Multiple DMX inputs to a system

[Shez]

Just a quickie...

If a studio has DMX inputs in two or three different locations (that will always be used individually, never together) how is that normally wired up? I imagine the ideal would be a merge unit? Presumably just linking them all together would be a no go as it would act like a passive split?

 

All pearls of wisdom gratefully received <_< (both from an ideal and a it-works-in-the-real-world perspective)

[peter]

I've seen installations where there is a patch bay installed and a couple of short link leads let you move control from FOH to Auditorium, for example. Nice and simple, but effective.

[LXbydesign]

My venue has a "DMX IN" point in 4 locations. but on the DMX patch I only have a distro so right now is only possible to have one desk plugged in at one point to control everything (running through the distro) or its is possible to have one desk patched direct to dimmers - and another desk patched through to movers and / or other DMX devices etc.

 

In answer to your question, yes I think to save you keep repatching DMX lines all the time , just get a merge unit - that way all your DMX inputs are always plugged in and then the O/P on the merge goes to dimmers or a distro which then feeds out to dimmers and other DMX lines etc.

 

So what you will end up with is all your DMX IN sockets "live" so wherever you plug your desk in - you will always have control of the rig - and it also means you could have someone else maybe using another desk or riggers control for example at the same time

 

Hope that makes sense

[bruce]

I'd guess that using a DMX merge might be unnecessary complication.

 

I've seen it done several ways. One was a patch-bay arrangement, where there were several "inlets" in control room, rear of auditorium, on stage, even in a box. It was wired as a star - at the "focus" of the star was the DMX splitter/buffer which then fed out to dimmers and FOH bars etc. Simple and intuitive - just re-plug an XLR to select a different source..

 

Another arrangement I've seen was a "chain". A wall plate in control room adjacent to lighting desk with XLR-5M socket, which was wired through to splitter/buffer and then dimmers. In auditorium, there was a second XLR-5M, which was connected to a short "dangling" XLR-5F on the same control room wallplate. To move desk to auditorium, just unplug it and plug the short dangling cable into the adjacent socket. Again simple, but probably not practical for more than a couple of hops. And no use if the "link" is in a public/exposed area, cos anyone could unplug it.

[Wol]

Sorry if this is blatantly simple and I've missed something, but could all of the sockets not just be connected together? No need for an active DMX merger unit, or any form of patching. My old school used to have 3 different DMX inputs (stage, auditorium, box), and then one output next to the dimmers with all the inputs just connected together.

 

Why would connecting them together (passive merge) act as a 'passive split' anyway? Remember that the DMX input is a 'master' on the chain, and it's the receivers that need to be split properly. If a DMX source is plugged into one socket, then the other input sockets are just.... pieces of wire with no load on them. No current will flow, so nothing will be electronically different apart from a fractional amount of inductance from the extra wire. If you then turned these DMX inputs into *outputs* by using a gender changer, then the two sinks on the chain won't quite appreciate it, but with a master I see no reason not to connect them all together.

 

I will draw a diagram if people don't quite get what I'm on about!

 

Again, apologies if I've missed something!

[David A]

That is correct, Wol.

[JDP]

Does it have to be DMX? If your system has Artnet, Hog Net or MA Net, its even simpler, just re-plug the desk by a network point and your done, as system IPs will stay the same.

 

Otherwise, I like Peters idea about a patch system. I guess you could look at Mergers, but that will be near on £200, you can make a patch system for much less.

[Dmills]

just.... pieces of wire with no load on them. No current will flow, so nothing will be electronically different apart from a fractional amount of inductance from the extra wire. If you then turned these DMX inputs into *outputs* by using a gender changer, then the two sinks on the chain won't quite appreciate it, but with a master I see no reason not to connect them all together.

 

If those pieces of wire are too long, they will act as unterminated transmission lines and you will get reflections from the open circuit far end confusing things.

 

It is not just inductance, but capacitance also, so you will get current flowing, at every point until you get to the end of the line, where by definition current is zero which implies that the energy is reflected back up the line. This is basic transmission line theory.

 

Further, these is nothing that says that a DMX master must be at one end of the chain. Provided it is not back terminated, there is no problem with putting it in the middle (with a terminator at both ends), what you must not do is tee a cable with a long common part of the tee as this causes an impedance discontinuity at the junction and thus reflections. In fact RS485 (the underlying electrical signalling standard that DMX is built on top of) is actually half duplex with multiple masters by design. DMX does not use this capability (RDM does, but that is a bag on the side IMHO), but it does allow for this sort of thing.

 

Personally, I would go with the patchbay or (possibly as easy) a 6 position rotary switch.

 

Regards, Dan.

[Tomo]

Sorry if this is blatantly simple and I've missed something, but could all of the sockets not just be connected together? No need for an active DMX merger unit, or any form of patching. My old school used to have 3 different DMX inputs (stage, auditorium, box), and then one output next to the dimmers with all the inputs just connected together.

Because that's evil, bad and wrong.

 

Evil: It can stop working and the system go 'nuts' for no apparent reason.

 

Bad: It's hard to find the problem when it turns up.

 

Wrong: The specification for DMX and for EIA/TIA 485 specifically state this should not be done.

 

There are three ways to do this, in order of increasing flexibility:

 

1) A single 'daisy-chain' of plugin points. The 'upstream' connector is removed to allow the console to plug in.

- Cheap, but means a downstream connector could be accidentally unplugged and you lose control.

 

2) Multiple plugin points come back to a patchbay. Use a jumper lead to connect the pluging point you want active to the dimmers.

- Slightly more expensive, but makes it much easier to use more than one Universe.

 

3) Ethernet system. Every plugin point is actually a DMX-to-Ethernet converter.

- If a good protocol is chosen (basically 'anything except Artnet'), then multiple consoles can all be plugged in at the same time without problems.

- Either one console is higher priority, they are on different 'virtual' universes, or they merge HTP, or any combination thereof.

[Shez]

If those pieces of wire are too long, they will act as unterminated transmission lines and you will get reflections from the open circuit far end confusing things.

 

This was exactly the reason that prompted me to ask. Dare I ask how long is too long? In terms of pieces of string perhaps?

 

Background: A new build, with the infrastructure put in by a company whose pedigree may be questionable. I'm still waiting for a schematic of how they wired it up; I may pop some plates off the wall in the meantime to see what I can see deduce. I've not seen any evidence of a merger or patch (or a splitter to feed the outlets on each bar for that matter...)

[gnomatron]

We have a small venue - the Pleasance Cabaret Bar for those of you who've served time at the edinburgh festival - that has some odd DMX topography. As far as I can tell, the DMX line for the dimmers starts at a patchpoint FOH, runs via a second FOH patch, into the dimmers, out of them and into a third patch point onstage. Any of the 3 happily control the dimmers - though obviously with only one controller plugged in - as it's a fairly small space, and there's no clever merging or anything going on.

 

There's a separate universe for the intelligent fixtures that seems to do basically the same thing, too.

 

It's giving me a bit of a headache, however, as I want to patch the intelligent fixtures in the room into the same universe as the dimmers without losing the option of onstage or FOH DMX input. Looks like a patch bay is the only way to go.