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DMX512 - Behind the Revision |
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Background
The Need to Revise
The Revision Process
Status
New Concepts & Structure
What's In It for End Users?
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Background
Before DMX512, most manufacturers had their own dimmer control protocol that were proprietary and
incompatible with other manufacturers' equipment. Therefore, if a console from manufacturer A failed,
you had to get another manufacturer A console to run your manufacturer A dimmers. Manufacturer B's
console would not work. As the industry grew and shows became more complex, something had to be done
to resolve this situation. In 1986, the USITT Engineering Commission sponsored a session at the Annual
Conference & Stage Expo in Oakland, California. From that session, a project started that resulted in
USITT DMX512 - Digital Data Transmission Standard for Dimmers and Controllers. Minor
revisions were made in 1990. (This project also resulted in the formalization of Strand's multiple versions
of the CD80 protocols into one standard - USITT AMX192).
The expectation was that manufacturers would continue to use their own proprietary protocols but when
a user needed to mix manufacturers, the user would switch over to DMX512.
DMX512 was intended to be a lowest common denominator protocol. Little did we know it would become the
primary protocol and help transform the industry.
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The Need to Revise
So - "all was good with the world" as they say, right? Unfortunately, not.
1) The original and 1990 versions of DMX512 cover only data used by dimmers and do not address the
use of DMX512 for scrollers, gobo rotators, strobes, ballasts, automated fixtures and anything other
than dimmers. In practice this standard has been used by a wide variety of devices. DMX512 and
DMX512/1990, with their lack of error detection, is NOT appropriate for some applications - such
as pyrotechnic control and scenery automation. The new standard does allow for error detection, but
specifically states "DMX512 is not an appropriate control protocol for hazardous applications."
2) Many manufacturers did not follow the standard precisely, or inconsistently interpreted or completely
ignored the references to EIA-485. EIA-485 is the standard on which the physical part of DMX512 is based.
This could mean that in localized or single manufacturer systems, everything operates fine; however,
equipment could fail to work correctly when connected as part of a large or multi-manufacturer system.
For example, when manufacturer A says its systems are "isolated" and manufacturer B says its systems are
"floating," what do they really mean? Also, there have been revisions to the EIA-485 since 1990.
3) One of the biggest problems is the 5-Pin XLR connector. Despite the standard being very explicit that
the required (and only) connector to be used is the 5-Pin XLR, some manufacturers used 3-Pin XLRs. This became
a nightmare for many users, as they had to have adapters to make their systems connect.
4) Some manufacturers, while using the 5-Pin XLR, decided to use fourth and fifth pins for DC power, not signal
data as specified in the standard. This caused some compliant equipment to be damaged. (For more details,
see the Hazard Alert Page).
5) To add to the confusion, DIN, a German standards organization, was developing its own and potentially
incompatible version of DMX512. (The USITT standard, while the de facto industry standard, was not formally
recognized by the international standards community. German regulations did not allow the use of a
non-recognized standard when "proving " a system.)
In 1998 it became evident that additional updates to the standard were necessary and that formal
recognition through an internationally recognized standards organization was required. The USITT DMX512
Subcommittee, sponsored by the Engineering Commission, issued a Call for Comments in March of 1998 in
order to solicit recommendations for changes to the standard. USITT then transferred maintenance of DMX512
to the Technical Standards Program of ESTA
(accredited by ANSI as Accredited Standards Committee E1, Safety and Compatibility of
Entertainment Technical Equipment and Practices with ESTA as its Secretariat, aka ANSI ASC E1.)
In August of 1999, an application to make DMX512 an International Standard
under the auspices of the International Electrotechnical Commission (IEC) was approved by IEC Technical
Committee 34 (TC34): Entertainment Technology - USITT DMX512-A (IEC 62136). This process is no longer
being pursued. |
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The Revision Process
The revision effort was taken up by the ESTA Technical Standards Program's Control Protocols Working
Group, which appointed a task group to act on the proposals received in response to USITT's Call for
Comments. At this time, ESTA registered this project with ANSI. The official title is E1.11,
Entertainment Technology — USITT DMX512-A — Asynchronous Serial Digital Data Transmission
Standard for Controlling Lighting Equipment and Accessories. The Task Group prepared additional
proposals they saw as necessary to resolve some issues arising from public proposals. The Task Group
also established several goals as part of its work:
- Produce a consensus document following proper standards procedures.
- Make editorial updates to DMX512 appropriate for current times.
- Add technical features while maintaining a balance with backward compatibility
[Note 1]
including formalizing the management of Alternate START Codes.
- Identify among the proposals received which ones belong in an Application Note or Recommended
Practice instead of the standard.
- Take actions on these proposals and deliver a report and a draft of a revised DMX512 standard. The
draft would then be continued through the ANSI Accredited standards development and review process.
At the end of 1999, USITT and the Control Protocols Working Group issued a Request for Comments on the
first draft of a revised and updated version of DMX512 (well over 300 comments were received).
Commentors were reminded that proposals that compromise backward compatibility unfortunately would have to
be rejected. This was followed over the next four years by three formal public reviews on subsequent drafts.
ESTA and USITT urged interested parties to offer constructive comments, both positive and negative. In
total, there were eight full draft revisions prepared, each with several "sub-versions."
Over 955 formal comments were received over these six plus years, and each one was addressed and resolved
by the Task Group.
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Status
This new version of DMX512, informally known as DMX512-A, has been taken through
the rigorous American National Standards process for establishing consensus.
On March 18, 2004, the Control Protocols Working Group of the ESTA Technical Standards
Program (ANSI ASC E1) voted to accept the comment resolutions for the last (third) public review and to accept
the draft standard as an American National Standard.
The American National Standards Institute (ANSI) approved the new version of DMX512 on November 8, 2004. It
is now available for purchase as a PDF
from ANSI's
Electronic Standards Store. A hardcopy version is now available from
USITT and
from PLASA.
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New Concepts
A number of new concepts were introduced in this draft:
- Structural changes, including the use of annexes (in part to comply with IEC format).
- Addition of an Introductory Overview clause to expand on the scope statement and aid people to better
understand the DMX512 standard.
- Definitions and Terminology.
- Physical Layer Details / Earth Grounding Practices.
- Formal classification of topologies used with the second pair - known as Enhanced Functionality.
Pins 4 and 5 remain dedicated to data
[Note 2].
No power on the 5-Pin XLR connector is allowed.
- Manufacturers may chose to Protect DMX512 circuitry above "default" EIA-485 levels
[ Note 3].
- Required declarations of various parameters and Marking of Ports.
- System Information Packet (e.g., Checksums) - Optional.
- Alternate START Code Management (for more information, see the Alternate START Code information page on ESTA's Web Site).
- Cabling has been removed from the document and will be in one or more separate standards documents
(BSR E1.27-1 -- Portable Control Cables for Use with USITT DMX512/1990 and E1.11 [DMX512-A] Products -
has been approved by the E1 Accredited Standards Committee. Final ANSI approval is expected in the spring of 2006.
The companion document to USITT DMX512/1990 - Adam Bennette's Recommended Practice for DMX512, will
continue to be available while another Task Group (working under the auspices of the Control Protocols
Working Group) develops an updated version. The advice within the Recommended Practice is still
applicable. If there are any conflicts between the Recommended Practice and
DMX512-A, the standard applies.
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Table of Contents
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DMX512/1990
Section
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DMX512-A
Clause
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1 - Scope
2 - Applicability
3 - Cross-Reference
4 - Electrical Specification
5 - Data Protocol
6 - Data Format
7 - Timing
8 - Receiver Tolerance
9 - Connectors
10 - Cable
11 - Marking |
1 - General
2 - Normative Reference
3 - Definitions
4 - Electrical Specification & Physical Layer
5 - Nominal Operating Characteristics
(Including Topologies)
6 - Protection
7 - Connection Methods
8 - Data Protocol (Including Format & Timing)
9 - Receiver Performance
10 - Marking & Disclosure
11 - Protocol Implementation Conformance
Statement (PICS) |
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A - Non Preferred (Alternate) Topologies
B - Enhanced DMX512 (the 2nd Data Link)
C - Higher Protection Levels (DMX512-A
Protected)
D - Reserved Alternate START Codes
E - Alternate START Code, Manufacturer ID, &
Enhanced Functionality Registration
F - Protocol Implementation Conformance
Statement (PICS) for
Annexes A through E
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What's In It for the End User?
The ultimate goal of this process is a revised, more precise standard, resulting in more reliable and compatible
systems, with recognition by formal standards-making bodies. More importantly, End Users will:
- not normally need to change their existing practices.
- not normally need to modify compliant "legacy" equipment.
- be better able to troubleshoot and resolve problems when they do occur.
Karl Ruling, ESTA's Technical Standards Manager, states:
"E1.11 is an significant improvement over its predecessor, USITT DMX512/1990, in that it clears up some of
the technical ambiguities in that original, very short document, and it opens up some extensions that will
allow for new protocols, such as E1.20 (RDM). E1.11 has been written, however, with the intention of not
making products that fully comply with USITT DMX512/1990 instantly obsolete."
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Note 1:
Backwards Compatibility
It is important to note that due to the vast amount of equipment already installed and operating for years,
a number of proposals had to be rejected. While many of these proposals did not suffer from any technical
shortcomings, in accordance with the third goal they had to be rejected because they were not backwards
compatible and would have immediately made most of the equipment installed obsolete.
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Back to the Revision Process
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Note 2:
All versions of DMX512 state that the second pair, Pins 4 and 5 of the
5-Pin XLR connector, are defined as:
PIN 4 - Optional Second Data Link Complement (Data 2 -)
PIN 5 - Optional Second Data Link True (Data 2 +)
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Back to New Concepts
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Note 3:
EIA-485 is the standard with which the circuitry of DMX512 equipment is required to comply.
Since EIA-485 specifies only transient, not steady-state protection, the only amount of steady-state
protection currently required by default is the levels which allow the equipment to operate. Protection
beyond that is prudent, and most manufacturers do provide higher levels but to varying degrees. This is
not as simple as it may seem, because some methods of providing higher protection can affect data
transmission/reception performance.
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Back to New Concepts
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