DMX cabling, lengths, types etc Options

[Jivemaster]

The standard for DMX always has and still does specify 5 pin XLR many appliances still have 3 pin esp in the disco world. Even beware RJ45 and TRS Jack!!!

Wiring

1 - 1 - screen
2 - 2- data
3 - 3- data

some disco kit wires data+ and data- either way round You will need to check and may need reversing adaptors.

pins 4 & 5 (if present!) may likely, have voltage on them, be NC, or in fully compliant modern DMX-512A systems be used for remode device monitoring. You may need 3 - 5 pin adaptors with and without pin 4 & 5 continuity.

Cable. True DMX cable is available from Terralec and Canford and many others. You will get away with mic cable many times but it WILL bite back. I have seen DMX sent down the audio snake in arena events but a digital (ca. 7 volt signal) does play havoc with mic level signals.

Terminators. the Spec says you must use one, practise is that it is best but not always necessary, and some fittings have auto or manual terminators built in. A terminator is a 5 (3) pin XLR with a 120 ohm resistor across pins 2 & 3 its that simple.

[Tom Baldwin]

A terminator is a 5 (3) pin XLR with a 120 ohm resistor across pins 2 & 3 its that simple.

Not forgetting it should be 1/4W rated, not the 1/8W sub miniature types you often see these days.

[Gilly]

pins 4 & 5 (if present!) may likely, have voltage on them, be NC, or in fully compliant modern DMX-512A systems be used for remode device monitoring. You may need 3 - 5  pin adaptors with and without pin 4 & 5 continuity.

As I understand it the DMX512A standard specifies pins 4+5 as a second data link as Jivemaster and others have said, but RDM (when it finally lands properly) will be able to work half duplex down a single pair (ie pins 2+3) and pins 4+5 will allow full duplex between the desk and the receiver.

[Dave]

Actually the DMX/1990 standard documents their use as "optional second Data link Complement (Data 2 -)" and optional second Data link True (Data 2 +)"  Therefore Avo would be within the standard, however I dont know about CP/pulsar

AFAIK, Pulsar are the main mis-user of the standard - their dimmers output a dc voltage on pin 5 to power their small desks. I'm not actually sure who else breaks the rules - perhaps others can post so we can start a DMX rogues gallery!

[kalmatthew]

Actually the DMX/1990 standard documents their use as "optional second Data link Complement (Data 2 -)" and optional second Data link True (Data 2 +)"  Therefore Avo would be within the standard, however I dont know about CP/pulsar

AFAIK, Pulsar are the main mis-user of the standard - their dimmers output a dc voltage on pin 5 to power their small desks. I'm not actually sure who else breaks the rules - perhaps others can post so we can start a DMX rogues gallery!

I can see that making a mess of equipment that has those pins connected to accept data. Hmm, I'm glad that most manufacturers opto-isolate, the DMX from the rest of the desks.

[pedant] wouldnt it be a non DMX rouges Gallery [/pedant]

[Mystic]

AFAIK, Pulsar are the main mis-user of the standard - their dimmers output a dc voltage on pin 5 to power their small desks. I'm not actually sure who else breaks the rules - perhaps others can post so we can start a DMX rogues gallery!

I should double check my pulsar demux regarding this as this could be the cause of some malfunctioning output on my pearl (where the secondary outputs are on pin 4&5...). If it happens to be the case I promise a photo of the demux for your gallery...

[Olie]

I do not believe anyone has answered the original question of how far you can send DMX before it attenuates too much.

The spec says 1000m and up to 32 units. However this will depend on many factors.

1) The DMX Source
2) The cable
3) The connections
4) The fixtures connected
5) Termination
6) Surrounding conditions (Noise/Three Phase)

So the practical answer is that DMX goes as far as the light that is not doing what it is supposed to do and probably not even that far because DMX errors are not always noticeable on the fixture.

Good practice is to use the correct cable as already discussed, always terminate the line and use splitters in plenty supply.

If you are experienced with DMX then you tend to know what you can get away with, but until then I recommend you follow good practice.

An Example: It is bad practice to send DMX down a CAT 5 cable because it dose not have the correct impedance, but experience tells me that it works most of the time.

Hope this helps!

[Dave]

I should double check my pulsar demux regarding this as this could be the cause of some malfunctioning output on my pearl (where the secondary outputs are on pin 4&5...). If it happens to be the case I promise a photo of the demux for your gallery...

If you look here you will see that the Pulsar 18 and 36 channel demuxes (they call them universal interfaces as they do other things as well) do indeed output DC on pin 5 of the XLR. They say it's current limited, though. If this is what you have, it might be a good idea to give it a bit of minor surgery commonly known as the snip.

They also say that DMX should be terminated with a 100 ohm resistor. Bit pathetic really.

They even make a power supply to produce this voltage so you can run a Pulsar desk off a proper dimmer. The instruction manual on the web site for this doesn't actually mention the XLR connectors that are visible in the photo, so they must be a relatively recent addition.

Does anyone know of any other manufacturers? I'm pretty clued-up when it comes to theatre kit (and I'm not aware of anyone else who breaks the rules), but I'm not familiar with the dozens of el-cheapo DMX disco lights whose manufacturers may not comply with the specs.

Dave

PS I was just wondering how many people have two-pair DMX cables, as I've never actually seen one. I didn't realise a Pearl uses the second pair - do any other manufacturers?

[Rob]

I've noticed this before on pearls. I'm not quite sure what the advantage is over just having four sockets numbered 1 - 4. I guess it's done so you can run two streams down one line and split it further down the chain to save cable.

I'm not sure if a Hog II uses pins 4&5 when driving overdrive units - the manual doesn't say...

[Mystic]

PS I was just wondering how many people have two-pair DMX cables, as I've never actually seen one. I didn't realise a Pearl uses the second pair - do any other manufacturers?

I did for two reasons, first because I had 2 pair wires and 5 pins XLR so I soldered everything in the right place thinking it might be useful one day (but I underestimated pulsar evil side ). second because my pearl outputs a second DMX on pin 4&5 and I thought it might be good to have it split at the far end of the DMX run (so I made all my cables with 2 pairs).

[Mystic]

I've noticed this before on pearls. I'm not quite sure what the advantage is over just having four sockets numbered 1 - 4. I guess it's done so you can run two streams down one line and split it further down the chain to save cable.

This is true but there is one other explanation : the photo is one of the new pearls (and azure). older pearl like mine only have two XLR 5pins sockets to ouput 4 DMX universes. So the primary goal was to save XLR sockets I think. However they were the only ones to do this so they may have ended up in adding the two sockets afterwards to please the users and make all those XLR5 Y cables useless...

[Brian]

An Example: It is bad practice to send DMX down a CAT 5 cable because it dose not have the correct impedance, but experience tells me that it works most of the time.

This is not what ESTA found; to quote their report...

Data obtained from all three of these test sessions confirms that, in most respects, UTP and STP Category 5 cable can be expected to perform at least as well as EIA-485 rated data cable for DMX512 applications.

The full reports can be found here.

[Joe_606]

Only 5 pin is recognised as DMX cable, not 3 pin XLR's. Originally everyone used 5 pin for DMX fixtures but then some clever people came up with the idea that DMX could be transmitted over short leaps through standard 3 pin XLR mic cables. So 3 pin was fitted to cheaper DMX fixtures, slowly everyone started to use it because there customers found it better to use cheaper 3 pin XLR cables and also because (mobile djs especially) are more likely to find spare 3 pin XLR’s laying about that they aren’t using on the sound system they could use on a DMX rig. But as I said officially there is no such thing as 3 pin DMX cables, if you look at some of the long established professional manufactures like Pulsar and Clay Paky they only have 5 pin outputs. For instance the Pulsar Masterpiece, and all of the Clay Paky fixtures.

Sorry if this has aready been said.

Also, as long as you use high quility cables and connectors, it should reach the maximum distance. If the cable is sheilded you shouldent get interfearance either

[RumbleO]

(Beaten by JSB and PaulEARS on this well-worn topic, so I'll add my £0.02 worth here...)

By the book - 5 Pin XLR's are the required connector for DMX, 3 Pin XLR's are excluded (not allowed).
However 3 Pin XLR's are very popular, if not predominant. I recently purchase some Stairville Led Par56's, Mobile color 150 and some scanners. All have 3 Pin XLR's on them. As far as practical differences are concerned, you will probably find 3 Pin XLR's are cheaper but that's about it. <snip>

I had exactly this issue as the Showtec equipment - Desk, Dimmers and LED PARs - are all 3-pin.
I ordered up a couple of half-metre lengths of 3-to-5 and a half-metre length of 5-to-3 of
DMX Lighting Cable DST 110 Black .

The first 3-to-5 is effectively an accesory to the desk (to bring it up to (5-pin) standard.)
My long run is 5-pin.
The 5-to-3 accomodates the ShowTec fixtures (with short runs of 3-to-3 between them)
and the second 3-to-5 has the DMX Terminator attached and dongles from the final fixture.

Thus my universe is complete, but I know that if I meet proper kit (e.g. the Frog or Avolites desks, Strand Dimmers, or hired movers) I have the correct DMX cables to let me swap them in place or add them before or after the Showtec fixtures.
And secondly if the venue has their DMX in place I am converted to 5-pin to connect through it.
My strongly held view is that 5-pin IS the standard. The fact that everything I own is 3-pin is my issue and this resolves it.

HTH.
Owen

[martinw]

I'm not sure if a Hog II uses pins 4&5 when driving overdrive units - the manual doesn't say...

The Hog2 only uses pins 2 and 3 to talk to the overdrives. It works by sending data to the overdrive at 500Kbs (twice the rate of DMX). The overdrive receives this and then spits out 2 streams of normal DMX.

Martin