DMX over mains cable

[Tezzachs]

We are looking at making our lighting install more perminant and the powers at be (those with the money) have seen the price of DMX cable and said "use mains cable its exactly the same". But is it?

 

The run length would be 100m give or take and we have:

 

12 par cans

6LED pars (3 chan each)

3(4 if I get round to reparing the other one) mini movers (5 chan each)

2 large movers (11/14 chans)

and a smoke machine.

 

Will using mains cable to run DMX cause me problems in the long run or shouldi try and push for the extra cash and do things properly?

 

Thanks in advance

[david.elsbury]

Uh, well, DMX cable is a shielded twisted pair, whereas mains flex is unshielded, not twisted, and with a rather different characterstic impedance.

 

It might pass signal, it will almost certianly be prone to interference, and will probably stop working at the most inconvenient time.

 

If the price is really an issue, and this is for an install, Cat5 cable is the same impedance and is proven to work as an alternative cable. Use one pair for the data +/- and another wire for the ground.

 

I'm assuming this isn't a wind-up??

David

 

Edit: If you really are studying "electronics and communication engineering" then I'd have thought you'd be right up there with knowledge of transmission lines, and data networks. Those should inform your question...

[pritch]

It seems like a strange way to cut corners on a job, that one. I think the phrase "do it right, or do it twice" springs to mind. Sure it might cost an extra £50 or whatever, but when you look at the cost of someone's time to reinstall all the cabling once it's let you down at a critical moment once too many...

[Ynot]

Will using mains cable to run DMX cause me problems in the long run

No - it will cause you problems in the SHORT run - in fact very likely as soon as you turn the system on!!

 

...shouldi try and push for the extra cash and do things properly?

Not 'should' but MUST.

 

This is really a no-brainer, I'm afraid.

[J Pearce]

I'll second the recommendation for Cat5. It's also simpler to install as most jobbing sparkies know how to pull in cat5 and will be happier working with it than a strange cable they haven't used before. Just make sure they understand the ring topology of DMX (rather than the star topology of Ethernet) unless you want to spend what you've saved on cable on a DMX splitter to really do the job properly.

[themadhippy]

van damme 278-401-000C 43quid from cpc,job done

[lightsource]

The powers at be (those with the money) have seen the price of DMX cable and said "use mains cable its exactly the same". But is it?

 

Thanks in advance

 

There is no similarity whatsoever between mains cables and DMX. Quite simply, one is a power cable, which uses a lot of copper to move electricity from one place to another, the other is a sensitive data cable, which needs to be 'in tune' with it's surroundings, ie correct cable, connectors, splitters, terminators etc.

 

It's like comparing The Incredible Hulk to Cheryl Cole :D

 

I also don't see how DMX cable seems to be more expensive than mains ?

[jono9691]

A friend just pulled a heap of Mains cable used for DMX out of a club, needless to say the DMX aspect of the rig was totally screwed.

Just Don't.

 

Cat5 doesn't solder well and has almost no ability to shield against interference. (blue install cable)

It's also solid core and breaks easily from reuse. (no good for cable stock)

 

Push hard to do it properly, at the end of the day you're the one that looks like the Muppet when it all goes haywire.

Best of luck with the stingy bean counters :)

[Ashley R]

Use CAT 5 cable to install in walls to wall points, and then buy proper flexible cable to go from the wall to the fixtures. Also the neatest way of doing it I think.

[Jivemaster]

As a communications enginering student you should understand transmission line theory, in the ents business you need to realise that DMX is a high speed data system using a balanced line at about 240KBaud. DMX works perfectly when wired perfectly BUT is prone to fail (usually due to impedance mismatches along the cable) when out of spec cable is used.

 

Plan the DMX installation carefully or it will fail, and they usually fail during critical events.

 

I'd suggest that you get a good DMX cable and pull it in with the other data cables to the stage, Then use a splitter so that you can have copies of the same data in several places where it will be useful.

 

Cheap (Chinese!) lighting equipment sometimes has poor DMX receivers and can have it's own idiosyncrasies, don't have a suspect DMX line as well

 

12 par cans Mains? DMX to the dimmer rack

6LED pars (3 chan each) Up on bars? DMX to the bar end

3( mini movers (5 chan each) DMX to the bar end

2 large movers (11/14 chans) DMX to the bar end

and a smoke machine. DMX to a floor trap with the smoker power.

 

(4 if I get round to reparing the other one) Don't shaft yourself trying to do repairs (FOC?) for someone who will not do the install right -you will catch the flack for everything.

[jonathanhill]

Who are these 'powers that be' that have given this misinformed direction? Technical or administration?

[mrboo]

I dont think Buliding / power / local regs will like you using main power cables for this use as it far to easy for some one to think they can plug it into the mains. you can`t be tight on the data side on a install as it the only bit of a insall you will never need to redo as once done right the cable will out live many lights that will use the data line.

[Tezzachs]

Replying to several people at once:

 

David: no wind up.

 

To the people who commented on my course: yes I am studying Electronic and Comm. Eng however, data networks and transmission lines are 2nd year so I haven’t covered them yet.

 

It turns out that 'the powers that be' are the people from the church that owns the building, (still not sure why but that’s not for public discussion), one of which is a network engineer so god help anyone who has a network installed by him.

 

Jivemaster: par cans, and led pars are on stands, at the minute, all of the movers are along the back of the stage.

 

I’m tempted to say "do it your way but don’t moan when I say 'I told you so' when it doesn’t work" but I can’t see it winning me any points.

 

I will do it properly, even if I have to put up the imaginary 'extra cost' myself and claim it back in free food and drink for life, or add onto my charge for anything else they want done.

 

With regards to the 4th small mover, it works fine but none of the green LEDs in it work.

[Brian]

...one of which is a network engineer so god help anyone who has a network installed by him.

 

Just ask him why he doesn't use mains cable for his networks? After all, CAT5 cable is the same as mains cable isn't it?

[Tomo]

From the top few online suppliers that turn up on a quick web search:

 

100m solid-core Cat5 spool, £20

100m 1.5mm² twin-and-earth, £50.

100m 0.75mm² 3-core mains flex, £50.

100m flexible one-pair DMX cable can be had for around £40. (Though that's fairly thin so I wouldn't tour it.)

 

So Cat5 is much cheaper cable, and even DMX cable may be cheaper than mains flex anyway.

 

With Cat5/5e/6, you must use IDC terminations at the wallplates either end, never solder as that will break.

While IDC XLR-5s are more expensive than solder-bucket ones, it's still often a cheaper installation.

 

Note that "Microphone" cable is also wrong, and if the supplier says it's "Microphone and DMX cable" then it is probably really microphone cable and not actually suitable for DMX as they want different characteristics.

- However "Digital audio cable" is often equivalent.

 

As the guy's a network engineer he'll be able to get Cat5/5e/6 cheaper than the above anyway.

 

There is a standard pinout for DMX-over-Cat5, see the Blue Room Wiki.

 

Then you only need a few DMX patch leads, which you probably already have anyway.

 

The general issue with DMX over 'funny cables' is that there are a lot of cases where it'll appear to work.

 

- It's a digital signal, so it gets through right up until the moment it doesn't.

The way DMX works also means that even if you're actually losing the vast majority of the DMX frames to interference almost all the time, the first few channels may well appear to work.

Thus a lot of people think they can use whatever they like as "it's always worked for them" - because they've only ever used really short cables and the first few channels.

 

There have been a few threads on this before, eg this one.

 

A book that's pretty useful: Recommended Practice for DMX 512: A Guide for Users and Installers (2nd Ed) by Adam Bennette.

It's only a tenner from L&SI. (The first edition doesn't include RDM so it's fine if you don't see yourself using that.)

 

I've even seen one 'DMX' LED driver that works fine if it's channel 1 to 4, but got progressively worse at higher addresses. The manufacturer hadn't ever done DMX (or indeed LED drivers) before and simply didn't understand.