DMX Terminators

[jackblakemore]

I have set up a lighting rig and want to know if splitting the DMX line is good/common practise. I have done this as it stopped me from having to run a massive cable from the penultimate fixture to the last one and it SEEMS to be working ok. Obviously in this case there will be two fixtures at the end of the line, do I need to use a terminator on both of them??

 

Thanks

Jack

[bruce]

It breaks the spec, and is working by accident and good luck rather than by design. You should not rely on it - it is certainly not "good/common practise".

 

In this situation, an active DMX splitter would be the way forward.

 

There's lots of discussion on this in the archives - have a search....

[pritch]

Depends on how you've split it. If you've made a Y-shaped adaptor, this would be considered bad practice but hey, if it works, it works. Terminating each leg would in effect halve the resistance of the terminators, but in effect we're outside what specified in the DMX standard with this arrangement anyway.

 

If you split the line correctly using a buffer / splitter, then yes, you should terminate each leg.

[Tomo]

What you really want is a DMX optosplitter - it's a dedicated, powered box that has one DMX input, and two or more DMX outputs.

Sometimes a DMX passthru as well, although this 'output' is less useful than the buffered outputs.

[soundo26]

I have set up a lighting rig and want to know if splitting the DMX line is good/common practise. I have done this as it stopped me from having to run a massive cable from the penultimate fixture to the last one and it SEEMS to be working ok. Obviously in this case there will be two fixtures at the end of the line, do I need to use a terminator on both of them??

 

Thanks

Jack

 

Here we go yet again!!

 

DMX should only be split using a buffered splitter NOT a Y lead. there should be one DMX chain with the last fixture being terminated.

 

the output from the desk usually goes straight into the dimmers then from the dimmers to the fixture chain, this not absolutely necessary but is usually done because desk output is 5pin and dimmers are 5 pin, many of the fixtures these days are using 3 pin (which is not strictly DMX standard) but connecting in this order reduces the use of 5-3 pin convertors all over the chain as far as is possible.

 

All DMX cabling should be made from screened twisted pair data cable NOT mic cable.

 

Terminators should be made using a 120 ohm termination plug which are extremely easy to make.

 

It bothers me that much of the attitude in the replies is "hey if it works" etc. Any other way of connecting is wrong, if it is wrong then don't do it! If you want to do the job, do it right and professionally, not like an amateur, bodging it and hoping for the best!

 

Moderators, this comes up so often here along with many other old chestnuts, wouldn't it be worth having a "basics section" in which these sort of issues could reside for perpetuity to save them cropping up ad nauseum?

[bruce]

Moderators, this comes up so often here along with many other old chestnuts, wouldn't it be worth having a "basics section" in which these sort of issues could reside for perpetuity to save them cropping up ad nauseum?

 

Do you mean something like the Lighting FAQ ? - specifically the bit titled "Do I need to terminate my DMX line? ". Perhaps adding a statement about "no branches" is required.....

 

It bothers me that much of the attitude in the replies is "hey if it works" etc.

 

Ummm - as one of the first people to reply, I take exception to that! If you read the replies, you'll see that only one says "hey, if it works". (and it wasn't me... :D ) - and even that posting points out that it's bad practise and outwith the standard.

[soundo26]

Moderators, this comes up so often here along with many other old chestnuts, wouldn't it be worth having a "basics section" in which these sort of issues could reside for perpetuity to save them cropping up ad nauseum?

 

Do you mean something like the Lighting FAQ ? - specifically the bit titled "Do I need to terminate my DMX line? ". Perhaps adding a statement about "no branches" is required.....

 

It bothers me that much of the attitude in the replies is "hey if it works" etc.

 

Ummm - as one of the first people to reply, I take exception to that! If you read the replies, you'll see that only one says "hey, if it works". (and it wasn't me... :blink: ) - and even that posting points out that it's bad practise and outwith the standard.

 

sorry it wasn't meant to be a dig at you, it's a general thing across the board I have about bad workmanship, no offence intended!

 

Also, I know there is the FAQ section but judging by the amount of times this and similar topics appear, just wondered if something more prominent could be arranged, just seems that many peeps don't read the faq.

[GaryP]

but hey, if it works, it works.

I have done several presentations about DMX over the years and in these I offer a parallel.

Do you have insurance on your house / flat? Most people will answer yes.

When was the last time you had to claim on that insurance? Most people will say never...

Therefore you don't really need insurance do you??? Well you could go without insurance and save money but then one day........

 

Now people use DMX with Y splits and use mic cable for DMX because it has always worked and it is cheap. But then one day.....

 

You should never use mic cable for DMX.

You should always use splitters instead of Y splits.

You should always use splitters that provide isolation if your dimmers, intelligent lights, console or other DMX devices you are controlling may come off different power sources

All the above costs more. Call it insurance.

Because the cost of the show not happening can be very high in more ways than one.

 

Regards,

 

Gary

[soundo26]

Well said!!!