DMX Explained and Wiring Install Info
What is DMX
DMX512 stands for Digital Multiplex with 512 channel capability. DMX512 now usually referred to as just DMX, is used to control many different devices such as theatrical lighting, high power dimmers, intelligent lights, moving heads, fog hazer & smoke machines, effects lights (FX), LED fixtures, as well as 0-10 volt LED drivers. Many devices now can control or generate a DMX signal to control these lights such as lighting controllers, lighting consoles, computer programs with PC USB dongle interface or USB controllers (which are software driven), wall switch controllers, 0-10 volt converters, DMX multi merge mode mergers, RS232 to DMX converters, and many other devices. DMX was originally designed for theatrical environments but has expanded beyond theaters to uses in architectural lighting, Churches, Christmas lights, synchronized to music shows, electronic billboards, stadiums and arena style concerts.
DMX512 uses EIA-485, or RS-485, a serial asynchronous protocol. RS-485 employs a differential or balanced signaling across the two wires. When DATA- is low DATA+ is high using a 5 volt difference. The DMX signal operates at a 250Khz baud rate. The DMX packet is made up of several parts: BREAK, MAB (Mark After Break), Start Code, SLOT, and MTBP (Mark Time Between Packets). This packet is repeated many times per second. The SLOT is the 1-512 channel value(s), 0-255, being sent to the fixtures and equipment in a daisy-chain configuration also known as a universe. The amount of times it repeats depends on the amount of data slots or channels that are being sent in addition to the timing of the other portions of the packet. For proper operation and device responses, a packet needs to be repeated a minimum of 30 times per second. If a valid DMX signal or packet isn't received within 1 second it's considered 'no data'.
DMX uses two wires to carry the signal. Pin 2 is DATA - (negative) and pin 3 DATA + (positive), and utilizing a 3rd wire as a shield on pin 1. The interfacing connections are typically XLR style connectors and the pinout is the same pins 1, 2, & 3 for both 3 and 5 pin XLR styles. XLR input connectors are male and output XLR connectors are female. A DMX output can source or drive up to 32 devices, daisy-chained from one fixture or device to another. By inserting a DMX Splitter, then each output can source up to 32 devices. The last device of a daisy-chain must be terminated with a 120 ohm terminator to close the loop regardless of the amount of fixtures in the chain. For increased reliability a DMX specified cable should be used as stated by the USITT DMX512-A standard. However, Ethernet cat5, cat5e, or cat6 cable is used often with much success.
A DMX universe is a single output of a lighting board or controller with up to 512 channels of control sourcing up to 32 devices, and by using a DMX splitter each of the outputs can source up to 32 for as many as desired. Each channel can be any value from 0 to 255. This value can be used to control not only brightness but color, patterns, gobo's, X & Y axis for example, depending on the device. If more than 512 channels are required, (more than 512 total points of control) then 2 or more universes can be used. Each added universe provides 512 additional channels of control and each requiring its own wiring distribution to its respective devices. Some controllers create multiple universes while in some cases multiple generating devices can be used. For example a DMX console could be used to control the theatrical fixed lighting while a computer USB dongle interface with lighting software, or an RS232 to DMX converter could be used for intelligent or moving lights. Each DMX universe can be daisy-chained to its respective devices or a DMX Merger with 2, 3, 4, or more inputs could be used to combine the universes into a single universe to send the DMX signal to all the fixtures. Check device specifications to determine rdm capability.
DMX512 does not incorporate error checking or correction and should never be used where human safety is involved. And DMX should never used for control or part that hazardous applications such as pyrotechnics or movement of theatrical rigging. False triggering, electromagnetic interference, static electricity discharges, improper cable termination, excessively long cables, or poor quality cables can cause unwanted responses.
Remote Device Management (RDM)
RDM allows for a DMX universe to be a bi-directional network (over the same two wires) to communicate with specific devices. If a system is RDM capable, then the connected devices have the ability to decode specific device information with an RDM start code (0xCC) followed by a unique device ID. The controller will then release the data lines, and wait for a response. The device will then send the requested data back to the controller. After a certain amount of time the controller will begin sending it's normal DMX data. This short data reversal doesn't affect the other devices connected and continue to operate normally. RDM allows the controller to discover, determine what's connected, as well as set specific device parameters or retrieve diagnostic information from the device(s).