The RDM standard is now published, and products are coming on the market that use the methods described in the standard to collect information about equipment on a DMX512 network and to set parameters such as a DMX starting address and operating modes from the lighting control console. Officially called ANSI E1.20-2006, Entertainment Technology-RDM-Remote Device Management Over DMX512 Networks, the standard builds on USITT's DMX512/1990 and its update, ANSI E1.11-2004. Products that support RDM will have features impossible without it, and older products that don't support RDM but conform to the DMX512/1990 or ANSI E1.11 standards won't be affected by it running on the network; those conforming products simply ignore the RDM messages.

Unfortunately, not all “DMX512” products on the market actually conform to the DMX512 or ANSI E1.11 standards. These non-conforming products may flicker or behave in unexpected ways when RDM is used on a network. The problem is that they don't check the START Code — the first word — of a DMX512 message. With the original DMX512 standard and ANSI E1.11, the first word of a message that contains level data sent by a control console to a dimmer rack or other controlled device is a null START Code; it has the value of zero. With the original USITT DMX512 standard, any START Code other than zero (an Alternate START Code) meant that the message had some special meaning not defined by the standard. ANSI E1.11-2004 defines the meanings of many of these Alternate START Codes. ANSI E1.20, RDM, uses the Alternate START Code of CCh (204 in decimal) to indicate that its messages are RDM messages. However, in the early days of DMX512 and for a long time after that, there were rarely any messages on a DMX512 network that had a START Code other than zero. Some manufacturers designed their products with the assumption that there would be no messages with non-zero START Codes, so their products don't read the START Code at all. Problems start when these products read the RDM messages between the control console and the controlled RDM devices as messages meant for them. It's a bit like opening an Adobe Acrobat file with Microsoft Word. The results are interesting but not very useful.

What's to be done? Well, if you put RDM equipment on a DMX512 network, and your older equipment goes crazy, your older equipment probably is not checking the START Codes. One solution is to get rid of your older equipment and buy new, but there are less expensive options. Most of the RDM splitters on the market have some provision for blocking messages with non-null START Codes on one of more of the ports. That blocked port can be used to isolate the non-compliant equipment that doesn't read START Codes. Other products to isolate non-compliant equipment may come on the market as RDM is used more widely.

If you are buying new DMX equipment, then it would be wise to check if it reads the START Codes. Anything that supports RDM will, and anything that is claimed to be USITT DMX512/1990 or ANSI E1.11-2004-compliant should — it's not compliant if it doesn't — but it doesn't hurt to check. Even if you never plan to implement RDM in your lighting system, there are a number of manufacturers with registered Alternate START Codes. You can see the listing of them at www.esta.org/tsp/working_groups/CP/DMXAlternateCodes.php.

One way or another, equipment that uses Alternate START Codes may be on your DMX512 network soon.