Why do we need RDM?

The DMX512 standard has served the industry well for over 17 years. It made possible the independent development and sale of controllers, dimmers, and moving lights. DMX512 is a simple system of transferring “live level data” from a console to a dimmer, moving light or fixture accessory. Clever schemes have been developed to extend this to other types of control such as lamp on/off, fixture recalibration and the like.

As systems using DMX512 have become increasingly more complex, the limits of these schemes have been reached. Further, DMX512 is “talk only”: there is no cross-manufacturer method of returning information from fixtures or dimmers.

RDM allows explicit commands to be sent to a device and responses to be received from it without the use of typical “set a value on channel 1 and then wiggle channel 10” schemes. Functions such as lamp control, recalibration requests, and fan control become standardized commands for all devices. Further, since RDM is bi-directional, devices can send back confirmation of the command as well as responses that indicate state or status: lamp condition (on or off), lamp hours, temperature, voltage, etc.

In the past, proprietary control systems have been developed to provide these capabilities. RDM offers both users and manufacturers a standard protocol on which to build interoperable consoles, controllers, intelligent fixtures, dimmers, and fixture accessories, which can communicate with each other.

Manufacturers, however, are not limited to the standard commands. RDM provides for manufacturer-specific commands that can be defined as needed in order to cope with the requirements of introducing new equipment.

What equipment is required for an RDM control system?

A basic RDM system consists of a controller and one or more RDM devices. The controller can be part of the lighting console, a separate box that connects in line between the console and the controlled devices, or part of an Ethernet DMX512-type distribution system. An RDM Controller could also be part of a gateway to an ACN system. As only the primary DMX512 pair on pins 2 and 3 are used, all existing DMX512 cable remains usable with RDM. Existing DMX splitters and opto-isolators will need to be replaced with RDM capable bi-directional units.

How does RDM work?

The RDM protocol allows data packets to be inserted into a DMX512 data stream without adversely affecting existing non-RDM equipment. By using a special “Start Code,” and by complying with the timing specifications for DMX512, the RDM protocol allows a console or dedicated RDM controller to send commands to and receive messages from specific moving lights, dimmers, color scrollers, and other RDM enabled devices.

Each RDM device is assigned a Unique Identifier (UID) by its manufacturer. This UID is composed of the Manufacturer’s ID and a “serial” number to uniquely identify the device. The RDM controller or console can search for and identify all of the RDM devices connected to it using a process called “discovery.” Once discovered, the controller can communicate with devices individually or in groups by manufacturer.

All of this RDM communication takes place during the time between standard DMX512 “Null START code” frames of level data and is ignored by existing DMX512 compliant equipment. At any time, all RDM traffic may be disabled, removing any speed penalty or compatibility issues with non-compliant equipment in a show critical environment.

When can I have RDM?

The RDM standard was released in 2006, and manufacturers have been implementing it in their products since then. Having said that, just because your product was purchased after 2006 does not immediately mean it will work with RDM.

Manufacturers are adopting the RDM logo on their products to indicate RDM enabled firmware is installed.  You may even already have it!

Who made RDM?

RDM was created by the Control Protocols Working Group, a group of manufacturers with a particular interest in control protocols within the entertainment industry.  The basis of the RDM protocol was the High End Systems Talkback Protocol, which was implemented in a number of High End Systems products.  A need was identified for cross compatibility of these messages, so a group of engineers from manufacturers across the industry decided to create an official industry standard for bi-directional communication.  In 2006, the RDM protocol became a formal ANSI standard (ANSI E1.20-2006).

How much does RDM cost?

The good news is that RDM is generally free of charge, either as an upgrade or already installed in new products that you’re buying.  Certain manufacturers do charge for software updates, so please check with the manufacturer of your equipment for exact details.

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