Can I connect 2 or more DMX cables to make one long one?

[Franchise]

Hi,

Can I connect 2 or more DMX cables to make one long one? (I am in the process of putting together/ordering a system. I see that there are some 6 packs of DMX cables on ebay. I am wondering if I could connect 2 of those cables to make a long one).

Thanks.

[JEC]

Daisy-chaining many cables together in a single line? Yes, it's no problem at all.

[DynamoBen]

Works fine I do it all the time. The only limit is you want to stay under 1500 feet. ;)

[Macrosill]

1500 feet. That won't work for my display! lol

[JJGross]

You can go over 1500 feet, but you'll need a repeater. Another way is to parallel DMX lines, but if you use a ton of fixtures you'll still need a DMX repeater to keep the signal strong. You'll obviously also need to terminate at the end of everything and make absolutely sure you separate the ground conductor from the plug barrels to prevent ground loops. I've read some people who say you absolutely can't parallel DMX lines, but they've obviously never looked inside a standard DMX stage lighting fixtures like color changers, washes or intelligent lights. They're mostly paralleled internally between the input and output jacks, that's why there are limits on how far you can go without a repeater. They're not buffered from unit to unit and the source can't go forever without degradation.

Lots of people use standard XLR mic cables for DMX with good results, but to make sure you don't get ground loops resulting in random artifacts, you really need to cut the ground-to-barrel solder joint that most mic cables come with standard. Obviously at the 1000 feet range, your cables can't be the cheapest things you can find. Those work fine for your averag bar band light show where random accidents don't really mess with the show, but not something synced to recorded (especially well known) music.

Cheers,
- JJG

[P. Short]

Sorry, but I consider adding parallel lines to be an ineffective way of increasing the distance of the DMX lines. It is far more likely to decrease the distance than to increase it. The reason for this is that at 1500 feet the one-way propagation time of the signal will be around 2-3 uS, which is much greater than both the bit-time and the rise/fall time of the signals. This means that things are in the regime where transmission line effects are quite important (as important as DC resistance characteristics). Doubling up the DMX cables will create signal reflections at the point where the cables double up and where they reduce down, possibly causing bit errors to be observed at various points along the cable. If the entire cable is doubled, then the terminator impedance no longer matches the cable impedance, and you will get reflections off the far end of the cable.

There are doubtless cases where this doesn't cause any problems. Since it is difficult to predict and describe which setups will cause problems and which will not, and there is no real reason in the DC case for trying it, I would not recommend that anyone use this technique.

--

Phil

[DynamoBen]

Alright folks let me separate fact from fiction. Technically speaking DMX can go up to 5000', however most systems are designed to stay under 1500'. The "parallel" that is being discussed isn't the right terminology. Inside a fixture or a dimmer the two DMX connectors are tied together, this creates a Y. Y-ing a DMX line is a “no no” however what is happening in the fixture/dimmer isn’t a Y its actually a "stub" and needs to be as short as possible. As far as the limit on the number of DMX receiving devices, this is 32. Some transceivers allow more than 32 devices on the line however this is a good limit if you have a mix of different transceivers.

As far as running two lines from one source, this is not a good idea. By doing this you are creating a Y, unless it’s under an inch its not a stub and you will run into data problems. In regards to Mic cable, you never want to you mic cable. Mic cable has the wrong capacitance and resistant for DMX. Many vendors use 3-XLR connectors which is fine you just can't used Mic cable on those connectors. I've fielded way too many support calls where Mic cable was the source of the "flakiness." You really want to run your data down the proper cable type and it should be shielded. In a pinch cat 5 will work but be careful its not shielded and you may run into issues.