Specifications

“Lighting Systems Made Easy” Page 32
may only contain the minimum 3 cables required to carry the DMX signal. In this instance you
will not get continuity when checking between pins 4 and 5. The DMX specifications currently
do not define a function for pins 4 and 5 but some manufacturers use these pins to transmit
read back information from a device. If a device at the end of the DMX line functions as de-
sired, this is a good indication the signal is being transmitted correctly. When failure occurs at
the end of the line, backtrack along the line to the first device functioning correctly. Place a test
unit, which can receive the DMX signal, on the output of this device, this will determine the
beginning of the failure. Test units that receive the DMX signal and displays a readout of the
signal in a percentage value, are beneficial to confirm changes from the lighting console are
received along the line. Individual devices can be tested, without a connection to a lighting
console, with DMX transmitting test units.
These test units are compact and portable, making it easy to check the system at any location
of the network. An error occurring in the middle of a line often indicates signal reflection, a
distortion of the DMX signal. A distorted DMX signal can cause the receiving device to trigger
unexpectedly. Signal reflection can be compared to an echo effect, the signal is transmitted to
the end device then bounces back up and down the line. A terminating resistor at the end of
each DMX line will eliminate signal reflection.
DMX networks can be limited, a single lighting console connected to an opto-repeater that
sends the signal to the dimmers and scrollers. They can also be expansive like the DMX
network installed at the Canadian Broadcasting Corporation Studio 42 in Toronto. The CBC
designed this system for a wide variety of applications. This network offers the ability to con-
nect lighting consoles scrollers and automated lights into a prewired network and then route
their control via computer to an electronic DMX patching panel.
The heart of the CBC system is a computer controlled DMX network patch panel that allows
multiple input DMX signals to be distributed to specific DMX outlets in the studio. The com-
puter software allows patch configurations to be saved and recalled when needed. This sys-
tem offers many diverse configurations. Beginning from the lighting console, or multiple light-
ing consoles, the DMX patch operator decides the control route for the specific console depen-
dent upon the location of the controlled device with the studio.
As an example let’s take three devices and follow their path. First we can assign dimmer
control to the desired lighting console, again dimmers are controlled in blocks of 512, so we
could have separate consoles controlling separate 512 blocks of dimmers. Dimmer control of
the same block of 512 can be shared between two consoles with the aid of a DMX merge box.
A DMX merge box combines both input DMX signals into a single DMX output. Devices will
respond to the input console that has the highest output level, so if device 194 is set to 50% on
one console and 70% on the second console device 194 will respond to the 70% level. It is
also possible to cross DMX ports and assign individual devices to the specific lighting console
with a separate electronic DMX patch unit.
The benefit of this complete system is evident when the DMX output control from an auto-
mated lighting console is mixed with a conventional lighting console output to control specific
dimmers within the same 512 data stream. This of course eliminates the need for a separate
dimmer system for automated lights requiring external dimming. Along with the standard