Using a DMX splitter

[steventaylor]

I am about to use a DMX splitter for the first time. Do I have to terminate each branch from the splitter or if the splitter is the end of the DMX chain does it self terminate? I'm a little confused and in fact at present my DMX chain ends with a MAC250 and I haven't needed to use a terminator on this.....is this normal?

 

 

Thanks

 

 

Steve

[Ynot]

Ideally, yes - if you have anything plugged into the output of a splitter you should terminate the end fixture.

It's probably unnecessary to terminate the unused outlets, but won't do any harm to do so.

[lxdad]

Hi,

 

Basically you should run a cable from your desk to the splitter and then off out of the isolated outputs (not the THRU) to your fixtures. Don't worry about termination unless you have problems, it is very over-rated!

 

I hope that helps.

 

LXDad

[paulears]

Backing up your PC is also very overated, until it crashes and you realise it would have been good practice. DMX termination is often not worried about until jst a single fixture/cable in the system goes a little astray, and the whole thig starts to play up, leaving a huge amount of faultfinding to do!

[Cormac]

DMX termination is usually a poor fix for bad cable,improper use of turnarounds 5pin>3pin and 3pin>5pin and more than likely bad fixtures.

You should not have to use a termination plug unless as a last resort and it is not standard practise.

More often than not 5 pin DMX terminators create more problems than they solve.

just to add most modern DMX opti splits do not require DMX termination and it will cause problems by doing so.

 

C

[paulears]

RS-485 has some quirks - one bit says

Ideally, the two ends of the cable will have a termination resistor connected across the two wires. Without termination resistors, reflections of fast driver edges can cause multiple data edges that can cause data corruption. Termination resistors also reduce electrical noise sensitivity due to the lower impedance, and bias resistors (see below) are required. The value of each termination resistor should be equal to the cable impedance (typically, 120 ohms for twisted pairs). Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance.

 

My thoughts are that if the designers felt it was a good idea, put it in the spec, and recommended the use of terminators - it seems silly to diss them simply because most times, you can get away with it. I don't always bother, but from time to time I have had some odd things happen (and yes, there probably was the odd polarity or 5-3 adapter in the line) and the terminator worked?

 

Only today somebody banged on my door asking if I had one to cure a problem. I assume it worked, he hasn't been back!

[Peter Chivers]

DMX termination is usually a poor fix for bad cable,improper use of turnarounds 5pin>3pin and 3pin>5pin and more than likely bad fixtures.

You should not have to use a termination plug unless as a last resort and it is not standard practise.

More often than not 5 pin DMX terminators create more problems than they solve.

just to add most modern DMX opti splits do not require DMX termination and it will cause problems by doing so.

 

C

 

Your opinion does not appear to agree with most of the industry, nor the standards recomendations.

 

You have to terminate because missing or faulty termination is probably the biggest problem in DMX512 systems. EIA–485, the electrical basis for all versions of DMX512, requires the transmission line (i.e., the DMX512 data link) to be terminated. This is normally done with a resistance matching the characteristic impedance of the cable (see the Cable FAQ) which is ideally matched to the circuitry. Most consoles and the input/output ports of splitters have built in termination. Many receiving devices have a termination switch engaged when it is the end of the data link, disengaged when it is not at the end. Check the manufacturer documentation.

from http://www.usitt.org/standards/DMX512_FAQ.html#FAQ_14

 

Peter

[mark_s]

Basically you should run a cable from your desk to the splitter and then off out of the isolated outputs (not the THRU) to your fixtures.

 

Just out of curiosity and to further my knowledge a bit, what's the problem with using the through?

[tokm]

Just out of curiosity and to further my knowledge a bit, what's the problem with using the through?

There's nothing REALLY wrong with it. Just it's not opto-isolated like the rest of the outlets are. Just doesn't give the same protection from DMX errors that you'd get if you went from an output. In most cases its just a looped cable inside the unit from the input..

 

Generally I only use the through if connecting across to another splitter.. Only to a DMX device if I really have to.

 

If you have a free properly isolated output why not use it!

 

RE the whole termination thing, I've always found its best to stick one on the last fixture/dimmer. Why, because why not.. If you have them, use them by default and only remove if they do for some reason cause problems. Regarding not using them/not bothering, well, its a bit like the whole DMX/Mic cable thing. Everything could be fine... or it could not. Just hit or miss.

 

Tom

[soundo26]

DMX termination is usually a poor fix for bad cable,improper use of turnarounds 5pin>3pin and 3pin>5pin and more than likely bad fixtures.

You should not have to use a termination plug unless as a last resort and it is not standard practise.

More often than not 5 pin DMX terminators create more problems than they solve.

just to add most modern DMX opti splits do not require DMX termination and it will cause problems by doing so.

 

C

 

I don't know what sort of level of experience this poster has, but this is bad advice, according to the USITT DMX standard which is written by the body who invented the standard, DMX lines must be terminated. Just because you can get away with not terminating on many occasions doesn't warrant the kind of comment posted here, you must use termination and it SHOULD BE standard practise. Just as well as you should also be using proper data cable not mic cable.

On another note, do be careful with some DMX splitters which have a phase reversed output which should not be used at the same time as the standard outputs (if the fixtures are not phase reversed).

[Ynot]

On another note, do be careful with some DMX splitters which have a phase reversed output which should not be used at the same time as the standard outputs (if the fixtures are not phase reversed).

Interesting statement - why would you say that?

Not one I've heard before...

[peternewman]

He may be referring to some which from the looks of the panels have a passive Y split between phase reversed and normal. So if you need a phase reversed output you don't need an external reverser. However using both would be equivalent to using a Y split on one output, with a phase reverse on one leg, which would be a bad thing.

[Tomo]

just to add most modern DMX opti splits do not require DMX termination and it will cause problems by doing so.

There is exactly one situation where using the correct terminator will cause problems, and that is when one data line in your cable is broken.

In this case, it's merely flagging a real fault!

 

In all other situations, the use of a 110-130 ohm resistor across pins 2 and 3 at the end of the chain prevents the majority of data problems at source.

 

Problems due to a lack of terminator are also rarely seen at the end of the line - you'll usually see them at one or more fixtures along the chain, and which one may vary depending on what data is going down the line.

I have seen many problems caused by lack of termination, and none caused by proper termination.

 

Finally, the other reason why you should always terminate is that it makes faultfinding easier:

If you're not sure if the cable you've got is properly connected at the far end, unplug it from the console/splitter and measure the resistance between pins 2 and 3 at the console or splitter output.

If you get ~120ohms, it's connected. If you don't, then it's either not terminated or not plugged in somewhere.

[Cormac]

Your opinion does not appear to agree with most of the industry, nor the standards recomendations.

Hi,

just because someone does not agree with most of the industry does not make them in-correct as far as I'm aware? Please correct me if I'm wrong.

Some standard recommendations as we all know are perhaps a little outdated and do require some updating but being able to copy and paste is of course fantastic.

I am going from my own experience as a lighting tech and can say I don't use terminators anymore and if I do it is very very rarely.

Maybe I deal with equipment that works or am possibly using equipment that is of a standard that does not require them(terminators),granted my work may not be on the same level as others posting who may require 1 terminator for every moving light on an event but every show is different.

I'm not trying to get "away" with anything it's just not standard procedure on the jobs I do which range from small corporate events up to larger events/festivals with 200+ fixtures.

If anyone needs clarification of the sort of experience of work I do I'd be more than happy to give them a list of events I've been involved in or a link to my website as this seems to be an issue with some posters but I'm sure that won't satisfy some in any event.

 

Kind Regards as always C

[Bryson]

It doesn't mean that they are automatically incorrect. But it means that they are much more likely to be incorrect...