INTRODUCTION TO DMX
INTRODUCTION TO DMX
Chromateq software and hardware solutions are designed to facilitate the process of designing and programming DMX-based lighting systems.
DMX is a communication protocol that makes it possible to send digital commands from a control device to digitally addressable lighting fixtures with unique identifiers (DMX addresses) that enable individual control of each fixture’s capabilities and make possible the creation of complex synchronized lighting effects, scenes, programs and shows involving multiple fixtures.
Becoming familiar with the terminology used to describe DMX lighting systems will be helpful to understanding the functionality and features of Chromateq control solutions, which are based on industry-standard rules for controlling DMX lighting.
One of the initial steps when deploying a network of DMX lighting fixtures is to confirm the settings on the physical fixtures.
DMX Mode: Advanced fixtures may offer multiple « DMX modes » (i.e., a selection of different fixture profiles with various DMX channel quantities and layouts) with the possibility to choose a simpler profile (with fewer DMX channels) or a more complex profile (with a greater number of DMX channels).
Note the number of DMX channels used by the fixture in the chosen DMX mode
DMX Starting Address: Fixtures that include an LCD or LED display will typically include a menu for setting the DMX starting address, which will be displayed as a number between 001 and 512 :
Fixtures without displays may use DIP switches and a hexadecimal system for setting the starting address. Chromateq software includes a visual tool that aids in calculating the correct combination of switch settings for the values between 1 and 512 (available in 8 or 9 bits addressing). In the below example, the DIP switches are set to DMX starting address 004:
Unique or Shared Addressing: If all fixtures share the same profile (e.g., all are 3- channel RGB fixtures) and individual control is not required, it is possible to set the fixtures to the same DMX address. Otherwise, individual fixture control requires that each fixture be set to a unique address and that no DMX fixture channels overlap.
Another critical initial aspect when deploying a network of DMX lighting fixtures is following best practices for clean data transmission.
The DMX standard specifies cable runs up to 4,000 feet (1,200 meters, this assumes zero losses or termination problems) but in practice potential cable lengths are affected by cable quality, the characteristics of the DMX fixtures being daisy-chained together, and other factors.
For cable runs over 1,000 feet (300 meters) a repeater/booster may be needed. Placing a terminator at the end of each cable run (+Data and -Data lines) will help avoid degraded data transmission. Best practice calls for twisted-pair, shielded, low-capacitance data cable, not audio cable.
Good-quality, individually screened or shielded twisted pairs minimize crosstalk; additionally, a characteristic impedance of 120 Ohms (Ω), three or five inputs, and a flexible, tough jacket suffice for many needs. A characteristic impedance between 100 and 120 Ω is usually sufficient, with 120 Ω being the common nominal rating. Desirable cable features conform to DMX512 standards that include impedances of 85-150 Ω, low capacitance, and foil and braid shielded twisted pairs.
Using standard DMX cable instead of microphone XLR cable ensures smooth transition of data, and can compensate for mixers with less output power. Cable should be durable enough to withstand tough conditions and still provide powerful signal transmission.
DMX system requires channels to be correctly mapped and networked in order to minimize interferences or reflection errors. The object is to use as little DMX cable as possible, in the shortest feasible lengths, while creating a network capacity that exceeds the needs of the venue.
Faulty fixtures or incorrect fixture settings can complicate troubleshooting DMX networks. For example, a moving head set to operate as a « master » fixture will disrupt a DMX network and should be set to « slave » mode.
DMX-512 has a number of timing variables. A DMX signal sends a constant stream of frames, with each frame containing a value for every channel. Some lighting fixtures may have problems with a high frame rate.
In the Chromateq software, there are four values available to configure the DMX signal parameters which will affect the speed of the DMX signal:
"Break" and "MAB" (Mark After Break) expressed in microseconds (µs), "Period" setting for the data frame rate in milliseconds (ms) and "Delay" that adjusts the time between rendering the frame and outputting to DMX.
The default frame rate is 25ms (i.e., a 40 Hz refresh rate).
Lowering the frame rate may solve issues with lighting equipment that is incompatible with the default frame rate. But in general, most issues with DMX networks are related to cabling, connections and fixture settings. As such, adjusting DMX signal parameters should be one of the last considerations when troubleshooting DMX networks.
Once all fixtures have been set to their intended DMX addresses and modes, and stable communication established between the controller and the fixtures on the DMX chain, programming can begin.
Programming DMX lighting typically follows this basic process:
Glossary of Terms
Below are descriptions of common terms that are key to understanding the basics of DMX programming:
Channel 1: Dimmer
Channel 2: Red
Channel 3: Green
Channel 4: Blue
Channel 5: Shutter / Strobe
The set of attributes of a fixture’s DMX channel functions may also be referred to as its « personality ».