DMX Signal Feedback Mystery

Generally when using different types of fixtures I will always use a splitter and run a different line for extra fixture type. Some different fixture types don't play nicely with others. I always try and keep dimmers off of a mover and LED leg of DMX, just because there is a lot going on inside a dimmer that can cause issues. SCRs, chokes, transformers, all kinds of power running through them. Sounds like something in the dimmers don't like the LEDs. I would get a splitter and that should fix the problem right there.

I guess what gets me is that the issue still occurs even when the dimmers are powered off. As far as I know, DMX signals just pass directly through the units passively when they are powered off. All the fixture is doing is just 'tapping in' to listen to the signal and not necessarily modify it when powered on.

It does make sense though, that all of that power could be inducing interference and even so after being powered off because of components like the capacitors take a while to discharge.

I looked into the splitter/isolator, but I'm not ready to jump on that yet until I know that all of the equipment is functioning as it should.

Everything is fine when plugged directly into the controller, correct? The other issue might be the controller isn't putting out a strong enough single. How long is the line in question?

Also, whenever you have a connection, there is greater chance for interference then if it was just a single line. This is one of the reasons why it is considered a better practice to run one 100' line over 2 50' lines, or 4 25', etc etc.

No two DMX lines are the same, so you can be getting interference from any where, and just not realize it. At least DMX is more robust then audio or video. I feel sorry for those guys some times, j/k .

There again, every line will be different. As I stated before, I have had issues with more then 6 or 7 LED fixtures strung together on a line. Sometimes I can get more, but it generally runs out at 6. These are Coemar LED Par Lites. With Elation Opti 30 LEDs and Design LEDs, I have done as many as 12 on one line and been fine.

So different fixtures react differently. Adding in other fixtures to the mix makes for more combinations, which might mean more headaches. Moral of the story, I have found it just easier to always bring at least one splitter with me to every gig I do, even if the clients aren't paying for it. It saves me cable and headaches, which in turn saves time and allows for a better show. Happier client means I can charge them for that splitter next time. Better safe then sorry as well.

Makes sense. Thanks for the thoughts. Another brain in on it helps out a lot.

Everything is fine when plugged into the DMX console as long as the dimmers are no where in the same daisy chain.

The total DMX run is a about 60 feet or so. The proprietary controller for the LED panels is within 5 feet of the controller and still acting goofy.

Good news is that I have a new LED controller on the way, as the manufacturer thinks that their controller may be bad.

I will certainly be specifying a splitter into all of my future work. It is interesting how this whole fixture signal incompatibility is like an unwritten fact in this business that never gets talked about until there is a problem.

While DMX is a protocol specified by USITT, there certainly seems to be a whole lot of variation in how a fixture treats the signal protocol.

I can only imagine what the issues would have been had the protocol been expanded into the full 2-way process that it was envisioned to be on 5-pin cabling.

Thanks again.

No problem. Every manufacturer will do something a little different. This is evident in the fact that some use 3 pin DMX, some use 5 pin, some have both, and now some have 5 pin and Ethernet. Even LEDs have this problem, some are 4 pin and others 5 out of the power supply.

Actually DMX signals are sent at a specific baud rate... LED's with complicated menu structures will not react correctly to a slow rate because of the speed of the information coming from the controller. It will work fine when using dimmers and moving heads because they only need one signal to get to a spot where complicated LED menu structure units usually use pulse with modulation witch need a continues signal

Baud rate isn't specific on DMX. Higher end controllers allow you to adjust the rate actually, though this is slowing fading away. Older style dimmers are a great offender of not being able to work with a higher baud rate, this would cause ghosting in the dimmer system itself.

LEDs do in fact work on a lower rate. When I use to work at a college, I had to set the baud rate to the lowest possible do to the old Colortran ENR dimmer racks. If not, the lights would ghost. When I used LEDs and movers at this setting, no problem.

DMX also has variable transmission speed as well. But again, only on higher end consoles can you adjust this. The fact that the light was freaking out and offering no control at all leads me to believe it is feedback or a signal strength issue which is unlikely do to the short run and use of 120 ohm cable. Also, the fact that he can plug in one at a time or whichever is first in the loop also seems more like a feed back issue. If it was the baud rate, which he can't adjust with his particular console at this install, he would still have some control or ghosting on the lights on the dimmer. And since this is a newer dimmer and console, the baud rate is most likely set very high.

No bashing intended DJ Scruffy, I am actually surprised that someone thought of that. Just wanted to clarify what happens with wrong baud rates is all. Excellent thought though.