Terminator, DMX

derekleffew

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A male XLR connector, either 3- or 5-pin, with a 120Ω, 1/2 - 2W (See Note1 below), resistor soldered across pins 2 & 3, plugged into the output of the last DMX device in the line. Easily built oneself (see http://www.bigclive.com/dmxtest.htm), or available for purchase from many manufacturers, one example: DFD Base Model, or (a fancy one with a "DMX-signal present" LED indicator: DFD Deluxe Terminator).

The terminator is required to prevent reflections from causing data transmission errors on all DMX streams, except those ending with an ETC Source Four Revolution® or SmartPack, which cleverly switch in the resistor if they sense nothing plugged into their DMX out port. See also this thread: http://www.controlbooth.com/forums/l...tion-plug.html.

For more information, see the "Dr. DMX" article, Why Terminate?.

Steps to build one's own:

1) Obtain a 1/2W, 120Ω resistor. (Available from any quality electronics component dealer: Allied, DigiKey, Mouser, Newark, etc.; or from The Shack, Cat#271-1108, $0.99 for a five-pack.)
2) Solder the resistor between pins 2&3 of a Male XLR (5-pin or 3-pin as appropriate) connector. Polarity of the resistor is not important--it doesn't matter which end goes to which pin. Leave pin #1 empty (no connection).
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3) Reassemble the connector and LABEL IT CLEARLY, as it will look just like an un-wired male XLR connector otherwise.

Note1: The wattage of the resistor used is often an area of confusion. From Doug Fleenor:
The DMX signal is nominally 5 volts. 5 volts across 120 Ohms
dissipates 0.21 Watts, so some feel a 1/4 Watt resistor is adequate.

The standard says the signal cannot exceed 6 volts when unloaded and
cannot exceed 5 volts when loaded with 60 Ohms (One 120 Ohm terminator
at each end of the line). The standard does not state the maximum
voltage when loaded with the more common case of a single 120 Ohm
terminator. If we use the worst case of 6 volts the power dissipated
in the 120 Ohm termination resistor is 0.30 Watts, hence a 1/2 Watt
resistor is required.

Although the standard says the DMX signal cannot exceed 6 volts, there
is always the possibility of higher voltages due to miswiring or
equipment failure. Since it is difficult to determine if a terminator
has been damaged by excessive power, Doug Fleenor Design uses 2W
resistors in its terminators to maximize the chance they will survive
fault conditions.

Resistors higher than 2 Watts tend to be wire-wound types which may
cause corruption of the DMX512 signal. Doug Fleenor Design has not
done any testing to verify this possibility.

Doug Fleenor Design recommends using 2 Watt carbon or metal film
resistors, however a 1/2 Watt or 1 Watt will work under non-fault
conditions.
 
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