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DMX-512 Background
DMX-512 is a digital data transmission standard developed by the United States Institute for
Theater Technology (USITT). It is designed to enable control of lighting equipment, originally
dimmers. DMX deals solely with the formatting of data for transmission and does not dictate
how the data is created or used.
Under DMX, signals are transmitted in much the same way a computer modem transmits data.
The Data, divided in to channels, is "Framed" using a start bit, high (1), eight data bits and
finally, two stop bits, both high (1). DMX uses no parity to check the integrity of the signal.
Instead, DMX relies on the ultra low probability of an error occurring in the same place when the
data is resent. The rate at which data is sent is fixed at 250k bps, almost four and a half times
faster that a 56k modem. This speed allows all data on a DMX chain to be updated more than
44 times every second.
The transmitted data follows a specific format. DMX allows for 512 channels each with eight
data bits, giving each channel the possibility of 256 values. When a data "Packet" is sent, all
channels are transmitted one after another. Even if the data on a specific channel has not been
changed, it must be sent. In a packet, a "start code" of all zeros is sent before the data to
identify the signal as a Standard DMX transmission. This start code is transparent to the user
and is handled by the controller.
The physical signals are transmitted using a twisted pair of wires and a common shield, a
configuration called Balanced. The controller and all receiving equipment are connected using a
“Daisy Chain" connection. The signal is jumped from the controller to a piece of DMX
equipment. From there, the signal is jumped to the next piece of equipment and so on until the
last piece of equipment is connected. No branches are allowed and the signal does not come
back to the controller. The final piece of equipment will have only one cable connection. As a
result, all equipment connected to the chain will see exactly the same signal whether it is first or
last. When connecting equipment, no particular attention needs to be paid to the order in which
the equipment is connected. Depending on the conditions and equipment, a line terminator may
be required. If there is any question, in most circumstances the addition of a terminator will not
degrade the signal. To make a terminator, add a 120-ohm resistor between the Signal Data
Negative and Signal Data Positive pins of a connector in the last piece of equipment in the
chain.
The DMX Standard calls for connections between DMX compatible equipment to be made using
5 pin XLR connectors. However, it is common to see fixtures with 3 pin XLR connectors as
these types of balanced or “Lo-Z” cables are common in the audio industry. In either case, pin
numbers are the same and carry the same signals.
Pin 1 -
Signal Common (Shield)
Pin 2 -
Signal Data Negative
Pin 3 -
Signal Data Positive
Pin 4 -
(not used)
Pin 5 -
(not used)
