Trouble with DMX signal. Could you help me?

[thebeardedweiss]

Here's my situation:

For the past several years I have used Elation lights in various places around our theatre. I run the DMX from the lights to the nearest mic jack and then connect from the snake (which terminates in our booth) to the light board.

Simple, right? No problems for years.

Suddenly, the signal doesn't transfer through the aforementioned path. (Meaning, they don't respond to commands.) I know the path is good, because a microphone plugged in to the mic jacks and then into the sound board works just perfectly. I also know that the lights are good, because they respond perfectly when I run a cable directly from the board to the DMX jack on the first light in the sequence.

I could just run 150' worth of cable down the hallway and through the fly rail to the lights that are currently mounted over the stage, but my Artistic Director is not fond of that idea.

Has anyone dealt with this kind of situation before? I'm baffled.

I called Elation's tech support, but they weren't able to offer any assistance.

Thanks.

 

[headcrab]

Three things I can think of:
By any chance did you accidentally leave the snake channel connected to your sound board after testing the line, or is the mic still connected at the other end?

Do the lights register the presence of DMX signal? That would nearly eliminate cable issues. It could be that the lights are addressed differently than you think they are.

Do you know where else the cable in the snake goes? If it goes through any intermediate connections, someone else, say IT, could have fiddled with it and disrupted your signal flow.

 

[Pie4Weebl]

Alright, with trouble shooting, the first question to ask is always "what's changed" did you add a new piece of gear to the mix or something which might trigger the issue? Do they not work at all, or is the output showing errors?

All things seem to point to an issue with that cable run. While audio may work on it, DMX is a much more sensitive signal because it is digital.

Have you considered finding the theatre's dimmer rack and then jumping DMX signal from that to the moving lights?

 

[mstaylor]

Running from the dimmers sounds like a much better solution than down the hall and through the fly. Make sure your cables are DMX not mic cables. I know that you have been running through the snake and never had a problem but it was always a crap shoot. I have done the same thing in a pinch and 99 times in a row it works, the 100th it does crazy things. Termination is important but should be related to your current problem.

 

[David Ashton]

one thing to check, if the plugs have pin 1 connected to the plug shell this will give you earth loop problems, which you cannot hear, as in a sound system, but still exist and will cause problems, go through and remove that link.

 

[NickVon]

A buddy of mine has some Elation design spots that he brings in when he designs shows at our venue. We run the dmx from an optosplitter to the OLD house audio snake then up to the instruments. What we discovered and deduced was that it works great. provided we use channel 5 7 and 11. (The only good audio channels before it became the old snake and after we got the New snake) DMX uses all 3 conductors if i'm not mistaken so the audio channels that had terrible ground hum, or noise, or radio broadcast, fail to work as a DMX run for the same reasons that there were terrible for audio. that the shield or third conductor is fubar'ed making the signal to be technical "sketch and poopy."

 

[Lotos]

Snake head ground lift?
Not all snake's include a ground lift... In fact, of the 6 or 7 kicking around the theatre here, I can think of ONE that has a ground lift per input... BUT... If yours does...

 

[FMEng]

So basically, you got away with doing something that should have been un-reliable or non-functional for awhile. Now you are surprised that it failed?

DMX needs to be run through impedance matched cable. Microphone cable is not impedance matched at all. A mis-match results in corrupted data.

Using a mic snake for DMX is hazardous. If someone forgets and plugs the lights into a mic preamp with phantom power turned on, smoke will get released.

 

[thebeardedweiss]

Thanks to everyone who's been lending their experience to my issue. I'll start the new trouble-shooting ideas that have been mentioned as soon as I get the chance.

"So basically, you got away with doing something that should have been un-reliable or non-functional for awhile. Now you are surprised that it failed?"

To the person who wrote that quote, I would like to say that it came across as somewhat snobbish. I had no idea that what I had been doing should have been unreliable. All I knew what that it was working flawlessly and that I designing some great theatre. Like all of us here, I continue to learn as I go through this life. Still, I appreciate your taking the time to help a fellow theatre artist with a technical issue.

Thanks again, everyone.

 

[mstaylor]

What you were doing that was unreliable was using an audio snake send to push DMX. While it can work it really isn't designed for it. DMX compliant cables are twisted differently and rated for 120 ohms.
Using noncompliant cable will work in many situations but can south on you at the worse time. It is always best to follow all the rules, DMX cables and terminators, so troubleshooting is easier.

 

[derekleffew]

thebeardedweiss, you may find a DMX tester, such as this:

Doug Fleenor Design - DMX512 Line Tester

beneficial in sussing out this sort of problem. If the MSRP of $34.45 is too much for you, but you are handy with a soldering iron, a link to instructions for building one's own, almost as good, is at
DIY DMX: Demux, Protocol Converters, etc. - ControlBooth .

 

[thebeardedweiss]

"Have you considered finding the theatre's dimmer rack and then jumping DMX signal from that to the moving lights?"

I'm not sure how to jump signal from the dimmers to the moving lights. I know I could go to the dimmers from the board via the second DMX output, but how could I go about sending that signal to the lights?

Thanks again.

 

[thebeardedweiss]

Let's start again:

Our theatre was not built with DMX lines running through the building with the exception of the one line running from the dimmer to the board in the tech booth. I wish to use the current Elations I have and other items I hope to purchase in the future. What is the best way to get DMX to the units that are located on the stage, approximately 200' and one floor under the tech booth? How have you done it wherever you are, assuming you're in the same boat as I?

 

[sk8rsdad]

Plan A - solve the current issue.

• Confirm you can run a single device via your existing audio snake.
• Assuming that test passes, acquire a DMX repeater/splitter and install it as the first device connected to the audio snake.
• Connect a single device to the downstream side of of the DMX repeater/splitter and confirm it still works.
• Connect the rest of your rig to the DMX repeater/splitter.
• If it continues to misbehave your fault is isolated to the stage rig.

Plan B - fix the facility

• It would help to know more about your existing console, dimmer rack, and future wishes.
• Failing that, install a proper DMX compliant cable run from booth to stage.
• Better yet, run CAT6 between booth and stage, and wherever else you might want a lighting controller or device then acquire one or more DMX gateways to convert between DMX and an IP-based protocol. Over time you can add a switch or router.

 

[GreyNetZach]

Hey I don't know how helpful I can be, but I know that sometimes if there is a coil somewhere in the line, it can cause interference. Also the longer the cable, the more resistance you have. In the past, I have run into problems when running 150 to 200 foot runs with cheaper boards. Signal deterioration can be a huge problem. Now I don't know if somebody has already mentioned this, or I guess asked this question, how many different cables are being used between the source (control) and the destination? Another problem I used to run into was when using three or more cables together, (especially lower end xlr cable), I had a polarity shift somewhere. SOmetimes it would sometimes it wouldn't. Ever since then I've opted to either run a single cable or two.

Hope this is of some help.

 

[church]

it is also worth checking that everything connected to the DMX signal is powered on - if not the unpowered RS485 receiver chips present an interesting impedance to the DMX signal creating some very interesting line impedances.

 

[kendal69]

This was like a three stooges film for me. All the lights were BRAN NEW so naturally they are perfect and it's NOT THE LIGHT. I bought new cable, same problem, I bout Feelnor equipment to the tune of a couple grand.

Out of 20 lights every time I hit a snag I switched out the light SAME PROBLEM, switch out again same problem, WHY , WHY, WHY would the signal only go so far then quit? A dozen people chimed in on site we all gave up and threw up our hands after changing cables, terminators, lights, power cables, ground, and on and on.

Finally when I had time I set up the lights in the warehouse and played the game all over again.

The problem was with four lights being bad from the 20. So every time I switched lights at times I was putting in ANOTHER bad light thinking that light was good, so it must be something else. Switch it again and randomly I would again put in a lemon.

What complicated things was that the four lights would take the signal but not pass the signal through. What that did was make me nuts because when a BAD light was at the end of the string it worked perfectly ( because it was at the end ) so I never suspected it.

After a lot of trial and error I pulled the four lemons and every thing worked like a charm. It was bad circuit boards on the four lemons.

I replaced the boards and all worked perfectly. For me it wasn't the cable, terminators, Fleenor, etc.etc. it was the last thing one would suspect on a bran new light.

 

[lightingguy1]

This was like a three stooges film for me. All the lights were BRAN NEW so naturally they are perfect and it's NOT THE LIGHT. I bought new cable, same problem, I bout Feelnor equipment to the tune of a couple grand.

Out of 20 lights every time I hit a snag I switched out the light SAME PROBLEM, switch out again same problem, WHY , WHY, WHY would the signal only go so far then quit? A dozen people chimed in on site we all gave up and threw up our hands after changing cables, terminators, lights, power cables, ground, and on and on.

Finally when I had time I set up the lights in the warehouse and played the game all over again.

The problem was with four lights being bad from the 20. So every time I switched lights at times I was putting in ANOTHER bad light thinking that light was good, so it must be something else. Switch it again and randomly I would again put in a lemon.

What complicated things was that the four lights would take the signal but not pass the signal through. What that did was make me nuts because when a BAD light was at the end of the string it worked perfectly ( because it was at the end ) so I never suspected it.

After a lot of trial and error I pulled the four lemons and every thing worked like a charm. It was bad circuit boards on the four lemons.

I replaced the boards and all worked perfectly. For me it wasn't the cable, terminators, Fleenor, etc.etc. it was the last thing one would suspect on a bran new light.

That recentlly hapened to me at one of my theatres....

I had 4 LED pars and 2 ADJ Mega Panels on my Expression 3's 2nd universe of DMX.

Here the Order I had my Fixtures in:

[LED Par 1]-------[Mega Panel 1]------[LED Par 2]-------[LED Par 3]-----[Mega Panel 2]

My first two fixtures(the Par and Panel) were fine,but after those teo fixtures everything was messing up.

After calling ETC and ADJ, it turned out that my Panel was sending dirty DMX to my other lights......


And thats the last time I will EVER buy an ADJ product!!

 

[mstaylor]

What did you do to fix the problem? I would try chaining all the LEDs then back track the panels at the end of the chain and see if it helps. If you already have a solution post what you did. I assume the problem was DMX rate that was the problem.