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Author Topic: termination of drain wires in a snake when both DMX and Clear Com  (Read 1452 times)

Brian Ship

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So I do DMX and Clear Com to the front of house thru 22ga digital grade 16pair snake cable.   I normally tie shield wires together in making two individual pairs into dual pair for the first six circuits in making semi-true five pin DMX.  I do the next two pairs for two individual circuits of Clear Com, and have the final two pair as spares in the 37 pin Socapex plug I’m using to terminate the snake.   Works well enough in isolation - I have only been told of one instance of someone thinking they were hearing DMX thru the Clear Com.  Could have been from anywhere in the system however.

My question is if someone should run Clear Com thru cir. 1-6, the semi-true DMX/2Pair with tied shield/drain wires at both ends, in making individual pairs into dual pair, would this cause a ground loop on the Clear Com?   Output boxes for the Socapex plugs have a dual row of 3 & 5 pin XLR panel mount inlets/outlets to make their use in the field easier.  This on the other hand could make it easy to plug Clear Com into something that even if pins 4&5 are not used would have a duplicated shield wire terminated on both ends of the snake in going to the same pin.

Given they are as a concept the same length (or within a foot at 400'), would there be a problem or would it only be a less resistance path by way of one drain doing the work and the second pair causing the ground loop?  I

n other words, by the NEC if both cables are the same length - or as a concept the same length given a span, they are both equal in making say two six gauge wires able to carry the load of about a 3ga wire when the same length.  Concept of this resistance could be as a theory that even if say in twisting of outer conductors around inner conductors, over 400' it's still only going to be about 12" difference in length which is almost nothing.  Does this say 12" difference over 400' equal a grounding loop for Clear Com?


Or am I best cutting all the female end second pair drain wire and only doing the drain wire pairing at the male end?  In fact given the lengths are not the same - and the second pair in making DMX normally comes from the inner core of the snake cable that's in reality shorter in actual length so it should in theory be a grounding loop correct?   16pair snake cable has about 10 pair wrapped around a core of 6 pair that are shorter in actual length.  

Normally I'm dual connecting both the male and female ends because it's a 400' FOH snake which is pulled thru a pipe from dimmer beach to the control area.  Given the tugging on the snake cable often tied to instead of the wee slightly more substantial 10/5 power cable loomed in with it, such snake cables take a lot of damage - especially on the male end, and the secondary pairs in making two pair DMX are the ones towards the center of the snake.  In other words, they break first in being shorter once the outer pairs un-wind from around the inner core of pairs..  My thought is that in dual connecting the shield/drain wires on each end of the cable, it at least ensures there will be some form of shield in common for making DMX to what's otherwise individual pairs of conductors.

Not so worried about the DMX line in tying together two individual pairs of conductors to make DMX two pair, never a problem so far in doing this, heck I have even sent DMX thru a garage type power supply pull down from the ceiling reel.  DMX will travel thru the brushes on a greased plate designed to carry 120v to some trouble light cord mounted reel from the ceiling.  DMX is fairly rugged as long as opto-splittered at hte far end.

This ground loop has not been a problem so far in happening this use of a looped DMX for Clear Com but could come up some day thus my wondering.
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Ron Hebbard

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Re: termination of drain wires in a snake when both DMX and Clear Com
« Reply #1 on: June 25, 2006, 05:09:19 am »

Hi Bri'!

Long time no ...

Let me see if I'm following you correctly:

- Each of the first 6 dual pair pairs terminate in both a 3 contact and a 5 contact at both ends with the 5 contact connectors having all contacts wired and the 3 contact connectors being wired in parallel with only the first of their respective pairs; number to number, 1 to 1 through 3 to 3.
Both shields tie to both pin 1's at both ends.  
These are intended for DMX but could, in a pinch, or mistakenly, also be used for ClearCom.

- The pairs intended by design for ClearCom utilize only a single pair and have only three contact connectors on each end.

- You're not questioning any possible crosstalk between the pairs designed for DMX and the pairs designed for ClearCom but only the possibilities of crosstalk when running both DMX and ClearCom within the first six dual paired pairs.

Am I with you so far or are we already separated by our common language?

I see where you're going with the relative length differentials between the inner core pairs and the outer layer pairs.
I also understand your concerns regarding length differentials between pseudo identically length matched pairs.

Just a thought; when the NEC worries about matching the lengths of parallel conductors, I suspect they're primarily concerned with maintaining equal voltage drops across power carrying conductors and are normally thinking in terms of essentially 60 Hertz, or variable speed drive, applications I doubt they were even thinking about potential noise contamination problems between the shields of data and/or analog or digital communications pairs.

In a power application, involving multiple SCR dimmer outputs routed through lengthy runs of rigid conduit with insulated 12 gauge solid (as opposed to stranded) ground conductors pulled in within each pipe and bonded to grounding bushings at both ends thereby creating ground loops within, and in parallel with, the pipes; I've personally found the 12 gauge insulated ground wires charred and burned off at the ends after several years of service and I can, upon request, still produce several other people who've also witnessed this so I can certainly see where your thinking is going and appreciate your concerns.

If we've gotten this far, and we're still understanding each other and in agreement, then, yes, I totally agree with not only your concerns but your suggested cure as well.
Yeah, connect one of each pair of shields at both ends and the second of each pair of shields at one end only with the male end getting my vote as well.

Good chatting with you again Bri'!
Yeah, these are exactly the kinds of forward thinking, head scratching, little gems I follow these forums for.

Toodleoo!
Ron Hebbard

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Brian Ship

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Re: termination of drain wires in a snake when both DMX and Clear Com
« Reply #2 on: June 26, 2006, 12:12:17 am »

It does go back years now doesn’t it.  You still the best on electrics I trust and it’s good to have your advice, but also hope to open this up to others willing to take on this concept question.

This concern about equal lengths is indeed once inner/outer core conductors my concern in should it be that someone plugs the Clear Com into the wrong circuit.  However you might have misunderstood the tying together of pairs.  

Let’s say for a universal system, all cables are wired on a 37pin multi-cable, 1-3 pins on it representing pins 1-3 for DMX circuit one, 4-6 pins 1-3 for DMX circuit two, 7-9 pins circuit three and so on.  In other words, in doing 37 pin, the first eight pairs of cable are all in order of pins 1-3 as the repeated action.  This way it does not matter how many channels of snake is once plugged into another snake, from six pair to 16 pair, or anywhere from 6 pair once plugged into a 16 pair snake will still convey at least the six circuit, eight or six semi-true DMX and two non but XLR.  

After this and starting at pin 25, I start doing the second pair of doing true DMX as repeated thru pin 36.  Pins 25 thru 26 are circuit 1 pins 4 &5, pins 27 and 28 are 4&5 of circuit two and so on.  The only thing shared in tying together these circuits to be as if one are the shield wires that for say in making pairs #1 and #9 have both wires tied into pin #1.  In other words, in this instance the drain of the outer wrap circuit #1 is tied into another outer conductor, but from #3-6 in having inner core conductors of a shorter length, the drain wires are tied together in terminating in the same place.  

For say circuits 1thru 2, given conductors - drain wires are the same length, as a theory given Clear Com both follows the path of least resistance and the secondary pair in causing this grounding loop - even if by way of NEC for duplicate cables is that if two are the same length or at least within reason the same length, it should not matter.  On the other hand, it could given it’s more or less a data signal.  This than two part than in question, given someone plugged into circuit 1 or 2 in that both are in all ways outer core wrap and the same length for the shared drain wire, would this cause a grounding loop?  Given two drain wires even if terminated on both ends to the same pin, would they work more as per the NEC of same length conductors, or more by way of crapped up data?  Than for pairs 3-6, given one shield is a different length as a theory, would this cause a problem even if only about 12" expected over 400'?

For DMX cable circuit #5 for instance, it would have over it’s five pins, Pair #7 going to pins in the Soco, #19 thru 21.  Than for the second pair Circuit #13 by way of drain wire also going to pin #19, than pins #4 going to pin #33 and Pin #34 going to pin #5 of the DMX.

It’s sort of in order, pins 1-3 of all circuits DMX or XLR going to the first 24 pins, than the final pins 4&5 following it by way of circuit number.  Pin #37 is a general ground for the snake cable which grounds the output boxes themselves  inlet and output just in case.  That’s the snake cable itself as terminated in 37 pin Socapex which I curse every time I get one to repair due to bad design of the plug.

Believe where we beyond this are more confused is where it comes to the output rack panels or truss snake boxes.  Figure as above a plug or outlet panel mount connector that attaches to the snake.  Each individual pair or conductor than say terminates into it’s appropriate place in a five pin panel mount plug or connector attached to the box.  Good rugged boxes, the box doesn’t fail, nor does the 16 panel mount connectors, just the Socapex - constantly.  Curse Socapex and the person that chose this plug.

In any case, the 5-Pin DMX in the box is than jumpered off to a XLR 3-pin panel mount also mounted into the box.  This thus circuit’s 1-8 have both 5-pin and 3-pin outlets for them in using pins 4&5 or not.  Granted circuits 7&8 don’t have a second pair tied in with them at all but they still also in doing if necessary fake DMX, have a 5-pin outlet.  Outlets themselves in the output box are just tied together 3-pin and 5-pin boxes.

Circuit #8 is reversed sexed in being the designated Clear Com circuit as a universal thing.  This granted FOH no doubt makes use of a lot of MM/FF adaptors in making up for two circuit Clear Com adaptors.  Still 6 circuits of DMX and two for Clear Com seems to be enough for most shows beyond the large ones where larger now Wirlwind 61 pin or dual snake shows are in use.
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Ron Hebbard

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Re: termination of drain wires in a snake when both DMX and Clear Com
« Reply #3 on: June 26, 2006, 05:43:28 am »

Hi Bri';

Yep!  Got it!  Understood and still with you.

Cutting to the chase

I'd suggest doing what I normally do in these situations.

In the controlled environment of your shop, create a test scenario and carry it to an absolutely silly extreme.
For instance, in this case, plug together a thousand or fifteen hundred feet of your multipin extensions, plug a breakout box on both ends; run the multicables through some of the worst environments you can find, through a welding area right in there next to the welders, through an area where someone's roughing in cues for their next world tour right in there with 400 amp three phase distros, dimmer racks and wiggle lights and any other evil noise generators you can muster and then, for your own testing convenience, route the far end back to where you started.  Connect one or two channels of ClearCom from end to end, stir gently and listen to see what you've got.
If you get away with fifteen hundred feet, great!  You're most likely safe to send out lengths of up to five hundred feet and sleep soundly.
If fifteen hundred feet exhibits problems, take off a few extensions and test one thousand feet.
Keep scaling your test length back until things become acceptable to you.
Finally, compare your satisfactory length to the lengths you are desiring to send out the door and make your decision.
Make sure your satisfactory length still passes the test when routed past all of your ugly noise sources.

Here's an actual example of test I ran prior to sending an AC servo based automation system to Frankfurt for 'Tommy' about a decade ago.

The manufacturer of the servo drive system suggested forty feet as the practical maximum length of cable between their motor-mounted resolvers and their motor drive electronics AND this was assuming said cable would be enclosed within it's own dedicated run of metallic conduit with specified clearances between this conduit and other conduits carrying the actual drive power for the related motor and a third conduit for any DC for mechanical brakes and home position limit switches.

We had already determined from previous tests and experience that we could safely and dependably employ runs of up to one hundred feet; not only without conduit but with three or four individually jacketed runs of shielded cables casually bundled together with tastefully applied wraps of regular plastic electrical tape.
So much for individual runs of metallic conduit with spacings in excess of two feet.
Granted, the accuracy required to dance pinball machines around a stage is appreciably less than that required to choreograph assembly robots on an automobile assembly line.

When we were tasked with sending out a system for 'Tommy' in Germany, we were required to supply two hundred foot lengths of motor drive and control cables and we really wanted to be sure we could sleep nights without having to worry about making any flights from Canada to Germany on short notice.
With this in mind, we made up a test length of one thousand feet.
Amazingly, this actually turned out to work pretty darned good.
We dropped our test length back to eight hundred feet and couldn't detect any appreciable problems.
We made up our two hundred foot lengths and put them on the boat to Frankfurt.
Our two hundred foot lengths were still working just fine when the show closed months later.

Bottom line:
There's what's supposed to work and what's not.
Create your own worst case test scenarios and trust your own results.

Toodleoo!
Ron Hebbard
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Brian Ship

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Re: termination of drain wires in a snake when both DMX and Clear Com
« Reply #4 on: June 26, 2006, 10:26:17 pm »

1) I’m in a base sense lazy.
2) Having more than one or two in the shop at any one time when it’s not having just arrived and needed for the next tour in doing so, very tricky.  Never got the mid-year slow down last year and have not had other than a constant touring season chaos since the winter before last.
3) My own having time to experiment is a goal but not realistic at the moment where my dance card floats from what takes my interest in immediate need to what’s needed by a certain date to get out the door I need to construct.
4) Clear Com is evil as is all types of Data.  I would much rather be working on any number of other wiring need to do’s or experiment with than be playing around with a Clear Com system’s not yet announced theorum.

Might seem less than caring but my time at work is minimum at all hours of the day and given I normally do electrical wiring, what time I spend on data types of wiring always is last on my list of stuff to get done first.  Gee, as a E-Mail today reported, a 400A three phase AC distro on X-Show blew up (again for this brand and after the back to the factory upgrade of all of them after the last series of them blew up), or work on the despised multi-pin type cable that don’t blow up, just stops working.  Above any experimentation with the snake which would probably be wise is two out of four types of AC distro - one of which by way of how it was done is a ticking time bomb for blowing up and another in just having been back to the factory for total re-wiring and now it would seem have a new problem.  Beyond this is 6-light Cyc lights in two types that are all in need of major service and in one case total re-structuring and lots of other types of stuff to work on.  Lots of projects to do, little time to get even 3:5 XLR adaptors constructed.  The FOH snake question is on the list above the box of rusted 360 fixtures found in a trailer however.  I’m however on a four or five year backlog for projects.  Even my new assistant in getting my more normal crap jobs or seemingly below me jobs such as fixing Soco cable is back logged also.  Can't even remember the last time I had time to fix a Socapex cable.

Got the worst environments in all cases you describe to test with but no time to do so including the welding shop and a few hundred yards of parking lot or “tundra” we call the space between lighting and other building departments.  A generator, various shows hung and running etc.  I average 60 hours a week since before Christmas.

This at the moment the Clear Com question is all about the theory at the moment in prep for something to work on in the near future or as I make new FOH snakes.
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Ron Hebbard

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Re: termination of drain wires in a snake when both DMX and Clear Com
« Reply #5 on: June 27, 2006, 02:30:33 am »

Hi Bri';

Message edited June 27th/06 to protect the innocent.
Sorry Bri'!

Toodleoo!
Ron
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