but I don't know if we have a spare one.

Anyway I will be interested in your responses bearing in mind I know what is right but I want to know why we do it that way in a daisy chain (with taps off the connections to the control in the fixture)

I am off to bed now because here in OZ it is time for me to go night night. I will see your answers in the morning.

Regards

Geoff
 

StradivariusBone

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My building is wired like that on the input side of the dimmer rack. It has three spots where you could theoretically plug in a console, but they assumed you wouldn't plug in more than one at a time. I don't see how that would work unless there's some kinda optosplitter built into the building wiring. DMX doesn't work well if you Y it off like that.
 
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Amiers

Renting to Corporate One Fixture at a Time.
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Gonna need pictures to visualize this.
 
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microstar

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Lawton, OK
If I visualize your socket connection correctly, it is like a 3-fer with the SWroom being the input and FOH bar, LX3, and Dimmers being the 3 output legs. Therefore you are creating not just a DMX "Y" (if you unplugged the Dimmer cable from the socket) but a DMX 3-fer. All bad in terms of DMX distribution. Your comparison that this is essentially the same as the DMX "Y" or stub within a fixture is not valid because the legs of the stub or "Y" WITHIN the fixture is only inches long where the legs of your "Y"/3-fer are probably many feet long which has much more of an effect on the DMX signal. Just a thought.
 

derekleffew

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And a darn good, and correct, thought it is, @m'star. While everyone says, "Never Y", the dirty fact is, every pass-thru does. A "T" is just a "Y" that has gone through conversion therapy.

I could have sworn that that Fleenor guy wrote a paper titled "Why not Y?", but it appears that's a Rane paper applying to audio, even though many of the points apply to DMX512 also.

From http://www.dfd.com/primer.html :
Splitting
Never split a DMX signal with a "wye" cable. Use a splitter device that buffers each line separately or daisy chain from device to device.

The problem with splitting the signal with a wye cable is that the signal going up one leg of the wye is reflected back down and corrupts the signal going down the other leg. The longer the legs of the wye, the worse the problem. A wye where each leg is 10 feet may work fine but may fail if the legs are increased to 100 feet. A lot has to do with the quality of the cable, the strength of the signal at the wye point, and how long it's been since you last made a sacrifice to the DMX god.

If you must split the signal, and you don't have a buffered splitter, do it as close to the source as possible, i.e. at the console. If you have an isolator available, place the wye at the output of the isolator. If you have two isolators available, place the wye at the input to the two isolators (which is like making your own splitter).
 

Rob

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On a river near Toronto
It is for jobs just like this that we created this product. I have seen many schools that have one DMX input on the wall at the back of the cafeteria, then it goes to a Stage Right DMX IN, then goes to the racks. If you run the console from the back of house, you're not using the 'Y', but if you use the SL input, it's a huge one and a very bad idea. This very short paper is another bit of reference material if you're interested.
 

Crisp image

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Eastern Victoria Australia
The odd thing is it seems like this is an install? I can't imagine someone building it this way.
Yes it is an install. However when the building was built in the 1970's there was not DMX so it was all dimmers and a 2 scene cross fade console. Since the introduction of DMX someone (or various people) have added DMX to the building and most possibly they "got it going" but it was not the right way. My challenge today is to figure out where the cable runs come from and go to and try and figure out how to fix it so we don't have any more issues. Having said that we are currently building a new PAC which should open October this year and this current building may be decommissioned.
I will try and get some photos of stuff today and also sneak a few photos of the build next door too.
Thanks for your replies so far.

Regards
Geoff
 

RonHebbard

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Location
Waterdown, ON, CA
Yes it is an install. However when the building was built in the 1970's there was not DMX so it was all dimmers and a 2 scene cross fade console. Since the introduction of DMX someone (or various people) have added DMX to the building and most possibly they "got it going" but it was not the right way. My challenge today is to figure out where the cable runs come from and go to and try and figure out how to fix it so we don't have any more issues. Having said that we are currently building a new PAC which should open October this year and this current building may be decommissioned.
I will try and get some photos of stuff today and also sneak a few photos of the build next door too.
Thanks for your replies so far.

Regards
Geoff
@Crisp image An inductive tone tracer would be your friend.
Think POSITIVE.
Test NEGATIVE.
Toodleoo!
Ron Hebbard
 

RonHebbard

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Location
Waterdown, ON, CA
@Crisp image An inductive tone tracer would be your friend.
Think POSITIVE.
Test NEGATIVE.
Toodleoo!
Ron Hebbard
@Crisp image If / when you're using an inductive toner to trace a shielded cable, begin with the shield rather than one, or more, of the shielded conductors. Toning the shield will allow you to quickly determine if you're in the correct room or rack.
When you think you've found the correct cable, change to continuity mode to test / confirm each and every individual conductor.

If / when you have an associate using a second toner, agree to use two distinct tones prior to stressing each others' sanity.
Think POSITIVE.
Test NEGATIVE.
Toodleoo!
Ron Hebbard
 

Crisp image

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Joined
Jun 18, 2017
Location
Eastern Victoria Australia
@Crisp image If / when you're using an inductive toner to trace a shielded cable, begin with the shield rather than one, or more, of the shielded conductors. Toning the shield will allow you to quickly determine if you're in the correct room or rack.
When you think you've found the correct cable, change to continuity mode to test / confirm each and every individual conductor.

If / when you have an associate using a second toner, agree to use two distinct tones prior to stressing each others' sanity.
Think POSITIVE.
Test NEGATIVE.
Toodleoo!
Ron Hebbard
Hi Ron,
I was using my inductive toner today and found a few things out. It worked a treat.
So what I did today was take the leg that went to LX3 off (the one in front here) and put a make socket on it.
20210110_160557.jpg
I then fed this socket form the output of the dimmer DMX.
20210110_094750_resized.jpg

What else can you see here? Well the big flap of tape is the 2nd pr of the cable that comes from the console via this spot and continues across to the switch room. Then it returns via the 1st pr to the socket. From the socket another cable goes somewhere to FOH bar that I am yet to find. I ran out of time to do any more today as we had bigger fish to fry.

So that is where I am at the moment. Thanks everyone for all your comments which I read with great interest.

Regards
Geoff
 

Crisp image

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Joined
Jun 18, 2017
Location
Eastern Victoria Australia
Oh and as promised some photos of our new build.
20210110_173501_resized.jpg

This is form the front and you can see the lobby slab poured. The tree hides the 28m High fly tower structure.
20210110_173547_resized.jpg

This one you can see the raked seating structure, the cat walks and the proarch.
20210110_173604_resized.jpg

Another view of cat walks, seating and the Bio box floor.
20210110_173645_resized.jpg

And finally the back end of it all. On the right where the sign is is the loading dock. we will be able to get a semi down there easily. On the far right is the current building we are working out of. Not sure what is going to become of it when we get the new place finished. The story changes each week.
Here is an artist impression of the new place
and this is a time lapse of the Fly tower going up.
Enjoy.
 

Amiers

Renting to Corporate One Fixture at a Time.
Joined
May 28, 2009
Location
Phoenix, Az
unsplit the wires and get yourself something like this.


Leave one side dead then patch cable on the front between the two legs.

you could even get away with just mounted XLR plugs and no iso and go raw. But unsplit the wires for sure and make a proper chain.

IF you wanted to do it right and leave it proper for years to come. This all the way.


It leaves head room for anything and everything you will ever need.
 

StradivariusBone

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Location
Space Coast, FL
Great shots of the new constuction! I've repurposed old wiring for DMX before. Depending on the type of cable, it may or may not be Kosher, but as Dr. DMX says himself it'll run on barbed wire if you need it to. In my experience making sure you're following best practices on termination and isolation will make up for bad cable in a pinch. I don't know a tech who hasn't at some point in their life run a leg of DMX up a mic snake 😂
 
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