DMX splitters.

[Jerome]

Hi to all,

 

Can somebody please explain to me what a DMX splitter does please, I guess it splits the signal but how does that work with the chains and addressing issues ? I've DMX'd a rig before but never used a splitter. When are they relevant?

 

Thanks

Jerome

[smalljoshua]

They work by receiving the DMX signal and then re-transmitting it.

 

The splitter acts as one device on the chain, meaning that on the DMX chain into the splitter you could have up to 32 devices including the splitter and the Desk.

On each of the splitter's outputs you can have a further 32 devices including the splitter.

 

I don't get what you mean by addressing issues, you can still only use 512 Channels on the Universe no matter how many splits it has.

 

Splitters are relevant when you have more than 32 devices in the DMX chain or you have DMX going to each lighting bar then your dimmers. You would then have one split going to each bar and a further split going to the dimmers.

 

Josh

[Simon Lyall]

They are also useful when you want more of a star topology. For example, you require the same DMX universe to go 4 different trusses, instead of linking each truss (which is sometimes not practical to wire) you can have a line going to each truss, with no need to link them together as such. This also has the added advantage in that the splitters have opto-isolated outputs, this means that if an error in the DMX occurs on one line on the splitter, it wont affect the others. This can help with fault finding, but is also quite useful insofar as one truss goes strange, not the entire rig.

 

 

HTH

 

Simon

[lightsource]

Remember that standard DMX is output-input-output-input on and on and on ( usually called a daisy chain) until the last device in the chain, which is then terminated.

 

A DMX splitter allows the signal to flow through many 'paths' so 1 input and say 4 outputs, all independant of each other.

 

For a simple example, if you had 4 T bars with a Dimmer pack and 8x Par56 on each , then under normal circumstances your cabling would be - lighting desk into Dimmer1, Out of Dimmer 1 and into 2, Out of 2 and Into 3 Out of 3 and into 4 then terminate.

 

One problem here, is if there was a DMX cable fault in between Dimmer 1 and 2, then all signal would be lost after the fault.

So in this senario, Dimmer 2, 3 and 4 would recieve no signal.

 

With a splitter, The chain would be Lighting desk to splitter, Splitter to Dimmer 1, Splitter to Dimmer 2 and so on..... with the exception of a cable break between the Desk and the splitter, then any cable fault would be isolated to a single dimmer. As each has it's own split from the desk.

 

Each path needs to be terminated at it's end.

 

The splitter has nothing to do with DMX channels, (REMEMBER DMX is purely data packets). It just recieves a data packet, then routes it down multiple paths, so is transparent in the addressing scheme of things.

 

As far as programming / addressing goes, the DMX splitter is totally transparent, it's purely a cabling thing.

[Jivemaster]

DMX as transmitted is just pulses of data. A splitter will replicate those pulses down a number of new data (DMX) lines.

 

Standard DMX rules apply, 32 devices per DMX line, use the correct cable, terminate each line.

 

Sometimes, especialy when using cheap kit, the DMX receiver chip loads the line oddly and you cannot get close to 32 devices working on a single line so use a splitter to create enough DMX lines. (I once had four colour changers that only worked in one physical DMX wiring order and consumed a whole DMX line! They are gone now!)

[Jerome]

Thanks for the replies,

 

I'm still confused :** laughs out loud **:

 

So if the splitter is a fixture in the chain, then how does the programme addressing work. I understand the splitter would have its own address. On a standard daisy chain rig as you say you can only have 32 fixtures which in the desk go in as 0-31 in the soft patch, yes? If you then put a splitter in does that make the signals coming off the splitter 33-64 if you used another 32 lights? And does each Chain coming out of the splitter allow you to use another 32 lights in a chain and terminated? If you have various links coming out the splitter how do you know what order things come in?

 

Thanks

 

Jerome

[Yorkie]

The splitter doesn't have an address, as lightsource said, a splitter is DMX transparent. Each chain coming out of the splitter can have (theoretically) 32 fixtures on it. All data sent along the single chain from the desk to the splitter will be sent on to each of the chain coming out of the splitter. You can have fixtures with adresses 14, 27 and 403 on chain one, and fixtures with addresses 8, 24 and 475 on chain two.

[HobitLight]

When we say that you can only have 32 fixtures on a DMX line, we mean any device on the DMX line, whether it be a dimmer, moving head, smoke machine, lighting board or splitter, regardless of what it does or if it needs an address.

 

A splitter basically has the same functionality as a Y-Lead for microphoes; i.e. it splits the signal. However, you cannot use a Y-Lead to split DMX and must use a splitter.

 

In other words, you can take a DMX signal from the lighting desk into the splitter and then take multiple DMX outs, each with the same signal. The other day I used a splitter to send a DMX signal to multiple dimmer packs on T-Bars because linking them all together would have meant that there would be a lot of DMX cable.

 

I took a signal into the pack and had four outputs:

 

1) To dimmer pack 2, daisy chained to pack 1

2) To dimmer pack 3, dasy chained to pack 4

3) To scanner 1, daisy chained to scanner 2

4) Smoke Machine

 

Hope this helps

 

EDIT: Beaten to it!

EDIT2:

Each chain coming out of the splitter can have (theoretically) 32 fixtures on it.

Is this still the case if the splitter doesn't have a built in bufferer (is that a word?) or do they all have bufferers anyway?

[Pete McCrea]

One analogy of a splitter could be a photocopier. You put one sheet in the top (your DMX in), and it then copies it as many times as you want (the number of outputs you have on the splitter). The splitter does this very quickly, for each frame.

 

In effect it takes the data in, copies and pastes to the outputs. So the same data appears on each output. It doesn't need an address as it's not reading the data or modifying it, simply passing it through. As the same data is flowing out of all the outputs, it doesn't matter where a device is plugged in, just as long as you don't exceed the 32 devices per line rule.

[Jerome]

Ok, thanks a lot guys much appretiated.

 

Suppose I need to see it work to get my head round it fully but what you've said makes sense to me now

 

jerome

[Pete McCrea]

I think you'll be mightily underwhelmed when you seen one at work. It'll probably be a black box with a power switch and LED on the front, with a load of XLR's on the back of the unit. They don't sing or dance or have flashy lights. They simply pass the data through to the outputs.

[indyld]

If you want to really get technical about the ins and outs of DMX systems, Adam Bennette's industry standard book on DMX has been updated and was released this week.

[Simon Lyall]

Robesperrie... I think reading the blue-room wiki might help you too.

 

DMX spits out its 512 channels (well... normally all 512) when we talk about fixtures, they could take any number of channels of DMX up, say 1 if it's just a 1 channel dimmer (IE controls 'one' light). For things like moving lights though, they take up many more channels (1 for dimming, 1 for shuttering, movement etc etc). You must remember that on each universe of DMX (so on each separate and distinct output of the desk) that you can only put a finite number of fixtures on it such that the total channels used is 512. Look at a pearl for example, and they have 4 outputs (A B C and D or 1 2 3 and 4...) this means you could potentially have 512 x 4 channels (2048).

 

The 32 channels per line rule is different to. This doesn't relate to how many channels each fixture takes, but more to the loading that each fixture puts onto the DMX line. We put splitters (sometimes called buffers) in. Each of these isolated outputs can take 32 further fixtures, great if you have lots of 'low channel fixtures' that all need to be on the same line. Remember to terminate each line though, this stops reflections coming back on the DMX line (think of an output with out a terminator on like a mirror) which can corrupt the packets of information so that the fixtures don't read them correctly and go all strange.

 

DMX signal is pretty much (in very very simple terms) doing this:

 

Channel 001 @ 001

Channel 002 @ 225

Channel 003 @ 050

.........................

Channel 511 @ 255

Channel 512 @ 010

 

 

and then it repeats this over and over. This is a very very simplified view of the signal, it's not really like that, but helps you understand what's happening along the cable. Every fixture 'receives' all 512 channels, it then picks out the ones that it needs. So a fixture that has a starting adress of 30, and takes up 5 channels, will be looking at 30, 31 32 33 and 34. Every fixture also outputs all 512 channels.

 

 

Hope this helps, this is by far not the be all and end all of DMX, and it is heavily simplified.

 

 

Simon

[Rob]

Is this still the case if the splitter doesn't have a built in bufferer (is that a word?) or do they all have bufferers anyway?

Splitters / buffers are different names for the same thing...

[Jerome]

Simon- thanks very much that helped a lot,

 

Indyld- ile take a look at that book.

 

Pete McCrea- I wasn't on about the fact that it may be shiny or pretty, I was on about the fact I may need practical experience with one, you know becaue it helps people! But thanks for the sarcasm... ile put it in my 'another dumb student' lunchbox with all the other things I hate.

 

Jerome