DMX Cable - Are they telling me porkies?

[Ashley R]

Hi BR

 

Below is an excerpt from an email I received, after I asked about some of what I saw as deficiencies in their cable, and well I cant tell whats true, from whats pushing the boundaries, and was hoping you lot would be able to.

 

Cable description: A Decent jacket (As you would find on cheaper Mic Cables), copper braided shielding (At a guess only 60-70% coverage), all 5 pins connected, but instead of the cores being twisted pair, all 4 follow the same twist (so they all twist in unison)

 

Price on this cable: arround $20 for a 10m pre made lead.

 

Heres the excerpt from the email:

DMX cable does not require a full 100% braid/foil shielding as it is a low rate data (0/1) communications method. A simple farrady cage braid is sufficient. We have tested up to 200m in cable length on a loop back DMX line thorugh a high noise environment with industrial motors and lighting ballists and found there was a zero error return rate of the data over a 4 hour period. As data is 0/1 and threshold driven, it does not require as solid an earthing isolation as mic cables. Tight twists are usually only required for high data rates above 100Mhz for long distances. DMX data rates are much lower. Regarding noise, our experience has found that it is not necessary a tight twist of a balanced line that improves noise rejection, but rather the close proximity of the two balanced lines to receive the same noise with as close amplitude and phase as possible. This basically means that the twist is critical for un-sheaded wires, but when in a sheath where the conductors and essentially locked together, the twist doesnt add any further noise rejection, just higher data rates. The higher the twist rate, the less flexible the cable is for winding, and the more prone the cable is to knots and kinks. Its that fine balance between a sufficient data rate and cable that winds well and is flexible/easy to use at gigs.

 

Thanks for your help guys

Ashley

[gyro_gearloose]

I asked about some of what I saw as deficiencies in their cable

So were you experiencing DMX errors when using their cable, or are you questioning it simply because it is made a bit differently from other cable? There's nothing wrong with asking questions when confronted with something you aren't sure about. Its how we learn, after all, but the reply you got sounds reasonable. If they've tested a long length of cable in an electrically harsh environment and had no errors, then I don't think you've got anything to worry about unless you are actually experiencing errors of your own.

[Ashley R]

Well yes I was questioning because the build is a lot different than other DMX cables I have come across. e.g. Not twisted pair, but all lines twisted in unison, only approximately 70% coverage in the copper braid, with no foil shielding underneath to make a complete 100% shield. Not because I was experiencing DMX errors, but then again using Mic Cable I haven't received any errors with it.

[Ynot]

A minor observation about all 4 wires being in the same 'twist' - that's largely irrelevant in most cases, becasue you're most likely only using 2 wires and screen (unless you're using RDM or sending 2 universes down that cable). The other pair of wires will have little or no effect on the pair that IS being used.

 

 

[paulears]

This is actually quite interesting because it suggests that DMX cabling would actually perform poorer if used for audio. We usually spend hours discussing that audio cable isn't good enough for DMX, yet the response suggests in this case the opposite - the DMX cable is NOT going to be good for audio - 5 pin connected audio cable isn't that common - but I do use some for stereo microphones, fitted with 5 pin XLRs.

[brainwave-generator]

This is actually quite interesting because it suggests that DMX cabling would actually perform poorer if used for audio. We usually spend hours discussing that audio cable isn't good enough for DMX, yet the response suggests in this case the opposite - the DMX cable is NOT going to be good for audio - 5 pin connected audio cable isn't that common - but I do use some for stereo microphones, fitted with 5 pin XLRs.

 

DMX cable IS lower quality than audio cable. It doesn't need to be as high, the integrity of the signal is not as dependent on it. Audio cables carry an analogue wave, DMX carries 1s and 0s.

[indyld]

you're most likely only using 2 wires and screen (unless you're using RDM or sending 2 universes down that cable)...

 

Just a quick pointer so as not to perpetuate the myth that RDM uses additional conductors. It doesn't, that's the point of using bi-directional comms down the usual conductors.

[Ynot]

you're most likely only using 2 wires and screen (unless you're using RDM or sending 2 universes down that cable)...

 

Just a quick pointer so as not to perpetuate the myth that RDM uses additional conductors. It doesn't, that's the point of using bi-directional comms down the usual conductors.

Well, I stand corrected.

I was indeed under the impression (I think after conversations in a BR topic a while back) that RDM required all 5 pins connected.

Thanks for that.

[indyld]

I was indeed under the impression (I think after conversations in a BR topic a while back) that RDM required all 5 pins connected.

Thanks for that.

 

Just to expand on that for the benefit of readers of this, RDM uses the same conductors as standard DMX512 (handy, seeing as much DMX cable stock is made with cheaper 3 conductor cable). RDM transactions temporarily break the DMX stream in order to send calls and receive responses from RDM devices. Once the transaction is complete, the normal DMX stream is resumed and RDM goes quiet. This way, the RDM portion takes only an average of 10-15% of the capacity (not 50%), leaving the rest for the DMX. RDM is only sent when required, else it's quiet.

 

The time when RDM traffic and processing is heaviest is during "discovery", the process for interrogating, finding and sorting out the RDM devices in the universe and subsequent setting of addresses.

 

Of course, while "old" 3 conductor cables can be used with RDM, splitters/buffers and any other gateways in the universe need to be compatible in order to let this traffic pass - so non-RDM buffers create a roadblock. However, compatible and non-compatible devices can be used in a configuration where the roadblock doesn't matter. But we lose the functionality of RDM after that block.

[Ashley R]

This is actually quite interesting because it suggests that DMX cabling would actually perform poorer if used for audio. We usually spend hours discussing that audio cable isn't good enough for DMX, yet the response suggests in this case the opposite - the DMX cable is NOT going to be good for audio - 5 pin connected audio cable isn't that common - but I do use some for stereo microphones, fitted with 5 pin XLRs.

 

DMX cable IS lower quality than audio cable. It doesn't need to be as high, the integrity of the signal is not as dependent on it. Audio cables carry an analogue wave, DMX carries 1s and 0s.

 

Hmmm (tm Ynot) that reminds me, ive got to get arround to making up some 5P cables for some NT-4's

 

Its lower quality? So Far its been near on near impossible for me to find some nice DMX cable which meets the following ruff guidelines, at a decent price: Multiple strands (More than 10 preferably) per core, Twisted Pair, Decent jacket, 100% Foil Sheild, 50-70% Braided sheild. So far, DMX cable has been 1.5x more expensive than Mic Cable, everywhere I look.

 

And RDM only goes down the first 3 pins, due to the large amount of 5pin gear which surpriseingly does not have 4 and 5 even looped. e.g. Jands Dimmer Racks, Multiple 5P DMX Cables.

 

Ive also posted a simalar thing on the ALIA forums, and one person has said, that the grouped twist, will stop external interference, but wont stop internal between the two lines. recitfyed that by connecting 2 lines to the one set of pins.

[Keeper of the Keys]

Actually EIA-485 does call for twisted pairs, and 200m is not the max distance for DMX-512 (I can't double check now but the max distance is 300-500m IIRC).

 

As far as RDM and the second pair, the newer standards actually define 3/4 modes of operation:

- not using the second pair (most common)

- use second for return (full duplex)

- use second for second universe

- use both pairs half duplex

 

The last two are basically the same, this is from memory so I may very well have made some terrible mistakes, when I can I'll double check.

[mac.calder]

 

Its lower quality? So Far its been near on near impossible for me to find some nice DMX cable which meets the following ruff guidelines, at a decent price: Multiple strands (More than 10 preferably) per core, Twisted Pair, Decent jacket, 100% Foil Sheild, 50-70% Braided sheild. So far, DMX cable has been 1.5x more expensive than Mic Cable, everywhere I look.

 

 

Search for AES cable. It is 110Ohm and generally closer to the specs you are after.

[Ashley R]

Actually EIA-485 does call for twisted pairs, and 200m is not the max distance for DMX-512 (I can't double check now but the max distance is 300-500m IIRC).

 

 

According to a few sites I found after a bit of googling, EIA-485/RS-485 requires Twisted Pair. and according to wikipedia, its capable of transmitting over 1km...

 

Essentially what I have been comparing all my cable choices to is Beldon series 8102 cable, and that's what im looking for (But without the Belden price tag attached), why did I chose this type of cable as a comparison, because its what everyone else is using.

[Keeper of the Keys]

1.2km is at 100kbaud, DMX runs at 250kbaud so it is less then 1km IIRC

[indyld]

As we are in the mood for talking DMX practice, can I point everyone to the excellent "Recommended Practice for DM512 - A guide for users and installers" 2nd Edition, by Adam Bennette and produced by PLASA/USITT. It's only a tenner and is really clear on the use of DMX and design of RDM compatible systems.

 

Regarding the distance question, the book points out that while EIA-485 is capable of operation up to 1km, it is recommended that line length should be kept below 300m, turning to repeaters/amplifiers for distances further than that. Conductor cross sectional area and voltage drop is obviously a consideration.