DMX

If you do not possess the required electronics knowledge to create a controller from scratch or do not the have the funds to buy a commercial system but want to create your own display there is another option. You can purchase off-the-shelf ready-to-use dimmers that are controlled by DMX. With Vixen's DMX plug-in creating a display is as easy as buying DMX controlled dimmers from a local music store, and plugging in a USB-to-DMX dongle to your PC. Now you have a complete system ready to be programmed and impress your neighbors.

What is DMX?

DMX (Digital Multiplexing) is a communications protocol created by the theatrical lighting industry to control stage lighting. DMX controls everything from dimmers to moving lights and has been the standard for lighting control for over two decades.

DMX 101 is a must read if you are new to DMX

Why DMX?

DMX is very robust and stable in harsh environments. DMX also allows you to control 512 dimmable channels and up to 32 devices all from one cable.

In addition, because DMX is sent via RS-485/422 hardware it can travel up to 1500 meters (~4921 feet). Finally, because DMX is the worldwide lighting communications standard you can purchase any brand of DMX dimmers, connect them and be dimming within minutes.

"Dimmers" are a type of control box that allows a person to plug in multiple lights (or strings of lights) and control their intensity using DMX. Each dimmer normally supports four separate channels, each channel capable of being set to a certain brightness.

In addition to dimmers, DMX is a great option if you want to have other lighting effects in your display. DMX can be used to control DMX-capable moving lights, color changers, lasers, LED light bars and spot lights, fog machines, snow machines, and more.

Where can I get DMX controlled equipment?

Buying

Because DMX has been around so long DMX controlled devices can be purchased through a number of different local and online sources. Locally most music stores sell DMX dimmers and effects to bands and DJ's. Guitar Center is an example of a national chain that sells inexpensive dimmers and effects. In addition, most major cities have theatrical supply dealers that sell dimmers and effects. Generally, the products they carry are of a higher quality and will be more expensive. As for online sources, EBay is an excellent source for used DMX equipment.

NOTE: When shopping for equipment make sure it can be controlled by DMX. There are a number of manufacturers that use their own proprietary protocol and require converters to gain DMX control.

Renting

You don’t have to buy equipment. Because DMX is a standard protocol in the theater industry, you can actually rent equipment. A theatrical rental house will rent dimmers, moving lights, cables, fixtures, and other effects at a weekly rate. Check your yellow pages.

NOTE: The theater industry uses different style of power connector called stage-pin. Your home and Christmas lights have what is called an Edison connector. Also higher channel count dimmers (usually 12 and above) generally have what’s called cam-locs for power inputs. These dimmers are generally hardwired into an electrical service panel with "tails." Keep these things in mind if you rent dimmers or fixtures!

Specifications

Channels:512 (all dimmable)

Devices:32

Topology: daisy chain

Transmission: RS-485/422

Speed: 250kbit/sec

Distance: 1500 meters (~4921 feet)

Connector: 5-pin or 3-pin XLR

Pinout: 1-Common, 2-Data Minus, 3-Data Plus, 4-Not Connected, 5-Not Connected

Setup

Connecting Devices

DMX connects devices to each other via a daisy-chain topology, which means that each new device is connected to the last in one long chain (also called a universe). A device can be anything from a dimmer pack to a moving light as long as it can receive DMX. For this article we will assume the device is a dimmer pack (dimmer).

Generally, there is a DMX-In and a DMX-Out on each dimmer. The DMX-Out from the previous dimmer connects to the DMX-In of the new dimmer. You can chain a maximum of 32 dimmers together on one run (universe). The DMX standard dictates that all DMX output connectors should be female XLRs and all input connectors should be male XLRs. While not part of the DMX standard some DIY and permanent installation equipment uses RJ45 connectors.

These are the typical and most common pin-outs.

NOTE: This table is not correct - the Ren48LSD has the same configuration/pinout as the other Renard RJ45 interfaces.

NOTE: "Pixels" in the table refers to a DIY project that can be found here.

At the last dimmer, a terminator needs to be connected to the output XLR. This terminator eliminates any electrical reflections on the line that may cause your dimmers to flicker or act unreliably. A terminator is just a male XLR with a 120-ohm resistor soldered between pins 2 and 3. You can purchase them preassembled or make one yourself.

5-pin DIY terminator

3-Pin DIY terminator

DMX Cabling Basics

Notes about Connectors and Cables

While many of the inexpensive dimmers use a 3-pin XLR, using a mic cable to connect your dimmers together is highly discouraged. Mic cables have electrical properties that are great for audio transmission but damage the DMX data transmission. Always purchase cable specifically designed for DMX, while it is more expensive it will ensure reliable communications between your dimmers.

In addition some manufacturers swap Data Minus and Data Plus on the 3-pin XLR connector. Refer to the user manual for the proper pinout for dimmer pack.

Addressing Devices

Once of the dimmers are connected to one another they need to be addressed. All dimmers come with some capacity to specify a starting address. Because dimmers are “listening” to DMX they need to know what DMX values to listen for. The starting address tells the dimmer to start listening to a specific series of values.

The starting address is usually set sequentially from one device to the next. For example, the first dimmer in your chain would be assigned starting address 1. If that dimmer had 4 channels then the second dimmer would have a starting address of 5 and so on. While numbering sequentially is convenient it is not required. In fact you can give any dimmer any starting address you choose. This is handy when you have two dimmers in different physical locations but want to control them as if they are one. For example, you have a long string of rope lights but you have them plugged into two different 4-channel dimmers. What you want to happen is when you turn on channel 1 you want channel 1 on both dimmers to turn on, making it look like one continuous string. Easy, make the starting address of both dimmers 1. Then when channels 1-4 are controlled both dimmers will respond in the same way.

Starting addresses are assigned by several methods. Most dimmers have address wheels or dipswitches. Address wheels are easy to use; if you want a starting address of 1 you set the wheels to “001.” However most inexpensive dimmers use dipswitches. Dipswitches are a little more difficult because each switch represents a binary value. See the owner’s manual or side of the dimmer for details on how to set the switches.

DMX output

Once you have connected and addressed all your dimmers you are ready to send DMX to them. We are going to assume that you are using Vixen for this article. Vixen currently supports DMX out via a USB-to-DMX dongle. Currently Vixen supports the Open DMX USB Interface and the USB DMX PRO. There are three different ways to obtain these devices. The first way is by purchasing them fully assembled from Enttec or via Ebay.

For the do it yourselfer you can purchase just a bare PCB from Enttec for the Open, order the parts from an electronics reseller, and assemble the device yourself. Or you can obtain the parts, create the PCB, and assemble it yourself based with the schematic provided by Enttec on their website in downloads. While buying the PCB from Enttec may be convenient you need to be sure that, you are capable of soldering fine-pitch surface mount chips.

If you are not comfortable with that level of soldering then your final option is to create one yourself from Enttec’s schematic. To make this process easier I suggest you purchase DLP-USB232M USB Adapter from DLP Design. This eliminates all the fine pitch soldering; all that needs to be added at that point is the 75176 transceiver, some resistors, and a LED. (basically all the DMX specific circuitry) NOTE: The author of this article chose this method and it works well.

Once the dongle has arrived or is complete, you need to install the FTDI drivers prior to using it with Vixen.

Vixen Setup

Once the device is connected and the driver has been installed you can set Vixen up to output DMX to your dimmers. There are two parts to DMX transmission in Vixen – one part is the DMX-512 plug-in and the other is the DMX Add-in. The plug-in is the same as any other plug-in when using vixen. It needs to be added to the sequence and/or program you have created for the values to be sent to the dimmers.

Each channel in Vixen represents a channel in the DMX universe. Each channel on Vixen can vary from 0-100% intensity, which then controls the brightness of the lights connected to the DMX dimmer at that individual channel. If you wish to more precisely control the intensity of the lights, or if you want to use intelligent lighting fixtures or other effects such as lasers, you may wish to switch Vixen to the "actual intensity levels" mode instead of percentage mode by clicking the # button on the toolbar (available in Vixen 2.0 and up). This change is needed because Vixen automatically converts a percentage of brightness (such as 50%) to the DMX equivalent (50% brightness would be a DMX value of 128); some DMX fixtures need precise values, not an estimated value based on percentage. (For example, a DMX color changer may need an actual value of 1 to make red, 2 to make green, and 3 to make blue. It is necessary to use actual values, because the percentages will not match up, therefore making it impossible to accurately control the color.)

The DMX Add-in (not plug-in) has a different purpose: it is used to start and stop streaming DMX when Vixen is opened or closed. This streaming option can be found under ADD-INS, DMX. This add-in exists because DMX by design is a streaming protocol. DMX devices expect to see information streamed to them all the times, if they loss the stream they will automatically turn off. While this may not be an issue when creating or testing sequences, it becomes a major issue when running a program with multiple sequences. If this add-in is not enabled you will see a noticeable flicker between sequences within in your program. This is because DMX does not stream between sequences with this plug-in disabled.

The DMX Add-in should only be enabled when using an Enttec Open DMX USB dongle (or clone); "intelligent" dongles – such as the Enttec DMX USB Pro dongle, the Lynx DMX dongle, the RPM dongle or the USB2DMX (Yada) dongle – handle their own streaming of DMX data on-board (don't rely on PC) so you should not enable the DMX add-in (enabling this add-in WILL causes problems with dongles other than the OpenDMX).

Start Programming

That’s it, you are now controlling your DMX dimmers, snow machines, strobes, and more, all via Vixen. Happy DMXing!

Going Beyond

If you do the math based on my above example you will notice that if you adhere to the 32 device limit and use 4-channel dimmers you can only get 128 channels total. There is a way around this issue.

The 32 device limit is not a DMX limitation it is a limitation of the RS-485 transceivers. The older 75176 set a limit of 32 devices because beyond that point the data traveling down the line would start to deteriorate. Newer transceivers have published a limit of 128 devices. While this does solve the problem, most manufacturers use the older 75176.

To get around this hardware limit you need to introduce another piece of equipment. It is called an opto-splitter (also called repeater, splitter, booster); this device takes a single DMX-in and retransmits it to multiple DMX-outs (like a network hub). Let’s assume the opto-splitter has 5 DMX-outs. Each output of the opto-splitter can have up to 32 devices connected that’s a total of 160 devices all controlled from the same DMX source.

Opto-splitters have a second purpose; they allow you to send DMX to different locations. Let’s assume you have dimmers in the yard and dimmers on your roof. You could daisy chain all of them together or you could use an opto-splitter. Then you would have one DMX run going to the yard and all the yard dimmers would be daisy chained together. A second DMX run would go to the roof and all of those dimmers would be daisy chained together. Opto-splitters are available from any theatrical supplier; you won’t find them at your local music store. This is somewhat of a specialty item because most bands and DJs don’t generally go beyond the 32 device limit or need multiple runs of DMX from a single source. However, in a theater you regularly go beyond the 32 device limit or need DMX in different locations in the facility.

A new opto-splitter is extremely expensive! A one-in five-out splitter will cost about $800. However, they are inexpensive to make about $10-30 worth of parts. Whether you purchase or build, a good opto-splitter needs to have a couple of things. All of the outputs should be electrically isolated from the input and from each other via optical couplers. You also want to make sure that each output has its own transceiver and power supply. These things will protect your expensive equipment from electrical damage.

Premade inexpensive opto-splitter

Fun Stuff Controlled by DMX

Doug Fleenor Design

Links

DMX for Dummies

DMX from Wikipedia

Why terminate?

Real world termination example with scope traces (video)

DMX-512 Primer (good read)

Comparison of DMX and LOR protocols (presentation)

Ujjal's DMX512 Website

DMX Protocol/Packet information

Tons of Lighting links!

Basics of Stage Lighting

DMX Packet and Timing