DMX 512 Protocol Standards 1: Introduction

Early in the evolution of computer-controled light boards, each manufacturer used  proprietary equipment and protocols to communicate between the control board and dimmers and other lighting devices.  This lead to incompatibility between different manufacturers equipment.  It was in this environment that the United States Institute of Theatre Technology (USITT)  published a standard entitled “DMX512 – Digital Data Transmission Standard for Dimmers and Controllers”.  Spun off from a USITT Engineering Commission session at the 1986 USITT Conference  in Oakland, California, the publication was an effort to standardize  the communication of control boards and dimmers.  Minor revisions to that standard were published in 1990.

In 2004, the American National Standards Institute (ANSI) accepted the DMX512  standards.  ANSI is an organization of industry and government representatives that determine standards for the electronics industry in the US and represents the US in setting international standards.

In 1998 USITT transferred maintenance and supervision of the DMX512 protocol to the Entertainment Services and Technology Association (ESTA) Technical Standards Program. In 2011 ESTA merged with the Professional Lighting and Sound Association (PLASA) under the PLASA name.  PLASA remains the primary international body representing technologies and services to the event, entertainment and installation industries and retains supervision of the DMX512 standards.

As stage technologies developed that could be controlled by a lighting console, the DMX512 protocol has been used to communicate with those new devices as well as with dimmers.  DMX 512 is now used to control  gobo rotator, hazers and fog machines, gel changers, auto irises, color engines, moving lights, LED fixtures and other devices.

What is DMX512?  Some Vital Statistics:

  • DMX is an acronym for Digital MultipleX.  In engineering terms, DMX is an asynchronous serial digital data protocol.
  • a single DMX link  can provide 512 channels of control in a single Universe
  • Each channel is divided into 256 steps dividing a range of 0% to 100%
  • The DMX signal repeats intensity instructions for each and all of the 512 channels as many as 44 times per second.
  • all 512 discreet channels of a single DMX Universe  can be distributed over a single appropriate cable
  • Each Device contains a DMX in and A DMX through connector and devices are daisy-chained from one-to another
  • Only 32 Devices may be on a branch/each dimmer pack or lighting fixture or other DMX-controlled device with a microcontroller is considered a device
  • Additional branches may not be created with a standard Y
  • Additional branches of the same universe can only be achieved by using a DMX Isolated Splitter (an active device that permits the splitting of a single DMX feed so that into more than one feed each of which may serve up to 32 devices)
  • Each branch of DMX  must be terminated at the last device in the chain to ensure reliable operation.  Some devices self-terminate.  Other devices require a DMX 512 terminator that resembles an enlarged XLR plug.
  • Only standard-compliant connectors and conductors should be used to ensure reliable and proper operation of a DMX512 network.

The original version of the standard is quite specific about what type of cable and connectors were to be used to carry DMX 512 signal.  it specified the use of 120 or 100 ohm 1 or 2 twisted pair cable and a 5-pin XLR connector. Each wire has a specific hook-up and a specified function as indicated in the table to the right.   A cable constructed in this manner with no additional breaks or connectors has a maximum length of about 300 meters without the need to introduce a repeater (a device that boosts the signal).

While the standard is quite specific, a few individual equipment suppliers have produced equipment that fails to conform to the standards:

Since the original standard only has functions for the Shield Drain and Pair 1 Black and White wires(Pins 1, 2, & 3),  leaving Pair 2 Green and Red as spare (for future development of the standard), some vendors have found non-standard uses for the spare wires and connectors.  Some actually have one or more of these wires carry current or serve to provide feedback signal between proprietary devices.  These non-compliant fixtures  should not be used in conjunction with compliant equipment, since they may damage any DMX 512 compliant equipment.

Other suppliers have  opted to install 3-pin connectors on their equipment (in an attempt to save money by installing the less expensive connector).  In theory, this connector used in conjunction with wire of appropriate impedance and capacitance  (and otherwise compliant with the standard) should provide reliable service.  However since the most common cable used with this connector is microphone cable, and since microphone cable  fails to meet the standard outlined in the DMX 512 standard, users are likely to attempt to use sub-standard cable and are likely to experience unreliable operation.

The preceding provides the basic information needed to begin to understand the DMX512 standards.  Further reading and understanding would be necessary to be fully conversant with the protocol or to build, specify or trouble-shoot DMX512 networks.  Of especial interest is recent developments in wireless DMX and in DMX over CAT5 wiring.

Links:

Organizations:

Web Sites and PDFs for further study:

That’s enough for now! Have fun!  Be safe!

SJM

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