DMX-Net RDM tm 

 The Efficient way to DistributeDMX / RDM

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DMX-Net RDM White Paper #1

DMX-Net RDM tm lowers the cost of providing multiple DMX / RDM outlets while providing a simpler user experience. 

Today’s trend in lighting systems is moving away from the large central dimmer room. Dimming is handled by small distributed dimmer locations combined with self dimming lighting instruments, such as moving luminaires or LEDs.

In the fall of 1998 Goddard Design introduced DMX-Net to help simplify DMX512 distribution. In 2011 we introduced DMX-Net RDM to provide for a simpler, lower cost data network with bi-directional capability.

What is DMX-Net RDM?
photo of DMX-Net RDMA system is constructed of small single port modules that either input or output bidirectional DMX data. The modules support the ANSI ANSI E1.20 protocol. Output modules conform to ANSI E1.20's RDM termination and biasing requirements. Inputs present a low electrical load (1/4 unit load) to the driving source. Each module is connected to other modules by a ‘private’ DMX/RDM back bus. The back bus also normally supplies the power for the modules; though the power supply may be distributed. The back bus is normally a pure daisy chain, which is configured and terminated at the time of installation. Since all inputs and outputs are fully buffered the end-user need only consider the network rules from the front panel XLR to device(s) attached to that module. For technical information on this product, please see the data sheets on Goddard Design Co’s website at

A key advantage of our DMX-Net RDM system over ones using conventional splitters or hubs is controller placement at any point along the networks. This placement can change with no reconfiguration of the system needed.

There are several ways in which DMX-Net RDM can provide a considerable cost saving over the standard methods of data distribution, while increasing flexibility and decreasing certain failure modes.

Consider a medium to large standard proscenium theater with a multi-universe control system. Multiple DMX512 /RDM controllable devices are located on multiple on-stage electrics, on a ceiling FOH position, and at other locations( box booms, cyc, wall positions, and balcony rail positions), and possibly a conventional dimmer room.

This type of venue is likely to have a console that can deliver multiple DMX universes on either DMX512 over EIA485 or Ethernet. Assume that it is desired to have at least 10 - 12 DMX consuming devices at each lighting position and that each position will normally be fed by one universe, which may or may not be shared by other locations.

OPTION 1: Run one DMX line from a RDM hub in the control room to each location. At each location install one passive output. Then daisy chain from lighting intrument to lighting intrument with loose cable.

This is a low cost approach The single 485 line will fit in a single 3/4" EMT conduit, and no active electronics is required outside of the control room.

However, reliability and ease of use are poor. A failure of any jumper cable will disable all lighting intruments further along the run. The long cable run from the control location can create a system where termination is important to achieve reliable network performance. The failure of one cable will disrupt the termination, possibly causing errors on devices closer to the console. Termination must be configured every time the setup is changed. The user must remember to install the termination slug in the last lighting intrument. If the lighting instruments have termination switches, the last one and only the last one should turned on. The termination slugs are soon lost. The loose jumper cables must be secured to avoid causing a possible trip and fall hazard.

OPTION 2: Take the single home run as described above and daisy chain it to multiple DMX connectors distributed across the lighting position. While cleaning up the loose cable problem somewhat, this solution may be worse than OPTION 1. A failure of any unit can still bring the system down. The nicely run cable hidden in the wire ways hides the fact that at every outlet used we are creating the dread DMX ‘T’. The users must know that only short jumpers can be placed between the installed connectors and the lighting intruments. Further ONLY THE LAST LIGHTHING INSTRUMENT OR OPEN OUTLET of the hidden system may be terminated; all others should be left open.

OPTION 3: Run 10 to 12 DMX/RDM lines from a central splitter/hub in the control room to each outlet at the lighting positions.

This will be more reliable than the single daisy chain solution above. It will requires few long jumpers and fewer daisy chains. As long as only one lighting instrument is plugged per outlet, or the daisy chain is kept short, termination rules are less of a problem. The lighting instrument or last lighting instrument plugged into any connector is terminated.

The electronics cost is higher than that in option one above but is still moderate. The change in installation cost, however, is not. Not only do we have the increased cost of purchasing and installing 10 or 12 runs of cable, the required conduit changes from 3/4 inch EMT to 2 inch EMT.

OPTION 4: Use an Ethernet protocol such as E1.31, ETC net 3 or ART-Net to each lighting position, then convert it to the native DMX512 /RDM 485 format for delivery to the lighting instruments.

This method may allow for the selection of any universe to feed the lighting position. However, unless more than one universe is required at each light position, the location of the protocol converter is often a matter of choice. 

Once converted from Ethernet to 485, the system designer is confronted with many of the problems listed above for OPTIONS 1 and 2.

One can install multiple Art-Net or ETC net convertors at the lighting position. However at $500 to $1000 each for the uninstalled convertors, this gets expensive FAST. Also you will need to run 12 Cat5 cables in a 2 inch EMT conduit or locate an Ethernet switch at each position.

Bring one or more native DMX512 Universes, or one Cat5 line to a lighting position. If Ethernet distribution is used, install one Art-Net or similar convertor. Provide one small DC supply to run the DMX-Net RDM system. Install a raceway with cover plates to take a single XLR and an LED. Alternately, install a standard single gang electrical box at each DMX drop point; DMX-Net RDM modules with cover plate install directly into the box. Ideally, place a DMX-Net RDM input module at the Ethernet decoder. (Being able to disable the Ethernet here assists in testing and maintenance.)

The advantages of DMX-Net RDM are considerable.

The user need only worry about the topology of the cables they install from a DMX-Net RDM output to the attached lighting instruments. Mis-termination issues will seldom affect the system performance; when they do, it will generally be limited to a single lighting instrument.
DMX-Net RDM decreases the cost of providing many DMX outlets.
DMX-Net RDM modules are much lower in cost than an Ethernet gateway.
DMX-Net RDM greatly decreases the number of home runs to the control room, decreasing cabling and conduit costs.
DMX-Net RDM requires minimal hardware and no software configuration

A recent installation in Texas provided 5 high schools with approximately 50 DMX-Net RDM outputs per theater. The savings in cabling installation and hardware costs were factors in the selection of DMX-Net RDM for the job.

For technical information on this product, please see the data sheets on Goddard Design Co’s website at

A further application note describing DMX-Net RDM in a black box theater will follow soon.

You can download a PDF version of this file.

Goddard Design Company
51 Nassau Ave.
Brooklyn NY 11222 USA
718 599-0170 718 599-0172 fax

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last updated Oct.28, 2014