Highest quality DMX cables for permanent installation? (without getting ripped off)

[gabrielrey]

Hi everyone!

I'm working on a permanent architectural installation that uses DMX. The final install will use one DMX cable, to go from a standalone DMX playback unit to an LED dimmer. The units will be right next to each other, so I'm looking for a very short cable. Quality and reliability are way more important than price in this situation -- I'm willing to pay a high price if it actually buys more reliability. But I'm not sure what a reasonable ceiling on price is. I wonder if someone with a lot of experience evaluating cables could help me out?

For example, I see this 1.5 ft Leprecon cable for sale at full compass for $86:
http://www.fullcompass.com/product/255574.html

That seems like quite a lot for a 1.5 foot cable. Is it worth it? Are there options which are cheaper but would be just as high quality?

Any advice would be much appreciated. I'm open to building my own cables if that's what will yield the highest quality, but prefer to buy because I don't have a lot of time to spare, and I'm also just a beginner at soldering, so not convinced my own skills will be good enough at this point.

The system will be running all night, every day for many years. There will be some heat from the dimmer board, hopefully not too much. Ideally the cable would function 50-100 years or more without needing to be replaced. So this is a situation where if, for example, doubling the price would get me just 5% more reliability and longevity, I'd seriously consider it. Especially since we're just talking about one cable (or maybe I'd order two so that one can be on hand just in case).

Separately (but perhaps related) -- I have the option of either installing a DMX connector on the dimmer board or soldering directly to it. I'd prefer to install a connector, as I will be testing the unit and then moving it (and this also seems to keep things more flexible/easier for the future, in case part of the system needs to change at some point). In this type of situation (a permanent architectural install that needs to last for years with minimal or no maintenance), do you see any downside to putting a connector on versus just soldering the wires directly to the board?

Thanks!

 

[JD]

If you are looking for 50-100 year reliability, then soldering is the only way to go. $86 for a 1.5 ft cable? Insane. In fixed locations where flexing is not involved, any cable that meats DMX specifications will work fine. The need for a cable to flex, and it's ability to withstand flexing (as well as mechanical abuse) is the reason some DMX cable is expensive. When you remove the need to flex, then the connectors will be what fails first.

For example, in fixed insulation audio systems, barrier strips (screw terminals) are the way to go on signal cables. This is because most connectors will oxidize at some point in time. The problem becomes worse if the connectors and never "exercised" (plugged and unplugged) which actually helps clean the oxide off.

 

[MNicolai]

You don't need an $86 cable. The wire is worth maybe a couple dollars, plus a little expense for the connectors and the time for someone to solder the ends of the cable. In this case, spending more money doesn't mean you're getting a better product. To that end, a 1.5' cable can be pretty awful in quality and still function fine because of DMX's tendency to be forgiving, especially on short distances. That's not to say you should go out and buy the crappiest cable you can find -- only that you don't need to worry about it.

As for duration -- don't expect your system to be in place for 50-100 years. Any devices you may connect to your cable are not likely to last more than 20 years (and that's being generous), and DMX as a protocol is unlikely to be around in widespread adoption for another two decades. Something better, more reliable, and more flexible will be developed.

To that end -- wire is cheap. Electrical contractors buy wire by the 1000' spool. You can replace a 1.5' length without any trouble for next to nothing -- any theatrical systems installer can probably give you a short length of DMX cable out of their scrap wire bin for dirt cheap if not free

Lastly -- a cable you install is more likely to require replacement because it's been made obsolete or because it's been physically damaged by the user or environment than it is to require replacement due to age.

 

[gabrielrey]

Thanks for the replies, that's all very helpful!

JD -- I looked at the board I'm going to use again, and it actually has screw terminal connectors, so I'll probably just use those. Maybe I'll buy a connector that I can hook up to them for testing purposes. The DMX playback unit I'm getting does have a connector though. It also has an option of a terminal block which breaks out various connections, but it looks like that's connected through a DB37 connector, so I'm not sure that actually helps eliminate connections which might oxidize over time. I'll look into that -- sounds like eliminating connectors is going to be important in this case. Would you recommend opening up the back of the unit and soldering directly to whatever the connectors are connected to? Or better to have someone plug/unplug the connections every once in a while (how often would be necessary)?

Any recommendations as to what kind of solder to use for long-term durability (especially in a situation where there might be some heat)?

 

[gabrielrey]

One more quick followup -- why would screw terminals actually be much better than connectors in terms of oxidization? Wouldn't you have the same problem with oxidization? Is it just that it makes it easier to just strip a little more wire and reconnect (versus having to solder a new connector)?

 

[Les]

What kind of heat are we talking about?

I'm going to echo MNicolai and say that your cable will likely be the most resilient part of the system (especially for such a short run that is unlikely to get trampled on), and will by nature be more reliable than any component connected to it (given you buy at least a decent quality cable).

I guess the most popular option would be Belden cable with Neutrik connectors, but really any good quality, shielded DMX cable you get from a theatrical supply company will work fine. DMX cables are unlikely to just all-out fail unless you go with the lowest quality you can find.

 

[FMEng]

JD is usually right. This is one of those rare times when I disagree. Some connector types are more prone to issues than others. Quality XLRs from Neutrik or Switchcraft* are insanely reliable, partly because they choose the plating material very carefully, because the contacts have a fairly large mating surface area compared to other connector types, and the strain relief is effective. In facilities with hundreds, maybe thousands, of connections, I can count on one hand the number of times I have had an XLR get flaky in a permanently installed system.

The problem with screw terminals is that most people don't know how to properly prepare the connections. Like soldering, it takes good tools, materials, and a little skill to do it right. What I often see is wires wrapped around a screw with frayed shield braid flying around. Not reliable in the least.

Go buy a $25 Hosa and it will serve you just fine. If reliability concerns you, have a spare on the shelf and you've still saved some money. Building cables from scratch can yield great results, but it takes some training and practice to do it meticulously.

*I stopped buying Switchcraft, so I don't know if their quality is still like the old days.

 

[techieman33]

As long as you stay away from ebay cables you should be fine. I agree with MNicolai and Les, the cable is the last thing you have to worry about. It's probably the least likely to fail of anything in the system. I would be most worried about the dimmer pack. Most of the "LED dimmer packs" are cheap dj grade stuff and prone to failure.

 

[n1ist]

I'd be more concerned about electrolyic capacitors drying out and lifetime of EPROM/EEPROM than cable type if you are looking for that kind of lifetime. Unless you go with custom aerospace-grade designs, it would make more sense to design for ease of service, and more likely, ease of upgrade...
/mike

 

[JD]

One more quick followup -- why would screw terminals actually be much better than connectors in terms of oxidization? Wouldn't you have the same problem with oxidization? Is it just that it makes it easier to just strip a little more wire and reconnect (versus having to solder a new connector)?

When connectors mate, the connection is made by a spring element that has a small surface area. Over the years, all the other areas oxidize, then if the connector moves slightly, the contact ends up on the oxidized area and connection is lost. With screw terminals, there is a high level of pressure and a larger contact area so they usually will not be able to lose contact. The best, of course, is solder.

That said, what FMEng said above is true, that better quality XLRs are darn good and that if done wrong, screw terminals can be a problem. What others have said is also true, it's hard to conceive of the electronics lasting that long! And yet, I am sitting about 100 feet from a 36Kw dimmer system that has been on 24/7 for the last 24 years! (Of course I have probably jinxed it now )

 

[Zel]

Just throwing this out there but since we want to save cost and DMX only uses two wire plus shield why not AES/EBU (digital audio) cable? it comes in at 110 ohms as opposed to DMX at 120 ohms but your rig will never know the difference.

 

[JD]

Just throwing this out there but since we want to save cost and DMX only uses two wire plus shield why not AES/EBU (digital audio) cable? it comes in at 110 ohms as opposed to DMX at 120 ohms but your rig will never know the difference.

Yes, that would work with no problem, although you can't call it DMX without the other two (unused) conductors! Go figure!
In practice, for large runs through conduit in a fixed install, Cat5 fits the bill and is dirt cheap. ETC sells the connector plates that take it from punch-down Cat5 to 5 pin XLR.
(Not to be confused with Ethernet converted from/to DMX)

 

[RickR]

And yet, I am sitting about 100 feet from a 36Kw dimmer system that has been on 24/7 for the last 24 years! (Of course I have probably jinxed it now )

The jinx should stay away. 24 years is along way from 50-100!

 

[epimetheus]

If I'm specing DMX install cable, especially if it is going from terminal block to terminal block, you can't beat Belden 9841 (or 9842 if you just have to have those extra 2 wires). This is an industrial grade cable designed for RS485, the underlying signalling technology that DMX uses. If you're going to connectorize it though, 9841/9842 is kinda thick and might not fit the strain relief of some connectors.

 

[RickR]

Those Belden cables are intended for installation where they won't get bumped, like inside walls and ceiling spaces. (I like 9729 myself.)

It may interest you to know that Cat5/6 (shielded or in conduit) is rated just as highly for DMX. I prefer stranded versions in racks. It's also a lot cheaper, not that it matters for 1.5'

 

[epimetheus]

Those Belden cables are intended for installation where they won't get bumped, like inside walls and ceiling spaces. (I like 9729 myself.)

It may interest you to know that Cat5/6 (shielded or in conduit) is rated just as highly for DMX. I prefer stranded versions in racks. It's also a lot cheaper, not that it matters for 1.5'

While I agree that the Belden 9841/9842 are intended for permanent installation, there is no reason that they should only be used where they wont get bumped. 9841/9842 is about the sturdiest cable you can get that designed for RS485 (and therefore DMX). Belden 9729 is actually 100ohm cable, and for that reason I prefer not to use it for DMX. I'm was more than a little surprised when ETC recommended that 9729 be used for permanent install due to ease of termination verse 9842. I don't see a good reason when running permanent install DMX not to use a cable that is designed explicitly for the signal being used and has the exact characteristic impedance that the DMX drivers and receivers expect. There are so many things that can go wrong with a technical system, why not try to prevent as many as possible.

There, I've stated my defense of RS485 and proper 120ohm cable.

My preference for an install these days would be CAT5e or CAT6 properly terminated to patch panels and jacks, with ethernet devices and nodes converting the selected ethernet protocol to DMX. This is a much more future proof design and even allows you to physically convert a DMX signal to RJ45 and patch through your system in a pinch.

 

[JD]

120 ohm.....

Just some quick notes about transmission line impedance;
You'll never measure it with a multimeter! At high frequencies (and in the case of long distance power transmission lines, low frequencies due to size), a cable does not act like a straight conductor. Instead, it works like a series of chokes and capacitors. At the measurement frequency, the resonate circuit has a "natural" ratio of volts-to-amps where it works best. That, is the cable impedance. Now, how much does that matter to us? If you are going to use the wire at a high frequency, a lot! DXM is based on using cable in the 110 to 120 ohm range. The terminators are usually 120 ohm because it is a very common resistor value to obtain. So, what happens if you use a cable outside this range? Probably not much. The further off it is, the more attenuation of signal occurs. Will 100 ohm cable work? Most likely. Will 600 ohm work? How about 60 ohm? The answer is it may. What you will have is a much greater likelihood of some type of problem. The bigger problem occurs when you mix cables of different impedance. You start forming the electronic equivalent of echo lines. To stop this nightmare, we all need to agree on one value. For DMX, it is 120 ohm. A 60 or 600 ohm cable is NOT DMX.

 

[gabrielrey]

Thanks everyone! I'm blown away by the amount of useful information here, and how generous you all are with your time and expertise. It is a huge help.

Since I'm not talking about much length (and this will all by inside a box in an A/V room), I will go with cable that is the closest to the DMX spec that I can get. I'm inclined to go with high quality connectors versus soldering directly, given my relative lack of experience with soldering. It sounds like I will need to worry about the longevity of the dimmer board and other components more than the cable, which is what I would expect, but good to have that confirmed. I'm using very high quality stuff, so that helps. I guess this will be a good test of the longevity of the equipment.