DMX Stage Lighting Systems

Wednesday, July 18th, 2007 - Stage Lighting Control - by:

If you have any experience of stage lighting you have probably used the DMX512 protocol to control your lighting rig. Apart from being able to control your dimmers, DMX lighting control is at the heart of intelligent lighting, moving lights and accessories. This article explains the basics of a full DMX stage lighting system while busting some of the myths surrounding DMX lighting control.

An Overview of DMX.

DMX XLR connector
Image by VeldaZ

Digital Multiplex (DMX) was developed as a standard digital theatre lighting control which replaced the older analogue systems that were used to control the dimmers in a stage lighting rig. Despite the advance in use of ethernet networking at the top of the professional lighting industry, DMX is still the best answer for most lighting control applications. DMX had the advantage in that a total of 512 (hence the name DMX512) dimmer channels could be controlled using only a 2-core and Ground signal cable. The other plus point to DMX was (and still is) the fact that a signal cable could be daisy chained from one DMX unit to the next and the next until all the lighting equipment was connected by one single chain of signal cables. This is particularly important when controlling moving lights as it minimises the cable required.

How does DMX work?

This article was originally called “DMX Stage Lighting Systems – Get Them To Talk” but, apart from being a bit of a lame title, this actually misses the point of the serial DMX signal. Unlike modern networks, DMX lighting fixtures and their controller do not all “talk” to each other. (If you would like a DMX system that does, take a look at our RDM DMX article)

When a DMX Lighting Control talks – everyone else listens!

Information about each “DMX Channel” (in the old days, a dimmer number) and it’s level (0 – 100%) is transmitted down the DMX “Universe” cable and each DMX stage lighting fixture, moving light or smoke machine listens for it’s own part of the signal stream and ignores everything else. The signal is then transmitted over and over in “packets”, giving a regularly updated stream for the rig to obey. The lighting console recieves no information in this one-way street.

In order for all the DMX stage lighting fixtures to have their own part of the signal stream, each one has it’s own “address” which is set on the fixture using buttons or switches. If a dimmer channel has a DMX address of 001 then it listens for the 001 part of the DMX signal then obeys the “channel level” value, 75% for instance.

Once all intelligent lighting fixtures, dimmers and accessories are connected up and “addressed” the lighting desk can control each part of the rig indiviually using their own unique DMX address.

Masters of the Universe

Back when we only controlled dimmers using DMX, life was simple. 1 DMX Channel = 1 Dimmer No. Then we started to use more complicated fixtures, moving lights and intelligent (?) lighting that needed more than 1 DMX channel per fixture. This means that fixtures are assigned a DMX “start address” which is the first channel in a sequential batch that the fixture listens to. If your fixture uses 6 DMX channels and you set it to a “start address” of 001 then it listens to channels 001,002,003,004,005 and 006. Your next free address for another fixture is then 007 because if you set it to 006 then the “channel overlap” would create a conflict of control. Setting fixtures to the same start address can be useful some circumstances and is a common method of DMX fault finding.

With many DMX moving lights requiring the use of 20 or more channels, those 512 don’t look too many now, huh? A few moving lights, 100+ Dimmers, Strobes and a couple of smoke machines and you’ve run out of channels already! The solution to this problem is to connect and address some of your equipment on a second DMX “universe”, a different signal stream with even more cables. Many lighting control desks have more than one DMX output these days and the principles of fixture addresses and channel numbers apply to this, and subsequent, universes. The first DMX channel on a second universe is also 001 and each DMX universe is a totally separate stream, independent of each other.

Myths busted about DMX stage lighting systems

  • It does NOT matter which order you connect up all the fixtures in a DMX chain, so long as they all have a signal going to them.
  • The DMX cable chain should NOT return to a create circular control signal loop. (If your lighting desk has a DMX IN connector, leave it alone!)
  • A DMX signal cannot be split into two using a Y-cord. A DMX splitter/buffer (somestimes called an Optisplit) is required.
  • If does not matter which order your fixtures are addressed in just as long as they are unique and don’t overlap. Make sure you find out how many DMX channels each of your fixture uses for control.
  • A DMX chain will usually work without a termination resistor although it is recommended by the equipment manufacturers and can solve some tricky problems with a complex rig. Here is a good article on why you should terminate your DMX lines.
  • The “DMX received” indicator on a piece of equipment does not neccessarily mean that all is well with your DMX signal.
  • You can set two pieces of DMX equipment to the same start address on the same universe without problems. They will both do exactly the same thing , however.

Hopefully, you now have a better understanding of how DMX stage lighting systems work. A good book on the use of DMX systems is Practical DMX available at Amazon or read our review of some other good DMX books.

This article is currently sponsored by Trace Lighting’s LimeLIGHT lighting control software, a feature packed PC based control solution.

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Rob is a Lighting Designer and Moving Light Programmer and currently Senior Lecturer in Technical Theatre Production at Bath Spa University in the UK. He is also the Editor of On Stage Lighting and runs workshops in stage lighting practice.

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