Help me troubleshoot a DMX issue


Well-Known Member
In my blackbox, I got a grant for all new LED fixtures a few years ago. They've been flawless. I set up 5 of our old incandescent fresnels on some edison dimmers as house lights; three on one, 2 on another to avoid overamping. They've worked great for four years now.

We open a show tomorrow and the two house-left fresnels on the one box started coming on randomly. It's not with a cue, just they sometimes come on in the middle of the scene. The board says they're off. Trying to command them up or down does nothing. Thinking I had a bad dimmer pack, I replaced it with another one. Same issue. I tried re-patching the address to a different channel, same thing. I tried choosing different addresses and re-patching - same thing. Tried a third dimmer pack - same thing.

My ME suggesting maybe we have too many DMX in the chain, but I only have 5 Altman Phoenix RGBW and both of those edison dimmers on the chain (so only 58 addresses) which has never been a problem before.

Early ION console
Donner wireless DMX to the grid, then wired between instruments
Instruments include Colorsource Pars, Altman Phoenix, and four Clay Paky Alphas
Leviton 4-channel edison dimmers.

Like I said, it has been flawless for 4 years. I would love to blame the wireless DMX, but I think it would cause more global issues instead of affecting just those two addresses on a single dimmer. Anyone want to venture a guess before I randomly light audience members in the middle of the performance tomorrow night?
Likely a bad cable, where a solder connection has started to go cold. Try swapping some of the Dmx cable between the wireless receiver and the dimmer pack that’s acting up.
I think you have verified whatever is going on that all of your dimmer packs react the same way. Nothing was mentioned about termination. Rerouting or swapping cables could help.
I'll try a new cable. I also had a friend lend me a couple cheap-o LED pars that I can swap in and eliminate the dimmers altogether if all else fails.

I have never used a terminator and it never gave me fits. I made one once to troubleshoot a different issue (which ended up being a bad DMX chip in a rented console) but never needed one. What is their purpose?
The proper use of DMX indicates a terminator at the end of all DMX runs. Some dimmer packs have a switch to throw if it is at the end. Other devices will need a terminator plugged into the output if it is last. Weird, unexplained stuff can occur when DMX runs lack termination. Is maybe interference what is happening here? Could be.
If you remember back to your high school physics class....a wave on a fixed rope reflects back out of phase....and a wave on a free end rope reflects back in somewhere in between fixed and free (that happens to be the characteristic impedance of the transmission the case of DMX512, 120 ohms) there is no reflected energy. When there is reflected waves, any receiver has no way of knowing if the signal it detects is the intended signal or a reflected one. Hence, why termination is needed.
If possible you can try running another line out of another output of your console to verify the problem. Remember that it will be by default on another universe (patch address 120 as 2/120 for universe #2 on the second output) and that you'll want to use known good DMX cable as I assume there will be quite some distance involved.
Another option is to use an OPTO-Splitter (Optically Isolated DMX Splitter) to split off before the first fixture, so you can have multiple runs to use to do diagnosis. Honestly depending on cable length, type of cables and number of fixtures you may want a splitter like this anyways. Always use an OPTO splitter as it will help reduce problems and prevent bigger ones than a splitter that is not optically isolated.

Agreed, termination should always be a thing... Built-in termination is a thing that exists but never rely on it. You would be surprised how many DMX issues I've fixed by throwing in a terminator.

When looking for a bad cable, well that can be fun... If the problem is showing on fixtures near the end of the chain running a fresh line as mentioned above makes life way easier. When all the fixtures are acting up it's easy to split the chain in half, is that half OK? If so the problem is LIKELY (not sure) to be after the middle point... Split what remains in half, does the problem start again? Well your problem is in that section!
The real issue is some of these issues only surface over the entire chain.

Good luck and God speed!
You'll often hear people say they don't terminate because it causes things not to work. Thing is, a system that's faulty and on the edge of failing (maybe running single ended as previously described) will be pushed into failing by a terminator, so that's actually a good thing.

There was a case on farcebook a while back where a guy said his system started to flash when he added a fixture. I asked if the line was terminated and he came back and said it hadn't been but he'd found a terminator, installed it and the flashing had stopped. "Do ya think that was it?" He asked.

Well, err, yeah.
So today I borrowed a terminator and it didn't fix the problem. I also realized that the dimmer pack that was acting up was not the last thing in the chain and the lekos after it weren't acting up. That's nothing definitive, but just more info. I replaced all the DMX in that chain and the problem remains.

I threw a couple of the borrowed LED cans and bypassed the dimmer pack. Problem right now is that I have no documentation on the cans and they're the cheapest possible overseas stamped steel DJ-style things. No brand name, no sticker with what they are, and I'm not yet a genius with an ION. I figured out the DIPs and got it to respond... but no matter which generic LED I choose from the list it isn't right. I got it to be blue, but I ran out of time.

For opening (which is going on right now and the audience is laughing like crazy in the background) I just tilted a par to light that part of the house and added it to the house-up cues, then programmed one of the movers to pan over and cover the dark spot I created. It's not ideal because there are two cues where the mover is used in another spot, so I had to alter the fade to avoid looking like motion. LD is here now to hopefully fix things before tomorrow.
Another option is to use an OPTO-Splitter (Optically Isolated DMX Splitter) to split off before the first fixture, so you can have multiple runs to use to do diagnosis. Honestly depending on cable length, type of cables and number of fixtures you may want a splitter like this anyways. Always use an OPTO splitter as it will help reduce problems and prevent bigger ones than a splitter that is not optically isolated.

Just had a group in a couple weeks ago to get a quote for a proper grid instead of the hodge-podge of truss and sched40 pipe hung from the ceiling. The quote will include a full grid with 8 nodes of cat6, 20 circuits, and cable management.... and an opto.
It is best practice to terminate DMX.
One of the first steps in troubleshooting a DMX issue is to terminate the line, or change terminators.
That being said, I don't practice "best practices" .
In 30 years, I have never regularly applied terminators.
I have only had one instance where termination actually solved a problem.
In 1999, we had 20 fixtures on about 1000' of DMX that had some flicker issues that termination fixed.

Maybe I'm just lucky!
So....just a thought to add to the Termination Determination Question - What should we be doing with a RDM system? Now we are talking about two way communication on the DMX line.

2.2.1 ANSI E1.11 - EF1.0
Systems that comply with this standard fall within the scope of E1.11 Annex B - EF1. Systems
that send data in both directions on the primary data link are classified as EF1. These systems
shall use the primary data link for both the NULL START Code DMX512 packets and also return
data controlled by the use of Alternate START Code packets.
Use of the secondary data link is beyond the scope of this standard.
One end of the primary data link shall be terminated as specified in E1.11.
The other end of the primary data link shall be terminated by a line biasing network. The
requirements for line biasing networks are in Sections 2.4 and 2.5.
Command ports shall include the line biasing network.
Command ports may be designed with means to disconnect the line biasing network. In this
configuration a line biasing network shall be provided by other means.
The idea of talking about "directions" when it comes to cable signal propagation is misleading. A signal is a change in voltage. Termination is required when the length of the cable exceeds the length of time it takes to transmit a bit of data. It's a bus topology where, in most cases, one end of the bus is connected to a device that typically transmits, such as a console. In an RDM system, any device could be the transmitter. That doesn't change the nature of the bus topology. A terminator is still needed under the same circumstances.

This article provides one of the most accessible explanations I have read about RS-485 signaling and termination. For those who may not know, RS-485 is the foundation of DMX.
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Everyone has suggested everything that I had thought of. There is one method of fault finding that may help you although it is a bit of a ball-ache. Take the last device from the line and stick it on a new cable direct from the lighting board and stick a terminator in DMX out on that device. If all goes well stick another device with a new cable and take the terminator from the previous device you just tested from the faulty line and keep on doing this. If you have a lot of devices on the line this can be a huge job especially if you have lights screwed up high. Hopefully you will be able to identify what is giving you grief. Hope this helps, best of luck (I've been where you are and it is bloody annoying) Keep us informed on how you get on please. :¬)
So is there a length of cable under (or over) which the bounce is a problem, or is it endemic to any length?

The reason I ask is that I have never used a terminator in the black box where the longest run is 25' of cable (wireless receiver in the grid to five 5' cables between lekos). I've also never used a terminator in the big proscenium where I have 150' from the booth to the pipes, then a various assortment of 10s, 25s, and 50s going across each batten. I have never had an ounce of DMX problems (with the obvious exception of this one)

I'm down for getting some terminators, in fact I've made a few for basically free, just trying to understand if there are some configurations where it would be more or less of an issue.
Every source feeding a transmission line has an impedance. Every transmission line has a characteristic impedance. Maximum power is transmitted when the source impedance matches the receiving impedance. When you're not transmitting power (i.e., feeding a signal into an open circuit) some of the power will bounce back fromthe infinite impedance of the open end of the line. How much and when is the issue. The higher the frequency/pulse rate is, the shorter the line which will have a reflected signal which will conflict with the source signal. In the case of the pulse rate of a DMX signal, if the line is short enough so that the reflected pulses arrive at a point on the line (i.e., the input of an instrument which is "watching" the pulses and using them to control something) while the greater part of the forward-moving pulse is still there, no harm-no foul. If the reflection is much later than that, it will "confuse" the instrument watching for pulses.

DMX-controlled devices "bridge" the transmission line (not match it) which is why you can have multiple DMX devices daisy-chained on the same transmission line. The source's matching impedance is not present in any of the devices using the DMX signal. (Unless the device has a "termination" option of some kind.) In short: Be safe, ALWAYS terminate ANY transmission line (DMX, audio, RF) with its characteristic impedance., either with a device having the proper input impedance or with a load resistor.

Sorry I do not have the brain to tidy this up right now. But here are some scope shots someone posted somewhere years ago. Shows you the differences in signal. The small "reflected" one shows you what signal reflections look like. The thing with a digital signal like DMX is that you can not terminate and do all sorts of things "wrong" and it can still work without a problem.... or it can not. Doing all the "best practices" is about mitigating the risk of it going wrong at the wrong time and messing up your show.
Notice how the unterminated signal has overshoots, this can really mess up the receiver depending on how "big" those overshoots are, and completely mess up reception of the packets. The Reflected you can see all the garbage that can start happening that will REALLY make the receiving device go for a loop.


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