DMX-512 is a standard for digital communication - originally developed to standardise stage lighting protocols, use has now expanded to include interior and architectural lighting, as well as special effects and other fixtures.
A DMX-512 network uses a bus layout with nodes connected to it in a 'daisy chain'. The DMX-512 specification calls for a 120ohm terminator across the primary signal pair at the last node.
Each DMX-512 network is known as a 'Universe' and can carry information for 512 channels. Each channel has a level of between 0 and 255.
The most recent edition of the full DMX-512 standard is known as 'E1.11 - 2008, USITT DMX-512-A' and can be downloaded from the ESTA website - ESTA Technical Standards Program
The original DMX-512 standard specified the use of a five pin XLR style connector, using the following pin configuration:
Pin 1 - Signal Common
Pin 2 - Data 1 -
Pin 3 - Data 1 +
Pin 4 - Data 2 -
Pin 5 - Data 2 +
A 2008 revision also allowed for the use of 8P8C eight pin modular connectors (RJ45) where regular connection and disconnection is not needed:
Pin 1 - Data 1 +
Pin 2 - Data 1 -
Pin 3 - Data 2 +
Pin 4 - Not Assigned
Pin 5 - Not Assigned
Pin 6 - Data 2 -
Pin 7 - Signal Common 1
Pin 8 - Signal Common 2
The use of 3 pin XLR connectors is technically prohibited by ANSI E1.11 - 2008, however many manufacturers have adopted the connector using the following pin configuration:
Pin 1 - Signal Common
Pin 2 - Data -
Pin 3 - Data +
Although these configurations are now standard, some manufacturers have used alternative pin configurations, so care should always be taken when connecting new equipment.
Generally, cables deemed appropriate for RS-485 transmission are suitable for DMX-512. The cable should be a screened, twisted pair (or two pair) with an impedence of 120 ohms. Suitable cables include:
Cat 5/6/7 cable - commonly used in the telecomms industry - is approved for use with DMX-512 under the ANSI standard, though it is generally recommended that this should be a shielded version.
The cable distance from the controller to the last connected node should be no more than 300m, and there should be no more than 32 nodes connected.
If the system design calls for more than 32 nodes then the use of active DMX splitters enables multiple outputs, each of which can accommodate 32 nodes. (See Artistic Licence Rail Split RDM)
As DMX-512 has no error checking nor correction, it is not suitable for safety critical or hazardous applications.
As higher channel counts became more common place, controllers began to output more than one universe of DMX-512. Art-Net was developed using a simple UDP based structure to transmit multiple universes of DMX-512 data over an ethernet network.
Art-Net supports RDM (Remote Device Management) which allows for remote monitoring and configuration of lighting fixtures, and the latest version of Art-Net (Art-Net 4) allows for control of up to 32,768 universes on a network.
sACN is an ethernet based protocol maintained by ESTA to ANSI E1.31 - 2018. sACN offers a Discovery process and also Priority settings, which can be useful when implementing a system with multiple controllers.
Each fixture will have a channel count depending on the features within it. The channel count can be as low as one for a single colour dimmed fixture, up to tens of channels for complex intelligent fixtures with high resolution functions.
A Universe can control up to 512 channels.
No. Dali is a very different protocol used mainly in commercial applications. DMX has a higher channel capacity on each bus (universe) and runs at a much higher refresh rate. This lends itself to applications where smooth fades or time critical triggering is necessary. DMX is polarity sensitive, whereas Dali is not.
Dali has advantages for fixture monitoring and emergency lighting testing, as well as being widely adopted by architectural fixture manufacturers, with many system accessories available such as PIRs, relays and wall plates.
It is good practice to use a 120 ohm terminator at the last node on each universe. The resistor should be connected across Data + and Data -.
The terminator prevents signal reflections which can manifest themselves as erratic fixture behaviour and flickering.
DMX splitters are designed into a system for two main reasons - firstly to enable the connection of more than 32 fixtures to a controller, and secondly to facilitate a more straightforward wiring layout.
A simple splitter can have multiple outputs that each send the same universe, or more complex versions can use an ethernet input (which recieves multiple universes of DMX-512 data) with each output able to be configured to a different DMX-512 universe.
It is worth noting that fixtures on different splitter outputs are isolated from each other. A faulty fixture can cause interference and control problems with others on the same output (whether they are before or after the fault on the DMX cable). So a system with fewer fixtures on a higher number of splitter outputs may be considered more robust.
Generally, specialist equipment is used to test an installation. It is possible to use a digital multimeter between Signal Ground and either Data + or Data -, where the meter should read between 1.5 and 3.5v DC, but this should not be considered as a check for valid or good DMX signal, more that there is a connection to the controller.
Some test equipment to consider:
The results of using an out of spec cable would be a degradation of signal quality, which could mean interference and flickering resulting in unpredictable control. The longer the cable run the more exaggerated the problem becomes.
Using a unscreened cable for example may seem to work at the early stages of an install, but later could become susceptible to interference from other systems such as air-con or motor control.
No. The DMX-512 specification calls for a daisy chain layout. Including any sort of star, loop or y-split connection on a DMX-512 bus can cause flickering and interference.