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Copyright © 2020 Tivoli 05.20.20
ADNM-DTV Flicker Free TV Studio Camera Installation Instructions
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Page 4 of 8
Why choosing the right cable is important
External disturbances
DMX is a balanced three wire system. Two wires carry the data signals and one wire acts as common reference. The advantage of a balanced system
is that external disturbance signals (EMI or electromagnetic interference) can easily be reduced. Both signal lines in a balanced system carry the same
signals with opposite polarity which are subtracted from each other at the driver.
DMX cables should have twisted pair conductors. This means that each pair of wires in the cable are twisted together. This ensures that any external
disturbance signal will occur equally on both signal wires (DMX in + and DMX in -). Since the driver subtracts signals on both wires, the equal
disturbance signals will also be subtracted from each other and are cancelled out.
The use of a shielded cable can further reduce EMI effects. The shield prevents external disturbances from reaching the signal wires. If a shielded
cable is used, do not connect the DMX shield to the mains ground.
Reflections
DMX works with high frequency signals. In an unterminated cable, thise signals will be reflected when they reach the end of the cable. These
reflections can cause eratic behavior lake random flashing lights, wrong brightness levels, etc.
To get the most reliable operation with minimal or no reflections. DMX cables should have an impedence of 120Ω as described in the DMX 512
standard. Cable designed specifically for DMX (or RS-485) applications is readily available and also contains a shield wire; for instance, Belden 9841
cable. In some installations Cat5 or Cat6 UTP cable can also be used. At the last driver, the cable must be terminated with a 120Ω resistor to prevent
reflections. Signal loss between the controller and the last driver must be less than 328 yards.
There is also signal loss because of the loading of the connected drivers. The DMX512 standard states that a maximum of 32 unit loads can be
connected to one DMX cable. One driver is one unit load. If the total unit load exceeds 32, a splitter, repeater or booster can be used. Note, however,
that repeaters, boosters and splitters may also add to the total unit load.
EMI reduced with balanced system
DMX
Signal
Split in
DMX + and DMX-
External disturbance on
DMX+ and DMX- wires
Subtract DMX+ and DMX-
to get the original signal
Subtract
Ideal Signal
Signal With Reflections
DMX Controller
DMX Driver 1
DMX Driver 32
DMX
Repeater/
Booster/
Splitter
DMX Driver
DMX Cable
Termination
Resistor
Termination
Resistor
More than 32 unit loads
RGB LED
RGB LED
RGB LED
Why Choosing The Right Cable Is Important
