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?
A 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 www.goddarddesign.com/rdm-net-1.html
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.
CASE STUDY:
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.
SOLUTION: DMX-Net RDM
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 www.goddarddesign.com/rdm-net-1.html
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.