Specifications

“Lighting Systems Made Easy” Page 31
address. Devices with identical addresses, from the same DMX outlet, will respond identically.
To achieve individual responses the devices must have an individual address. Imagine a room
filled with 512 people if they are all named “Bob” they would all stand up if you instructed “Bob,
please stand up”. If each individual had a distinct name you could instruct each person to
perform your commands simply by calling them by each unique name. This analogy applies to
DMX addressing of devices along a DMX line.
Attention to specific procedures and the correct cable type is important when transmitting
DMX-512. Failure in the DMX network can often be traced to poor installation practices. Use
cable designed to EIS RS-422/485 specification to carry a DMX signal. Connect devices in a
sequential order like a chain linking one unit to the next, starting from the console. A maximum
of 32 receiving devices per DMX line is recommended. Single DMX cable runs should not
exceed 4,000 feet. Install a terminator (resistor) at the end of the DMX line of devices. This
will reduce the possibility of signal reflection, which can create errors in the DMX signal. The
cable type used will determine the ohm value of the resistor. Some devices allow for self-
termination at the end of the line.
Distribute multiple lines of DMX data through an opto-repeater. This device creates a physical
break in the line by transforming the electrical signal into light, which spans a gap, and then it
is restored to electrical signals. This function is performed by opto-isolators (optically isolating
the electrical signal) within the repeater. This protects devices from damaging high voltage,
accidentally travelling along the network. It will also repeat the original DMX data to several
output lines. The input data is recreated at the outputs, eliminating distortion. The signal
leaves the opto-repeater as strong as it left the console. Using a “Y” cord (two DMX output
cables to a single input connector) degrades the DMX signal and should be avoided.
A simple DMX network would begin with the lighting console, the DMX 5-pin XLR output of the
console is connected to a cable designed to EIA RS-422/485 specification to carry a DMX
signal. If the signal were to be transmitted to multiple locations then the next in-line device
would be an opto-repeater, to protect and distribute the signal. Even when the signal is di-
rected along a single path the use of an opto-isolating device is a beneficial protective practice.
The signal continues along another cable, designed to EIA RS-422/485 specification to carry a
DMX signal, from the output of the opto-repeater to the first device you wish to control with the
lighting console. The device should be addressed with a numeric value between 1 and 512, to
decipher the corresponding numeric DMX data transmitted by the lighting console. Each
device is connected one after the other, using the input and output XLR connectors, similar to a
chain. The last device, on each line, should use a terminator (resistor) on the output XLR to
keep signal integrity. Following these simple practices should insure a DMX network that
operates correctly.
When there is failure within the network tracing the fault can be time consuming, test equip-
ment now exists to aid in network checking. They range from units that can generate DMX,
receive DMX to simple plug-ins that illuminate and LED when it receives voltage on pins 2 or 3.
Many manufacturers recognize the importance of visual assurance the device is receiving a
signal. Many devices now include an LED that illuminates when receiving the DMX signal.
Along with a visual inspection, the individual cable continuity should be tested. Pins 1, 2 and 3
of the DMX cable should be checked. Although the DMX standard is 5-pin XLR, the cable itself