On Monday, November 8, the ANSI Board of Standards Review approved "Entertainment Technology - USITT DMX512-A - Asynchronous Serial Digital Data Transmission Standard for Controlling Lighting Equipment and Accessories." Publication is expected in about one month.
The new standard exemplifies the 90/10 Rule: 90% of the job takes 10% of the effort; the remaining 10% of the job takes 90% of the effort. The older USITT DMX512/1990 has been very useful, getting most of the details right for communication between lighting control equipment made by different manufacturers. However, that standard left out some of the details that can cause lots of headaches. For example, the original USITT DMX512/1990 standard does not mention the need for data line termination and leaves that to be covered by the referenced EIA-485 standard, which does it only in very general terms.
ANSI E1.11-2004 requires termination, states where the termination should be, and gives the proper termination impedance value. The original DMX512 standard recommends some kind of electrical isolation to guard against the propagation of voltages outside the limits of EIA-485 but gives no specifics on how this isolation should be provided. The original DMX512 standard also doesn't mention how or where the signal common should be connected to ground, and the referenced EIA-485 is only a bit less vague and seems to have been written with the assumption that an engineer will be designing the entire EIA-485 system.
In the entertainment industry, lighting manufacturers more often make components that are assembled by end-users to create complete systems, and not all the isolation or grounding practices that may have been chosen by the different manufacturers work well together. Experience has shown that more guidance on isolation and grounding is needed, and ANSI E1.11 provides this.
The original USITT DMX512/1990 standard is six pages long. ANSI E1.11-2004 will be about 60 pages long when published. ANSI E1.11-2004 was developed by the Control Protocols Working Group with input from the wider lighting community via public reviews. Two of the CPWG's current standards-drafting projects, BSR E1.20 (RDM) and BSR E1.27-1 (E1.11 and DMX512 portable cabling) are available for public review now and can be accessed by anyone at www.esta.org.