DMX Misbehaving

[dalton godwin]

Hi All,

I'm a complete novice, and I do the lights for a local Ameteur Dramatics group that runs out of a small village hall. I'm probably going to use all the incorrect terminology here, so sorry for that. I've worked in I.T. for numerous decades, so I'm not completely clueless.

We have recently upgraded our lighting bar from what you might call an analogue system (power packs that vary the voltage to the different fixtures), to DMX. 

We've invested in a good controller and some LED fixtures, as well as a few moving head spots. However, I'm experiencing some really odd behaviour, both at the venue, as well as at home. The LED lights will be fine 95% of the time, but very occasionally they will flicker. I can stop the flicker by moving one of the sliders. This happens regardless of whether I'm using a physical DMX Controller (have tried 4 now), or a laptop & USB Dongle.

As I say, I've managed to replicate the problem at home and recorded it on the video here:

 

It's almost as if the controller can't make up it's mind what address to send. The setup in the video above is a chauvet obey 40, a single data cable leading to an unbranded moving head spot. Remember, the odd behaviour also happens on my non-moving head LED fixtures too.

Has anyone seen this before that could offer advice?

Reading around, most people blame the cables, and whilst I'll admit the cable in the video is actually an XLR microphone cable, the cables that I've hard wired into my lighting rig in my village hall, is definitely proper DMX cables and plugs. Fitting a Terminator on the last (or only) fixture makes no difference 

[cedd]

Could be a few things. First, check the DMX channel chart for the fixtures you're using. Some cheaper generic led fixtures combine multiple functions on to one channel. For example they might say dimmer is 0-100, then strobe is 101-200 etc. Your controller will be fading up from 0 to 255, so will go through the dimmer but then reach some other function. 

Next, check for fixtures (or the desk) being in some sort of sound to light or master/slave mode. Again, cheaper fixtures do this and one fixture will send DMX to the others on the circuit. The theory being that if they were in slave mode, they'd come up with their own (rubbish) light show. Useful possible for bands and DJ's. Utter pain for us. Go through and make sure none of them are doing this. Also, go in to the room and clap loudly. See if anything changes. This'll be a big clue. 

You could also do with a DMX terminator on your line. Some fixtures have them built in, but not all. 

I wouldn't be looking at mic cables as the first source of trouble. Yes it's not "right" but it's also something lots of people do without issue. 

[sunray]

2 hours ago, cedd said:

Could be a few things. First, check the DMX channel chart for the fixtures you're using. Some cheaper generic led fixtures combine multiple functions on to one channel. For example they might say dimmer is 0-100, then strobe is 101-200 etc. Your controller will be fading up from 0 to 255, so will go through the dimmer but then reach some other function. 

Next, check for fixtures (or the desk) being in some sort of sound to light or master/slave mode. Again, cheaper fixtures do this and one fixture will send DMX to the others on the circuit. The theory being that if they were in slave mode, they'd come up with their own (rubbish) light show. Useful possible for bands and DJ's. Utter pain for us. Go through and make sure none of them are doing this. Also, go in to the room and clap loudly. See if anything changes. This'll be a big clue. 

You could also do with a DMX terminator on your line. Some fixtures have them built in, but not all. 

I wouldn't be looking at mic cables as the first source of trouble. Yes it's not "right" but it's also something lots of people do without issue. 

I was going to suggest checking for master/slave functionality, I encountered some LED PARs which can run as DMX/master (Or stand alone master) & addressable slave. They got very confusing as the DMX channels coming out of the DMX master were increased by one to accomodate the control to the slaves. If the line wasn't terminated the regenerated slave control signals could leak back through the fittings.

Having been an audio person all my life I don't own any 'DMX' cables, other than 9501 etc which is the standard DMX grew up in I believe. I've found my self working with others within the industry and recent involvement with local AmDram has been found me becoming the 'everything with a battery or plug expert' within 3 interinvolved groups and have aquired a fair bit of lighting kit (still very much small time) 24 tungsten & 20 LED or so fittings and asociated 4ch dimmer packs & DMX desks etc and all has been linked with a wacky selection of various impedance audio cables and on more than one ocassion with 3 core mains type flex or no terminator. all of that to say I agree with Cedd the little system in your video is not going to be as a result of using the wrong cable.

[niclights]

I've looked at it a number of times but I'm struggling to match the description and symptoms with the video.  In the video I see the pan of a moving head changing between two values repeatedly and a display of a controller which changing between showing two values repeatedly (7/8).

I'm not familiar with the controller - I would guess these are step numbers of a chase or actual DMX values of a channel.  But, in any case, assuming the values relate to the output where 7 = pan position 1 and 8 = pan position 2 then it seems that the fixture is doing what the controller is telling it to.  I would not expect a fixture to be able to affect a controller in any way (other than being damaged by something going horribly wrong electrically).

Similarly, I would not expect 'flicker' to present on a fixture pan like this.  In my experience when you do have flicker on DMX it is too fast for things like pan (on moving heads).  LED's or moving mirrors, yes - in which case it tends to be mostly at the correct value with an occasional very fast flick to a zero value.  Not a regular change between two values.

So it seems, from the video, to be a controller issue.  But you say you have tried different controllers...

[dbuckley]

I’m one of those slackers that doesn’t terminate DMX runs, basically because we have opto-splitters all over the place, so having more than a handful of things on a line is unusual.  Trouble rarely occurs in such a system.

 

Until  it did, just last week.  A fogger lost its mind, and blasted the (small) stage, filling the auditorium, stopped only by a quick-thinking person hitting the machine power switch.  For a while I couldn’t see the stage.

 

Common  things occur commonly, so I assumed it was a show programming error.   But no, the following performance, instead of the machine doing a 5% slow fog, it went straight to flat out.  Fortunately, the innocent bystander was ready for the shenanigans, and hit the power switch.

 

After concluding there was no programm8ng fail, I humbled up to thinking “Do I need a terminator?”  Machine has been flawless since terminator added.

 

 

[Ynot]

Hmmm... (tm)
If a rig works fine for ages without a term in the end fixture and suddenly starts to misbehave, I personally would be looking at what has changed in that run to cause the need for it. It's unlikely that there's nothing else that caused the issue, and my money would be on a cable breakdown of some sort...

[dbuckley]

Different fogger.  I guess this one has some minor difference in hardware or firmware on the interface.

[Ynot]

Ah - so the change was that you replaced the machine, THEN had problems with data?

[Brian]

10 hours ago, dbuckley said:

Different fogger.  I guess this one has some minor difference in hardware or firmware on the interface.

There are two main problems with cheap-arse manufacturers who are too tight to do things properly...

1) Break Timing - each DMX frame starts with a break. You're meant to test for the line being low for 88us to detect a valid break; that's 22 bit times. Cheapskate manufacturers only test for a data byte of 0 accompanied by a framing error. That's only a test lasting 36us, or 9 bits times.

2) Stop bits - each 8-bit data byte is followed by 2 Stop Bits. The cheapskates only check for the presence of 1 of those.

There are also issues around grounding topologies at both the transmit and receive end.

There would be a lot less oddities if people did things properly; it's not difficult.