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DMX512-A
The following information is taken from the Artistic Licence website. It outlines the design goals of the DMX512-A protocol.
DMX512-A is the colloquial name for a new draft standard called "ANSI BSR E1.11, Entertainment Technology - USITT DMX512-A Asynchronous Serial Digital Data Transmission Standard for Controlling Lighting Equipment and Accessories". The standard is starting life as an American National Standard. The final document will be put forth for consideration as an IEC standard (Draft Standard 62136: Entertainment Technology - USITT DMX512-A); if accepted, it will then become an ISO - International Standard.
The development of DMX512-A is managed by ESTA. The group doing the work is called the DMX512 Task group.
DMX512-A was previously called DMX2000. The DMX512-A standard will replace DMX512.
The key features of DMX512-A are:
- The new standard is fully backwards compatible. This means that equipment designed for DMX512-A will work with DMX512(1990), and compliant DMX512/1990 equipment should have no problem with DMX512-A.
- DMX512-A defines higher levels of electrical protection for the data connection. This will benefit by reducing the potential for damage from static electricity or lightening strike.
- A specific definition of the earthing practises that are allowed. Most of the compatibility problems between installed equipment have been caused by bad or incompatible earthing practises.
- The use of pins 4&5 is specifically defined. These pins can no longer be used for anything but RS485 data. This means that Pulsar and Clay Paky are not allowed to connect power to these pins. Pins 4&5 are called the "Optional Second Data Link".
- The connector usage is defined more specifically. Most important is the fact that the 3pin XLR must not be used for DMX512. The main reason for this decision is to stop the confusion between DMX512 cables and audio cables. The 3pin connector was never allowed in the original standard ("Where connectors are used, the data link shall utilize 5-pin "XLR" style ..."), but language has been added to be emphatic about that point.
- Electrical isolation, also called optical or galvanic isolation, is now defined. DMX512-A does not mandate optical isolation. However, it does define the requirements for isolated equipment. It also suggests that the DMX512 receiver should be isolated, whilst the transmitter is grounded.
- The System Information Packet or SIP is a new feature aimed at large installations. SIPs are transmitted at low frequency, by the lighting console, interleaved with normal lighting data. The SIP contains information about the console such as software revision, but also provides information such as the number of times the DMX signal has been processed. This could be passing through a merger or patching system. The SIP data can then be monitored at any point in the distribution chain.
- Another new feature is the Text packet. This allows text information to be sent via the DMX512 signal. The key benefit is that 'black box' products that do not contain a screen are able to display text information such as operating status and software revision numbers.
- The installation test packet is a special type of DMX512 data that contains the worst case type of data. Worst case means data that is most likely to make a data distribution problem visible. The benefit of this is that it becomes possible to test a DMX512 installation with a known type of data.
- DMX512-A introduces a new system of unique manufacturer identification codes. These allow processing equipment to be identified from anywhere within the installation.
- Finally, DMX512-A lays the foundations for the RDM or Remote Device Management protocol.
This document is very much an overview of the key changes. A large amount of work has gone into this development, by a small team of volunteers. The standard is expected to be finished in 2002. However, being a consensus process, this is subject to change. Check the USITT Web Site for more info and updates.
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