Design Issues and Solutions Is this cable suitable for DMX

Crisp image

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Jun 18, 2017
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Eastern Victoria Australia
Following on from my last post "should I do this with DMX cable" I have been given some cable that is shielded twisted 2pr along the lines of Belden 8723 which has an impedance of 52 ohm and about 15 ohm resistance per meter https://catalog.belden.com/techdata/EN/8723_techdata.pdf
Can this be used successfully for an install DMX run from box to stage?
I am really interested in results here as I have quite a bit of it and a little project I could use it for.
Regards
Geoff
 
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RonHebbard

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Waterdown, ON, CA
Following on from my last post "should I do this with DMX cable" I have been given some cable that is shielded twisted 2pr along the lines of Belden 8723 which has an impedance of 52 ohm and about 15 ohm resistance per meter https://catalog.belden.com/techdata/EN/8723_techdata.pdf
Can this be used successfully for an install DMX run from box to stage?
I am really interested in results here as I have quite a bit of it and a little project I could use it for.
Regards
Geoff
@Crisp image [ 15 Ohms DC resistance per Meter !!?? Say it isn't so. ] I believe I read 15 Ohms per 1,000 feet in the link you provided.
I expect I'll be crucified for posting this; even at the risk of being turned into a pillar of salt, here goes:
DMX is an amazingly robust protocol. If it's easy for you, strip and connect the ends of your cable and try it. Test it in a NON CRITICAL situation, NOT DURING A PERFORMANCE. If you have 500 or 1,000' of the cable on hand on a reel or in an Un-Reel cardboard box, fish out both ends and test the heck out of it. Observe all termination rules and run lots of data through it. If RDM is a requirement, be certain you've tested that too.
If it works flawlessly at 1,000' and your intended application is 500' or less, it's likely you're safe to proceed with your installation.
If it basically works at 1,000' but is slightly erratic AND your proposed installation is less than 300', you're likely going to get away with it.
- Perform ALL tests prior to investing time and effort on your actual installation, the neat and proper installation you're planning to do WITHOUT breaking any other rules.
- Would I risk this on an install in another country six hours away by air when my boss's corporate reputation is on the line: NO WAY!!
- Would I try this at home when donating my time to a local amateur theatre group: IN THE BLINK OF AN EYE.
- Remember: You're reading this on the internet, posted by a stranger from the opposite side of our globe; few strangers are STRANGER than this guy.
- This advice is worth every penny you're not being charged for it.
This old geezer'll crawl back in his cave and NEVER admit to posting this.
Toodleoo! ( From another of the colonies. )
Ron ( I can feel lighting bolts about to strike me as I'm typing this. ) Hebbard
 
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JD

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North Wales PA
As Ron said, DMX is incredibly robust! However, compromised DMX has a habit of working just fine until 5 minutes into the show on opening night!
I would heed Ron's caution list above. If it is not mission critical, then try it, but have a backup plan. 52 is a long way from 110
 

MNicolai

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Comparing 8723 to 9451, it's not any worse than using standard mic cables. The characteristic impedance is one strike against it and the relatively high capacitance is another. Over long distances, the electrical properties of the cable will smear the clean edges of the digital signal and will cause "true"/"false" bits of data to turn into "maybe" or "the exact opposite true/false you intended". How long the cable is, what your environment is, and how forgiving the devices connected to it will all play into whether it's passable or not for you.

Good practice for a reliable, mission-critical solution? No.
Is there a fair chance it'll work? Yeah, if you're not taking it very far.
Is it going to suck if during a ballet performance your rig goes into "Last 5 Minutes of Any Korn Concert" on you in front of an audience and then you have to pull new, better cable to prevent that from happening again? Also yeah.
 

TimMc

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Feb 15, 2017
It will work until it doesn't. If you need/want to be a Cheap Sheep (a mattress store mascot), use CAT5e. It's only about 10-20 Ohms off of the DMX spec for characteristic impedance and is much closer to the capacitance of the spec as well.

Any cable suited for AES-3 digital audio will also work with DMX.
 

Crisp image

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Jun 18, 2017
Location
Eastern Victoria Australia
oops missed the 1000ft after the 15 ohm.
But thanks for the replies. Think I will give it a miss.
 
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eadler

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Sep 17, 2010
Location
Upstate NY
Cat5. From the official and testing side, there's this: https://tsp.esta.org/tsp/working_groups/CP/docs/DMXoverCat5_P3.pdf

From the nominal side, Cat5 spec is 100 +- 15% (85 through 115; 100+-5 at 100MHz, 95 through 105).
AES3 is 110 +- 15% (93.5 through 126.5)
DMX512 portable cables per E1.27 shall be 100 through 120 ohms but 120 ohms is preferred as it is standard for RS485.
High quality cable manufacturers keep their cable close to the spec's center. There's a chart of Belden Cat6 that shows a nice impedance line of random tests that show a low point around 97 ohms and a high point around 105 ohms. As you can see from above, anything that meets Cat5 (or 5e or 6) will meet AES3 spec. Cat5 will *likely* meet DMX512 spec (although it may drift out a bit)

In short: Cat5e is cheap and readily available and will more than likely work just fine.