Background
The Need to Revise
The Revision Process
Status
New Concepts & Structure
What's In It for End Users?
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.
The Need to Revise
So - "all was good with the world" as they say, right? Unfortunately, not.
- 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."
- 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.
- 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.
- 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).
- 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.
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.
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.
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.
Table of Contents
|
DMX512/1990 Section
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
|
|
DMX512-A Clause
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)
|
Annexes
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
|
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."
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.
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 +)
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.