How do you detect when a DMX cable (with a device on it) is plugged in?

ariesboy571

Well-known member
I don't have the cred to ask on Electronics Stack Exchange, or I would have done, and after quite a few searches with varying search terms (and not just a few "Hey Gemini" prompts) I've run up against a wall.
How in the bloody hell do you determine whether or not a DMX cable is connected to your rig? I have a feeling that the answer is almost literally under my nose.
Some background: at work, I work with Obsidian DMX repeaters, switches, and suchlike; the Netron series are able to tell when a cable is connected or damaged; when data is flowing normally, even when RDM packets are being sent (there's a little RGBW LED next to the port you're fiddling with, to tell you what's what). I probably don't need that level of functionality, but it did somewhat open a can of worms.
My trusty DVM is able to tell me quite a lot about what the DMX line is doing, while it's running. But I realized that DMX is not the only signalling system I might want to know the health of. I'm thinking voltage and current monitoring, for starters. Those would require sensors and a microcontroller of their own, but I'd like to get to a point that I can answer basic questions: what's the voltage (so I can extrapolate polarity) and what's the current (so I can extrapolate connected-ness).
I think I'm thinking about this correctly -- if expensively -- but...anyone know the answer to this?
It all started so innocently...
 
You could check the cable end-to-end with out of use, or you could check than an RDM capable device is responding from the far end if in use. DMX512 uses RS-485 signalling levels into 120 ohm termination. https://en.wikipedia.org/wiki/RS-485

I would not expect any DMX512 controller to be able to sense if a device is connected unless that device does RDM, and it might be able to sense if a terminated cable is connected (though why bother? its not standard circuitry in most RS-485 driver chips).
 
Hey Mark!
Here's what I've got so far:

1. A cable without anything plugged in to it changes the capacitance of the circuit, causing the capacitance to rise slightly. The amount of capacitance depends on the length of the cable and the quality of its insulation. This can only be measured when there is no potential applied to the cable AFAIK. (Caveat: If there is a way to measure this, I'd love to know what it is...) [UPDATE: there *is* a way to measure this. And I have the software classes that do it already...hot d4mn, I *knew* that would come in handy one day...]

2. If you connect a device (and there are actually termination resistors in it; some lights don't even have those) but don't power it on, you will (generally) see a relatively visible current flow -- 20mA per fixture with my Chauvet Par-Quad 7s, for example. If you then power those fixtures on, the current flow drops to about 5mA per fixture. Small,...but measurable.

I spent the weekend noodling a prototype design together, and got a test rig for it on the way here this morning. I want to take a dive into the dynamic characteristics of a DMX line in service. To be brutally honest, I've wanted to do this for *years* but could just never make the case for spending the time...until now. My current employment depends on DMX, my side hustles for the last 10 years have depended on DMX, so I think it's high time that I actually take a peek under the hood and have a deeper feel for what my livelihood depends on.

Also: I really, really wish that the people who design and build these fixtures followed standards. I realize it's costly to implement optical and galvanic isolation, and proper termination. Trust me, when I say that loads of LDs, L1s and L2s all over the world will bless your name, when the lights come on...
 
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Hilarious...you know, I didn't even know what was the proper name for such a thing, until you said it. It seems that you can't really use them on live, signal-carrying lines, though. I suppose that if you knew when a data sequence was ending, you could temporarily halt the data flow for a very short time (the length of your cable run X 2 nanoseconds, for example) to take a measurement? Although let's say that your DMX run is 250 feet long, so a reflection would come back in about 0.5 microseconds, and I'd bet that nothing on the line would even really notice the pulse...
 
About 0.7µs, not 0.5µs, with standard cables and dielectrics - your calculation works for air or vacuum :)

Just seeing the waveform on a 'scope for a live system will say a lot as unwanted reflections would show up.
 
Given the very light loads presented by RS485 receivers, I can't think of any way to distinguish between a terminated DMX cable with a receiver on it and one with none. Or many.
 
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