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Got an interesting phone call today. I'm not naming names, but it brings up a concern.

If you're using intelligent lights, dimmers or other things that you need to directly control, chances are you're using DMX-512 and some sort of DMX controller, be it hardware or software. There are a few things you need to stop and consider:

1: Please, and this is VERY important, you need to be using DMX cabling. While 3-wire microphone cable CAN work, it is rated for 1000-600 ohms and is designed for microphones. DMX cabling is 120-110 ohms and is designed for data applications. 3-wire DMX is wired identical to XLR microphone cabling, which only adds to the confusion. Just because it CAN work doesn't mean you SHOULD be doing it. Please use DMX cabling. The cost differential is small at best, not enough for you to make the wrong decision.

For years I ran my lighting via XLR cabling. Yes, it worked. I'm done with that, I'm only using AccuCable for my lighting DMX signal now. Proof that just because you did something for years doesn't mean you were doing it right.

On the phone call, the guy said he spec'ed AccuCable, but the electrical contractor installed XLR cabling instead, saying it would be good enough. Hmm, if that was me, I would have fired the electrical guy on the spot. Unfortunately, it sounds to me like things got out of control without the caller being directly informed of such decisions. Now the sheetrock is up and making the changes will be difficult. Thank goodness they ran conduit, there's still hope!

2: Use a DMX Terminator. These can be bought or made cheaply. Chances are you only need one, but if you're mobile, make a handful because they can get lost easily. I know, I've lost my fair share of them, but I also make my own, which doens't help any. While ADJ and Elation state any runs over 100-feet of total cable length should for sure be terminated, I go ahead and terminate all runs regardless of length. Better safe than sorry. Actually, sorry isn't the right term. I think the worse that could happen is your signal reflects back into the wiring and you get some wonky behavior, but probably no permanent damage.

3: Protect yourself. Protection can be in the form of an Elation Opto Branch-4 or DMX Branch-4. These devices do many useful things. They can accept either 3 or 5 wire DMX, and output either 3 or 5 wire DMX per "Branch" and there are 4 branches pre box. This also means they can repeat the DMX signal across multiple ports. Also, since they do some sort of optical isolation, electrical badness that may happen at stage won't come back and fry your DMX controller or computer interface, which is very important(but rather unlikely but it has happened). Not exactly cheap, but they are still affortable. Check them out. I own the Opto Branch-4 and I use it with MyDMX. Great combination.

Another good application with my Opto Branch 4, I have one run that goes to the stage, another run that goes to my far tree and floor mounted lights, another run that goes to my near tree, and a 4th run that goes to the back to my follow spots. This reduces a few cables and reduces some clutter. This sort of concept is more for what these units were designed for. But more runs means higher chances of something coming back at you that you don't want. Again, the Opto Branch-4 and DMX-Branch/4 address all those concerns.

4: In regards to MyDMX, it doens't put out the strongest of DMX signals, so using a DMX Branch/4 or an Opto Branch-4 ensures you have proper signal strength for DMX. Again yet another reason to buy one of those units. But, this can be said for other hardware interfaces for entry level products.

5: Interconnectivity. I just mentioned there is 5 and 3 wire DMX. The only thing you need is an adaptor cablem which you can make yourself, should you need it. There is no electrical magic, it's just a 3-wire circuit. Or, units like the DMX Branch/4 and Opto Branch-4 can do this for you. This isn't always practical to have your repeater handle this based on how you're running stuff. That's where you need adaptor cables. If you only run one type of DMX wiring, then you don't need this advise. But if you do make adaptor cables, make sure you make ones in the reverse pin terminations as well so you can adapt your way back out.

6: Polarity: Some lights are wired backwards. Seems odd, especially when the DMX specs state how things are supposed to be wired. This is an issue that can drive newbies insane with their new lighting. It's a tough one to diagnose. Some controllers have a polarity reverse switch, which can help troubleshoot or get the backwards-wired lights working, but then your properly wired stuff will stop working. The solutions are to make and LABEL and DMX cross-over cable(reversing 2 and 3 positions on one end). Make them in pairs so you can revert back to proper polarity. You have other choices. My choice would be to crack open the light and wire the connector properly. One clever uses chose to modify their Opto Branch-4 on one channel to be polarity reversed. While there is more than one way to address the problem, just be warned. LABEL your polarity reversing cables if you go that route.

I am fortunate that I do not have any polarity issues. However, sticking with ONE BRAND does not ensure you won't run into this problem.

Everyone's rig is different. Know your gear, and know you're wired properly and you're ahead of the game. Now go out and have some fun.
Original Post
I have heard of one case that a dmx opto branch 4 could have saved someone's ass. some club or venue had a nice thunderstorm going. Lighting struck near by and all the power went out. when the lighting guy turned his computer back on and tried to open compu live his interface would power on but would not output dmx. The lightning strike and the power going out sent a spike through the power outlets into his DMX line and blew his DMX chip in the interface. and I think one or two of his fixtures blew too. Had he had a dmx opto branch 4 the only thing he would have lost was one output on the opto branch 4. that is why they are optically isolated.
another handy tip for the opto branch 4 is if you have more than one you very easily throw it in the middle of the line anywhere and split it another 4 ways. So if you have 2 opto branch 4's you can do numerous splits of your DMX line. Hope this helps.
Sincerely,
I think Serra Ava told this story. And I am sold on the Opto Branch-4 for many reasons, all of them being the features this unit offers.

If the unit only saves you once, that's all it needs to do. Voltage going back to YOU(at console land) can KILL YOU at console land. Hopefully it won't, the gear is designed to fail before you get fried, but isn't it nice to play it safe?

And as Jingles said, you can do multiple splits. Say, another Opto Branch-4 at stage side and a run going UP to each truss flown. Nicer!!

The combination of the Opto Branch-4 and the DMX Branch-4(anyway you need to use them) is a fantastic combination.
I must have confused that with something else then. Either way, my whole motivation for getting it was based on forum discussions. First, the signal boosting due to MyDMX hardware not putting out a full strength signal was a major concern. Then, the rest just flat out made sense: splitting, repeating, boosting, isolation, 3/5 wire conversion(either in, and both out). It's just sounded foolish to me to not have one.

Either way, the opto Branch-4 is a great bit of hardwaer, and I think anyone of us who is serious about their lighting, this is a bit of "must have" gear. For what it does, man, it's cheap. Good gear, well made, durable, rugged and priced way low. Solves problems and expands options. Without this unit, my lighting couldn't evolve to where it needed to be.

Ideally, I'd like to own a second one to install stage-side, but right now, in my current configuration, there's no room in the rack that lives at the stage.
Yup, both provided electrical and opto isolation.

The Opti Branch is best if you've got rack space available. The DMX Branch/4 is ideal if you're mobile and don't have racks. Place this in a convenient location to handle your splits.

For example, I use the Opto Branch/4. I send a split to behind me for the follow spot locations. I send another run to the stage for those lights and the foggers. Another run for the one tree, then another run that crosses the room to another tree and some lights in the middle.

If I had the DMX Branch/4, I'd toss that on the stage and split the stage lights and the foggers to speed up wiring. That would be in addition to the Opto Branch/4 still doing the exact same job.

Both are good, both are recommended. I prefer the Opto Branch/4 because it is rack mounted and I have one. Wouldn't mind having a DMX Branch/4 as well.

Want to save money? Get the DMX Branch/4 and give up nothing as far as features and functionality. Want a bit neater and have rack space? Then the Opto Branch/4 is the way to go. Win-win either way.
What do you have on a rack onstage? Just wondering what the need of one would be.

Also, how do you test if a fixture has reversed polarity?

I have a terminator/tester (at the end of my chain) that was showing reverse polarity at a show last night, but I wasn't having amy issues except a faint random flickering on some foreign moving washes I use.

Can i put the terminator in each light an see a correct result? Like, run the software (mydmx) to one light and then terminate/testit?

I think I am having this issue too. I have an 18 light rig with movers, pars and effects. And, all 17 work well together and play nice, but my newest moving head which works great solo, when I put it into the chain, any where in the chain, bad stuff starts to happen. Some of the other lights drop the DMX signal, flash rapidly on and off, will not respond to the controller. What can I do with this new light? is the polarity reversed? Is it a simple as opening up the back and switching the wires on the dmx input and output? Or is it just the input, because when I put #18 at the end of the chain I can control, but my last 4 pars, 2 of them get knocked out. 

I can confirm it is not a bad DMX cable all are working, and I confirm it is not the power con adapter (I tried a different one and the same happened) So, now I have 2 choices the input jack is wired backwards  (but, why would it work solo? maybe a better question is can it solo if it is wired backwards? and should I attempt to switch the wiring myself?) or is there something in the light that is faulty (but then it would work at all and it does work perfectly solo). I am leaning back to the input jack...I need some really good advice, do not want to damage any of my existing fixtures, if I do this.

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