Charity installing DMX cable. Will it work?

[mark76uk]

Hello,

 

I am an IT Technician at a small college for students with disabilities. We recently had some money donated for sound/lights, and as part of this I plan to have a circuit of DMX cables/sockets installed in the common room (main venue for discos/theatre/etc.) I don't often get much time in the room prior to a show, so having DMX cable installed will reduce set-up time and eliminate the need for cables over fire exits, etc.

 

Anyhow..

 

I'm not completely sure of the robustness and signal strength of DMX. Could someone please reassure me that the following wiring will be ok? I plan to use Cordial DMX cables from Thomann.

http://www.thomann.de/thoiw2_cordial_cdmx_1_prodinfo.html

 

This is the scheme, with a female and male 3 pin XLR at each 'socket' position. I figured that this way DMX can be 'passed around' the room by plugging in fixtures/dimmers at any position, or just joining the female/male sockets with a short DMX lead.

 

Thanks for any input!

 

http://www.hinwickhall.ac.uk/external/soundandlightforum/DMX.gif

[Ken Coker]

Dear Mark

 

I think probably that you are trying to make a DMX "ring", which will probably work but is not may be the best way to go about it - DMX is a very robust protocol which will be more than fine for the distances involved here, but it really doesn't like being split by using a y-split or just jumping of the back of a socket; I would also advise that you use five rather than three pins in such an environment.

 

Give me a little while and I'll see if I can do a drawing for you; I imagine costs need to be kept to a minimum? Where are you based by the way?

 

Might be worth having a look at Nick Mobsby's new book - http://www.etbooks.co.uk/ will take you there.

 

Cheers

Ken

[Rob]

I think probably that you are trying to make a DMX "ring", which will probably work but is not may be the best way to go about it - DMX is a very robust protocol which will be more than fine for the distances involved here, but it really doesn't like being split by using a y-split or just jumping of the back of a socket;

 

I think you might have misread the OP - As I understand it he will break the ring at each point and install a connector on each side of the break. A short length of cable will bridge the gap when they are not in use to complete the circuit. I don't think he is contemplating piggy-backing the connections...

[j_b]

Looks like you've thought about this, and it will probably work well. You might want to consider one panel mount XLR and the other mounted on about 10" of cable with a gland to the box, that way, you'll never loose that otherwise essential patch lead!

 

Furthermore, if all your gear uses 3 pin XLR, then fit those. Why fit 5 pin then have to have a load of adaptors? 5 pins are a little more expensive than 3 pin too, but the adaptors will be a significant cost

[bruce]

Furthermore, if all your gear uses 3 pin XLR, then fit those. Why fit 5 pin then have to have a load of adaptors?

 

To stop people plugging audio kit into the DMX connections?

 

 

 

You might want to consider one panel mount XLR and the other mounted on about 10" of cable with a gland to the box, that way, you'll never loose that otherwise essential patch lead!

 

True. But it is much more likely to get damaged.

[mark76uk]

Hello Ken. Thanks for responding.

 

DMX is a very robust protocol which will be more than fine for the distances involved here,

Sorry, forgot to mention that the room is 20m x 8m and 3m high.

 

but it really doesn't like being split by using a y-split

I hadn't planned that, just a loop with either fixtures or a short patch cable continuing the loop at each male/female socket point. Then terminated at the last fixture, obviously.

 

or just jumping of the back of a socket

I'm sorry, I don't understand what you mean by this. Please explain.

 

I would also advise that you use five rather than three pins in such an environment.

Why would that be? Doesn't that mean we will need dozens of adaptors (an increase in resistance for the signal?) I planned to put in five core DMX cable, but only use 3 pin XLR sockets and adaptors. In the worst case, future-proofing, we can exchange the sockets and plugs. Currently though, surely most dimmers and fixtures (the cheap ones we will use) will be 3 pin? I think the 3 pin components and DMX leads are much cheaper.

 

Give me a little while and I'll see if I can do a drawing for you; I imagine costs need to be kept to a minimum?

OK, that's great. Thanks. Yes, you're right about the cost.

 

Where are you based by the way?

It's Hinwick Hall College, in Northamptonshire.

 

Might be worth having a look at Nick Mobsby's new book - http://www.etbooks.co.uk/ will take you there.

It looks interesting, although I'm not sure I'll have to deal with such issues for a long time :)

 

Thank you,

Mark :)

[bruce]

I'm not completely sure of the robustness and signal strength of DMX. Could someone please reassure me that the following wiring will be ok? I plan to use Cordial DMX cables from Thomann.

http://www.thomann.de/thoiw2_cordial_cdmx_1_prodinfo.html

 

I'm sure the cable is perfectly adequate. It looks like it's designed as a flexible/patch cable though - without seeing the spec it's not possible to determine whether it would satisfy your local building regs. Is the cable LSOH rated? Is there a LSOH requirement?

 

Bruce.

[mark76uk]

he will break the ring at each point and install a connector on each side of the break. A short length of cable will bridge the gap when they are not in use to complete the circuit.

 

You got it. Thanks :)

 

Is the cable LSOH rated? Is there a LSOH requirement?

Good question.

 

without seeing the spec it's not possible to determine whether it would satisfy your local building regs. Is the cable LSOH rated? Is there a LSOH requirement?

Good question.

 

Well, I found the specs: It looks pretty good to me, mentioning PLASA and USITT.

 

http://www.cordial-gmbh.de/en/produkte.php?kat=57

http://www.cordial-gmbh.de/upmedia/misc/CDMX%201%20en.htm

http://www.cordial-gmbh.de/upmedia/pdf/CDMX.pdf

 

Sorry, I can't get this to go in the correct place in the discussion thread!! Have tried five times!!

[lonfire]

hi,

if you want to save yourself some money I has used this DMX cable.. comes down to 40p per m in you buy 100m

 

works pretty well.. quite chunky though.. some of my longer cables I've split the two pairs and heat shrunk them and put 3pin XLR on one and 5pin XLR on the other.. means I can either have two runs of DMX in one cable or use either 5pin or 3pin depending on what kit I've got.

 

rgds

chris

[bruce]

Well, I found the specs: It looks pretty good to me, mentioning PLASA and USITT.

 

Electrically it looks fine. Just questioning whether LSOH is necessary for your venue. And that's a question that only your building services people can answer. And if they're anything like our building services people, they'll try hard not to answer :). As you may have gathered, it's one of my "pet hates" at the moment.... My gut feeling is that in this installation, it would not be required - but you need to confirm, otherwise some "jobsworth" fire officer might tell you to rip it all out....

 

The problem is that when PVC cable burns, it gives off lots of smoke and nasty fumes. In a duct or void, this can be a problem, especially if it's an air circulation space. You can get special LSOH (Low Smoke Zero Halogen) cable which doesn't have this problem. Of course, it's more expensive, and ironically it's less fire retardent than traditional PVC cables. But if it does burn, it doesn't give off as many nasties.

 

There is some debate within the cabling industry as to where and if LSOH cable should be installed. There is an argument that it's better to stick with non-LSOH in most circumstances, as it's less likely to burn!

 

I wish I knew the full answer!

[pritch]

For cabling, what about using Cat 5? The install stuff is dirt cheap, and designed for being chased into walls or whatever. Someone (USITT?) have a report available on trialling it for DMX, and the conclusion was that it's pretty good.

[Ellis]

One point not covered is that under the covers, DMX is an RS485 network. You should effectively have a single cable run from end to end (looped through any instruments.) At either end, you should have a terminator (a bit like thin ethernet [10-BASE2]). The controller usually has the terminator for one end built in and the last instrument should have a terminator connected to its loop-out connector. Just like thin etehernet, there should be no loops and no spurs.

 

If you install a ring, you should break it where you plug the controller in and attach a terminator to the (female) outlet from the other end of the network. Don't ling the grounds between the inlet and outlet, otherwise you will have an earth loop.

 

Any cable suitable for RS485 will do. In particular, as has been said, CAT5, or better can be used (and is covered in the new standard AFIK). Since you are an IT technician, you will probably have a good source if you do not have enough lying around.

 

HTH

 

Ellis

[Ken Coker]

Ok. I've got it now.

5 pin partly because it's what the DMX standard says, and mostly because it rather looks like an unsupervised sort of space where it will be only a matter of time before someone does the wrong thing with a bit of sound kit.

 

Other than that, I concur with the rest of the room.

 

Ken

[mark76uk]

THANK YOU EVERYONE

I think that about covers everything :)

 

:) :D :) :D :) :D :) :D :)

[strand600X]

Just to keep to industry standards I would certainly make all the DMX connectors 5 pin not 3 pin otherwise the noise boys will start using it and you don't want that