DMX

[sjbvision]

So,

 

What is the greatest cable length that someone has sent a DMX signal. Now this could be with/without an amp.

 

I need to send a signal 250m, I was going to put a DMX amp at each end, now I think that this will be good enough, although I do intend to test this before the event. The signal is being sent from a Pearl to Mac 2000s. Either post or pm me, if more details are need to answer this query.

 

Later

[andy jackson]

250 metres seems a decent enough length to the first fixture, depends on the distance there after tho.

the pearl should more than cope with it - just dont use mic cable to link the lamps! else everything will go tits up, also make a little terminator by soldering a 120 ohm resistor over pins 3+2

 

fingers crossed it will all work!

 

Andy

[BlueShift]

I thought 4000 feet was the official DMX protocol spec for max length of runs? (thats from source to furthest recieving unit in the chain)

 

if this is the case (someone confirm?) then 250m is easy (that works out at less than 1000 feet)

[bob_lamb]

Will be ok as long as you terminate it and use data cable (twisted pair) pro flex/tour flex etc, available from www.lightfactor.co.uk or www.aclighting.co.uk Also worth using a separate DMX line for different fixture brands. I have had a few un-explained funnies on this note.

[david.elsbury]

If you can, try making the data link one continuous bit of cable. Reason being that at every join you have to make, you will loose some of the signal- it will reflect back down the line on itself, and the connectors will change the impedance for a little bit- but possibly enough to make a difference.

Agree on the terminator. And if anything, place a DMX amp at the start of the run, not the end. This way it acts a line driver, instead of an amplifier which will amplify any noise or interference picked up.

How are you getting power to the moving lights and the desk? Same feed? Same earth? Different feed?

David

[Calder]

Sounds like a job for wireless DMX to me!

 

Calder

[sjbvision]

Cable runs hopefully will be continuous, I calculate total DMX cable run to be 3000 m!

 

Reason for amp each end is that the signal is getting bounced on again! Power feeds from different areas. At the moment power is 10 sync'd pairs of 135kva units, spread over a 2km site.

 

Radio DMX, hmmmmmm! Quick calculation is 25 transmitters 32 receivers! the cost is a little prohibitive on that one!

 

Thanks for all replies so far.

[david.elsbury]

Cable runs hopefully will be continuous, I calculate total DMX cable run to be 3000 m!

<{POST_SNAPBACK}>

Cool that should be good. Every so often, where you can (get power), put a splitter/amp to regenerate the signal, and it's sweet- another run of 4,000 feet or whatever.

 

Reason for amp each end is that the signal is getting bounced on again! Power feeds from different areas. At the moment power is 10 sync'd pairs of 135kva units, spread over a 2km site.

Fair enough, I would suggest making sure that the splitter/amplifiers you use are opto-isolated to ensure that if anything funny goes on with grounding or power imbalances that it's not your expensive moving lights that get affected, it's the splitter/amp.

Radio DMX, hmmmmmm! Quick calculation is 25 transmitters 32 receivers! the cost is a little prohibitive on that one!

Ah, yes. Can't say I'd have specified radio DMX on this one.....

 

Also worth using a separate DMX line for different fixture brands. I have had a few un-explained funnies on this note

Again, an opto-isolated DMX splitter would do the trick here.

 

Also don't forget the 32 fixture 'limit' on DMX. Another reason why you'd need a splitter. (geez, sounds like I make and sell the things, doesn't it... but I don't :) )

 

David

[Jivemaster]

Look into the earthing of the power system, as there are many generators there may be many earth potentials, and you could get signal failure due to earth loop currents in the screen wire, and common mode overvoltage, so aim to use an opto splitter for each "Earth Domain".

 

Perhaps also consider the possibility of a line being lost to damage can you plan any redundancy into the cabling esp the first one where a failure would cause total loss. Could you run a single DMX round to all the amps and then back to near the desk and leave it terminated there, then if the signal is lost by using a gender-changer you could drive the line from the other end. Is there a box that will taks two DMX's and pick the best so that you can actually network rather than simply daisy-chain.

[Jivemaster]

XTBA (,co,uk) have all sorts of usable boxes amps splitters smart routers etc which may assist you in planning a reliable system. There will be others also

[Bryson]

This may be so obvious you haven't thought of it, but does it all have to be on one desk? Most huge, spread out events use more than one desk and cue real human beings via radio. The cost is probably less than running and hiring and boosting all that cable.

[sjbvision]

The Whole Show is not on one desk it is spilt, yet still the cable lengths are massive, due to show area. But thanks for the idea. I appreciate all the help and info with this job and will no doubt post further as I encounter other problems.

Thanks again

[Mars]

As far as I know, the official specified maximum length for RS485 (the mother of dmx512) is 1200 meters, that's approx. 3600 feet. There are also issues about the amount of transferred data (distance reduction). Others like cable quality, parallel running with other electric conductors, &c. &c.

You will also need a telescope to view the stage.