DMX / XLR & Cables

[calpe]

I have been doing some reading on the DMX lighting and it appears some suppliers still sell equipment that need 3-pin XLR's.

From what I have read in 'Blue Room' it seems quite clear that DMX should be used with 5-pin XLR's. Or am I wrong?

 

One other point if I may is about the cable that should be used. Microphone cable be avoided.

 

I have come across this - http://uk.farnell.com/van-damme/268-610-000/cable-control-DMX-1-pair-per-m/dp/1972545 - whose actual impedance is 110 Ohms.

So I am assuming that a Terminator is not required seeing that the cable is of 110 Ohms.

 

Much appreciate is some help could be given.

[bruce]

While the DMX spec says 5 pin, on virtually all low end kit, 3-pin is the norm. They are cheaper and more common.

 

The spec also says use 110 ohm cable - but in practice most mic cable will work just fine.

 

 

Even if you are using proper 110 ohm cable, you still need the terminator, according to the spec. But as I have suggested above, DMX is a fairly tolerant protocol - most things will work just fine without.

[Jevans]

The DMX spec is for 5 pin to avoid getting mixed up with audio, which is 3 pin. (Or you can use RJ45s as well, but you won't often see that, and it's intended more for permanent install) Yes, many cheaper brands use 3 pin to save a few pennies on connectors - they're technically wrong but having a few 3-5 pin adaptors around is about all you can do, or make up some proper 3 pin DMX cables and clearly mark them so you don't get them mixed up with audio cables.

 

The cable you link to is the technically correct one for DMX, yes (other brands/styles of course as well) though in practice you'll find DMX will also quite happily travel over mic cable, speaker cable, mains flex, wet string, or anything else you happen to solder up to an XLR - until the moment it doesn't, which sods law will ensure is just after the show starts. Using mic cable with 3 pin connectors on is quite common, but not recommended as it's not intended for data, and you may find mysterious problems start happening when you start putting long lengths of cable in, or have lots of devices daisy chained etc.

Termination is quite separate - DMX travels over a balanced pair and requires line termination always - these are usually in the form of a single XLR plug which contains a resistor soldered between the data pairs. If you've not used the right cable in the first place using a terminator is probably a bit pointless as you're out of spec anyway.... In practice a) lots of pro grade fixtures these days automatically terminate the line if there's nothing plugged into their output b) If you are using proper cable you'll get away without termination most of the time, but YMMV - it is technically required.

[Charlotte_R]

The DMX specification states that connectors should be 5 pin. However, a lot of people use 3 pin microphone cables for the purpose because it reduces the number of cables that they need to own. Most of the time, on small rigs at any rate, it works (when I was working in my students union, we almost exclusively used 3 pin mic cable because that was the vast majority of our cable stock - our rigs probably tended to have about 20 intelligent lights in them on average).

 

The 110 ohm figure that you're looking at is characteristic impedance. This is a physical property of transmission lines, and not the actual value of the resistance of the cable used. So, you should use a terminator.

 

However, as I hinted at above, DMX is actually pretty robust. So, in practical terms, 99% of the time you can run DMX down 3-pin microphone cable without a terminator. But occasionally that doesn't work and you need to run it down the correct cable and put a terminator at the end.

 

HTH?

[matt c]

Personally, I wouldn't call DMX fault tolerant. Granted, it's better than two pieces of wet string, but it's a Time Division Multiplex over an EIA-485 physical layer.

 

If you wanted fault tolerance, you'd use a proper framed data packet, with a CRC on each packet. Given the required transmission rate, you'd probably discard bad packets and wait for the next one, with no error correction system (either retransmission or recovery).

[calpe]

Thanks to you all for such helpful information.

I've got some further reading to do....

 

I think ill stick with non-microphone cable to avoid any hiccups.

 

Interesting to read that the cheaper DMX's use 3-pin, surprising they have been allowed to use them, assuming their is a 'standard' set!

[IRW]

Interesting to read that the cheaper DMX's use 3-pin, surprising they have been allowed to use them, assuming their is a 'standard' set!

 

Yes...they'd better watch out for the cable police...

http://www.deeside.com/wp-content/uploads/2014/05/british-transport-police-cable-theft-1.jpg

 

http://news.bbcimg.co.uk/media/images/51289000/jpg/_51289548_jex_959317_de27-1.jpg

[sandall]

So that's why my trains weren't running !!

[bruce]

The electrons don’t really care what sort of plug they go through

[Yorkie]

The electrons don’t really care what sort of plug they go through

Is there a 'straightforward helpful post of the year' award?

[Jivemaster]

Rule number one of DMX is that it only fails at critical times. The spec says 5 pin XLR, The spec says 110 ohm cable, The spec says terminate the lines, the spec says daisy chain yet I know of one guy with a fanout of twin flex from a single XLR 3 pin, and it works! The further you deviate from strict compliance with the spec the more likely you are to have failures. A fully isolated splitter can be a get out of jail card because it can reduce the possible interactions on a line.

 

With fault finding the fault may not be where you think because sometimes a fault causes the signal to be reflected back along the line and the reflections can interfere with the signal somewhere else.

 

Other connectors are used in reality from data connectors to jack plugs. As a further gotcha some early kit used the two data cores wired the other way round (+- or -+) and just occasionally you need a crossover lead, fortunately it's getting much rarer as old kit fades from use

[calpe]

With thanks to all contributors.

[Don Allen]

The problem of 3 pin connectors used instead of 5 pin connectors was due to a small Danish company, Martin, using 3 pin connectors on their early lights. They also reversed the phase or polarity of the 3 pin xlr connectors, so you needed to carry a phase reversing 3 pin xlr cable with you. They stopped using their phase reversal after a few years.

 

The use of a 110 ohm terminator is a good habit to get into, as it shows up faulty cables, that would otherwise have let the DMX continiue to work, marginally, without a terminator on the end of the run. Always better to find faulty cables and swap them out before the show starts.

 

If you can use a soldering iron you can make your own DMX Tester and Terminator and Clive also mentions the Martin phase reversal.

[mac.calder]

To be fair to Martin, they did not release their initial lights running DMX - what they had was their own protocol which was running on top of RS-485 (like DMX) using a 3pin XLR with opposite pin out to DMX. Once DMX became the 'thing' they released a firmware update which allowed people to control lights using DMX on a 3 pin connector with a phase reverse cable. I would say more to the point the guilty party is chinese manufacturers using the cheapest possible connectors they could get away with.

[Mazz]

let's not forget High End, who used 3 pin well into the Studio Spot & Studio Wash era. They were quite expensive to buy new. Not cheap Chinese disco lights.