The 5 pin DMX Cable Religion

[Tom R]

Thinking outside of the box?!?

 

I agree that the standard may be wrong but the fact is, it is the standard. I was not arguing for or against the types of connector (though I cant see quite what makes 3 pin more technically advanced than 5 pin :s), just saying if they both had the same itd make life a hell of a lot easier! But thats just something we cannot change I guess...

[mumbles]

so I’m confused by why a 3-pin standard was not used from the outset.

Well, as soon as someone ballses up, and plugs up the cabling wrong, and you get phantom power going down to an LX desk, whoever connected it won't be popular!, and so there is an obvious benefit from telling them, apart.

 

Just as peter T said in post number 2,

I believe the 5 pin plug was used as a way of differentiating DMX signal cable from standard microphone cable as in big installations the two are not really interchangeable.

[Andrew C]

If you use 3 pin for microphone and 5 for DMX, then cables that look similar don't get used for the wrong thing. By all means use one pair cable, but 5 pin connectors would have saved a lot of grief all around if Martin (and a few others) had stuck to the standard!

[Tomo]

The second pair was originally going to be used for future improvements to the standard.

 

RDM was originally going to use the second pair - until somebody pointed out that nearly every installation doesn't have the second pair, so a method of interleaving it with the existing DMX data was created instead.

[Modge]

I thought that half the reason martin used 3 pin connectors was so as you could, if you so pleased, use MIC cable. It's as Brian says with using MIC cable, it breaks the standard, it could cause problems, but mostly people get away with it. They're just p.o.'d when it doesn't once every blue moon (that special blue moon that rises 10 minutes before the show when the other venue is using the splitter that would other wise have been a nice way of making the signal chain a bit shorter...)

[Ynot]

Whether 5 pin is right or wrong it is the standard and I just wish people to stick to the bloody thing so you dont have to work your rig around the amount of adapters you own and seperate rings for Martin fixtures to Robe because its easier with their different pins...argh rant over!

Unfortunately, whilst the 'industry' standard is and always will be 5-pin XLR plugs & sockets using correct data quality cable, there will NEVER be a 100% adherence to it for a variety of reasons.

 

Cost - 5 pin connectors are more expensive

 

Convenience - Even some of the big boys - Martin among them - use or have used 3-pin as THEIR standard, (although a lot of the newer stuff I believe has both 3 and 5's - the Entours do, for example) and as such it would be nigh on impossible for them to change everything obernight - so they won't.

 

Also on convenience - for those less picky light jockeys out there who don't really care about the quality of the DMX signal (I'm talking discos & nightclubs as an example, where the occasional blip on a scanner matters very little) then using mic cable - again cheaper & more widely available - is going to be their best option. (And don't forget a lot of the bigger clubs use a lot of Martin gear as well as the more 'budget' ranges).

[skier]

I know your all going to freak but, I only ever use Mic cable.

 

I regularly use it on rigs with no end of fixtures even up to 8 streams worth and never have problems. I have even run it over a couple of hundred feet with buffers on the end then a whole outside stage of fixtures over 3 streams with no problems!

 

It's cheaper than "proper" data cable works as good and it doesn't matter if another department runs over it with a genie and destroys it because its cheap!

 

The technical ins and outs are all very well but if you don't get any problems with it that side of it means nothing.

[Ekij]

Lets not start the "You do / You don't need 'proper' cable and termination" argument discussion again.

http://www.btinternet.com/~boxzone/Other/Smilies/Smilie_Argue.gif

If you don't use proper cable or don't terminate properly you may get away with it, but it's not recommended.

 

You probably have bad data packets, your lamps are probably trying to go to random positions/levels and then back to your set level again, possibly causing premature ageing, you just don't see it because it happens so quickly.

 

Edit This BR Thread covers DMX "why I might or might not get away with sub-standard wiring".

[dosxuk]

Explain the second pair, and why I should install and use 2–pair cable.

DMX512/1990 says the optional data link is for data – that's all. It doesn't say anything about the direction of the data or what kind of data, other than it conforms to the electrical specification of EIA 485.

 

[snip]

Several companies use the second pair for various legitimate functions which may be proprietary, but are not always. Some companies do not wire pins 4 & 5 at all, some wire them through.

 

[snip]

If you don't install 2–pair cable, you will may never be able to get any current or future enhanced functionality that may be offered in the future via the second pair.

 

 

 

In the early days, it was relatively expensive to have an entire set of data circuts sitting redundant for the future in equipment, so they were left out. Then companies realised they could save a bit more money by using three pin connectors instead, others decided they may as well use the other pins for their own use - Pulsar being a main one of these by sticking up to 48V down the other lines :D

 

 

 

It's complicated even more by companies like Martin, who had already released a range of lights which use RS485 (not DMX) to communicate with a controller, using 3 pin XLR's (and phase reversed to DMX). So they decided that as they already had all the electronics inside, they could support this new fangled DMXy stuff by just releasing some new firmware for the lights. Martin were not really on a cost saving mission when they started using 3 pin XLRs, it was more an attempt at expanding the useage of their lights.

 

 

 

I leave my final comment to USITT...

 

 

 

Is any connector besides an XLR–5 allowed?

No ! Using anything besides the XLR–5 connectors as prescribed in all versions of the standard is not allowed and defeats the interoperability the standard is intended to provide. Equipment using an XLR–3 connector is not compliant with the standard and should not be marked as such. In very special circumstances, the ANSI version of the standard allows for the use of a non–XLR style connector. One of the provisions for this is that the manufacturer must supply an adapter.

 

 

 

http://www.usitt.org/standards/DMX512_FAQ.html

[david.elsbury]

If you choose not to use "DMX" cable (IE, cable designed for DMX transmission. and sold as such, for example the Canford/ AC lighting stuff mentioned further up in the thread) for cost reasons, 110 ohm digital audio cable is an acceptable substitute and may work out cheaper that cable "labeled" for DMX.

 

Edit: 110 ohm audio cable is a twisted pair, plus screen type of cable. Therefore you would only connect up pins 1 through 3 in the 5 pin XLR connector.

 

Edit 2: Somewhat OT, but one of the venues I work at uses a Strand riggers remote, which uses 5 pins of a 6 pin XLR connector. They have adaptors, 6 to 5 pin. If the remote is used with cable with only 3 pins connected, it will not work. So all of their data cable is wired with all 5 pins connected. This way there's no confusion. And as said above, the standard says that 5 pin connectors are to be used, if it is to be a DMX cable. By all means make adaptors, but 5 pin connectors are the standard.

 

Hope this makes sense

David

[thinkoutside]

Thanks guys for all the history and the help. I’ve learned quite from all of this discussion but I would like to clarify just a bit more.

 

I respect the idea that a lot of people have money tied up in the 5 pin DMX standard my argument was that if another more technologically advanced/cost effective form of controlling lights came along (and one will eventually) I would see no reason for shooting down a change over.

 

BTW: I did notice post 2 about how a 3 pin cable could be plugged into the wrong device and my thinking was why not use a 3 wire cable with a different shaped plug then the standard mic cable. This would have prevented this from being a problem but would have been just as cheap to produce as the standard 3 pin cable is today?

 

I talked to someone in the industry today who told me that all that matters with guaranteeing the signal quality is that you use a 120 Ohm cable. David, if I’m not mistaken you said that 110 Ohm cable is a twisted pair with 2 cables not being used. Am I to understand that all cable that is designed for DMX transmission these 2 extra cables inside providing some sort of shielding?

 

What is it that makes a cable “designed for DMX transmission”? Is it the Ohm value, the extra to cables providing some sort of shielding, or a shield jacket around 3 or 5 cables that makes it as safe to use as your standard 5 pin DMX cable?

[Andrew C]

my thinking was why not use a 3 wire cable with a different shaped plug then the standard mic cable.

Two things at this point. Neither DMX or Mic cables are "3 wire" as I think you understand it. They are two cores with a screen. (Ignoring the second screened pair in true DMX) Have a look at "Balanced" in the Wiki. There is a slow move towards using CAT5 cables for DMX, and indeed using TCP/IP type protocols for distribution to nodes that then break out to DMX. I suspect that it won't be long before true TCP/IP dimmers and fixtures appear.

Please note, I am not a computer geek, and TCP/IP may well be the wrong acronym here!

Am I to understand that all cable that is designed for DMX transmission these 2 extra cables inside providing some sort of shielding?

What is it that makes a cable “designed for DMX transmission”?

The extra pair has no effect on the primary pair, it was a future proofing that was negated by people not following the standard! :D

The "Ohm Value" is a measure of the impedance, not resistance. The construction of the cable, twist, screening, low resistance and impedance all contribute to an effective cable. Add to that flexibility and robustness for temporary use, ease of installation for fixed wiring and then start trading one off against the other, and cost, to get the range of cables available.

 

My link above to Tourflex (click here) on AC lighting's site shows construction diagrams of both 2 pair and single pair.

[andy_s]

just to widen the discussion a bit, we have no installed DMX in our 6 month-old building. data is sent via ethernet to the dimmer racks. We have some DMX nodes which we can stick on the end of the ethernet to talk to DMX devices. Originally data and DMX outlets were specced; we decided to save money by not bothering with the DMX lines. Seems to work fine so far.

 

of course this is only relevant for installations: I'd probably agree that DMX is the way to do temporary set-ups.

[Tomo]

In my opinion, the DMX v Ethernet argument is already settled:

 

Send it from the desk to the end of the lighting bar as Ethernet.

Send it along the bar as DMX.

 

My reasoning is simple:

Ethernet is a Star configuration*, while DMX is a daisy-chain.

 

So if you controlled all your fixtures directly via Ethernet, you'd need one ethernet cable and one output from your stack of hubs/switches/routers for each fixture - expensive.

 

*This means that every device need a link straight back to a hub/switch/router, not passing through any other controlled devices.

(I'm ignoring 10 Base 2, as nobody uses it anymore)

[Liam FitzG]

SNIP...

 

of course this is only relevant for installations: I'd probably agree that DMX is the way to do temporary set-ups.

 

We use Ethernet over WiFi a lot on temporary work, especially on one night stuff. We generally put a set of DMX over WiFi transmitters (one for each universe) at the desk. Then pop one receiver on each truss, and no long data runs around venues or arenas to cause problems. The first few times I ran in data lines and DMX merge boxes as a backup, but at this stage I would be confident in the wireless system. Saves a heap of time and cable.

Other times we come straight out of the desk on Ethernet into a WiFi transmitter, and pop a node at the far end. Some of the Strand and ETC dimmers are running on Ethernet straight into the processor, doing away with the node.

It also acts as a form of electrical isolation for the desk, protecting it.

 

And to go back up the topic for a moment, my curiosity is peeked now, what is it that makes data cable, data cable and not another cable (eg mic cable). I note that in the BR Wiki the difference in the signals is said to be the frequency (data being much higher), also as said above both types of cable are shielded twisted pair (one pair or two). So is it the metal in the conductors, the diameter of the conductors, is there a difference in the twisting or the shielding? I have always found (particularly with younger technicians) that if you give them a reason not to do the wrong thing, rather than simply saying that's the standard; they are far more likely to comply.

 

Liam