RDM: What is it?

Remote Device Management, (RDM) is an expansion of the DMX512 protocol that allows bi-directional communication between a lighting controller and an RDM compliant device over a DMX line.

RDM was created to allow an RDM enabled Lighting console or an RDM controller to send commands to and receive messages from moving lights, dimmers, and other RDM devices in the rig. Commands such as device discovery, remote configuration, status monitoring, and management of these devices are now possible from the controller, because of RDM.

How it works

A DMX packet consists of these elements:
Break/Mark After Break/Start Code/Mark Time Between Frames/Channel Data/Mark Time Between Packets

The default Start Code for a DMX packet is 0x00. RDM packets, however, use the start code 0xCC, so they can be inserted between DMX data packets without causing interference with non-RDM compatible devices.

The DMX controller uses these packets to decide which device is allowed to send RDM data back to the controller. How does RDM tell which device is which? Each device is assigned a Unique Identifier (UID) by its manufacturer. The UID is comprised of the manufacturer ID and serial number. RDM simply uses the UID numbers assigned to uniquely identify the device.

What’s it for?

The RDM controller can search for and identify all of the RDM devices connected to it using a process called “discovery.” Once all devices have been discovered, and the controller receives no more responses, a list of all connected devices will appear on the RDM Controller. The controller can then communicate with these devices individually (Unicast Communication) or in groups by manufacturer (Broadcast communication).

The digital DMX512 protocol came on the scene in 1986 and quickly replaced the analog one-wire-per-dimmer control method used up to that time. DMX512 is unidirectional, meaning that communication flows in one direction. As a result, DMX does not contain a universal method of returning information from fixtures or dimmers to the console.

RDM, on the other hand, is bi-directional. The RDM controller can send out specific commands to any device on the line, and those same devices can, in return, send back confirmation of the received command. Using RDM, devices can also send back responses to the controller that indicate status for things including lamp state, lamp hours, temperature, voltage, operation modes, and errors.

RDM also allows the controller to initiate commands for device functions such as lamp control, recalibration requests, and fan control that are not manufacturer specific as well as broadcast commands to specific groups of devices based on manufacturer.

How will you use it?

RDM is integrated into the DMX signal, so it is primarily suited for permanent installations including houses of worship, retail, and architectural lighting as well as stage lighting for live performances.

Using this protocol has the potential to become an integral part of the lighting technician’s setup and maintenance routines for their lighting rig because it provides him/her with easier access to critical data, including:

1. Identification and classification of all connected devices
The RDM console can quickly identify all RDM devices connected to the console; as a result, this will simplify the patching process on the console considerably by eliminating the need to select fixture types from the library before patching

2. Remote addressing of these devices
A huge advantage of using RDM to remotely address a moving light is being able to avoid having to access onboard menus or DIP switches from the ground once the fixtures are at trim height. In essence, this means no more getting ladders out to climb up and address lights after they have been hung.

3. Configuration
Most manufacturers incorporate several operational modes in fixtures and other DMX devices. These modes are typically used to determine how many DMX channels are required to control the fixture (Standard mode or Extended mode, for example). These modes may also be used to disable or enable certain other fixture specific features like lamp power, color and gobo wheel functionality, onboard macros, etc.

RDM makes it possible to configure these modes from the controller at any time. It’s important to note here that the RDM commands are specific to the fixture type and are governed by how RDM has been implemented by a fixture’s manufacturer.

4. Status reporting during the operation of the rig
RDM devices can report status and error messages back to the RDM controller, offering information including lamp status and lamp hours, or they can alert the user to potential problems.

Things to consider when planning an RDM network
RDM has been designed around DMX-512. Without DMX present, there is no RDM. Also, commands issued by an RDM controller must receive a reply from the RDM devices within a designated time-frame. If a device does not respond within that time-frame, the controller will either retry its request or give up.

This is not a problem in a DMX system using 5pin XLR, as the delay between the device and the controller will be very short. But it could be an issue in Ethernet based networks where response times could take longer.

In order to ensure that responses from an RDM device reach the RDM Controller, it is preferred that there can only be one controller on a single line. If the lighting control system requires the merging of multiple sources of DMX, it is more difficult to utilize RDM because the responses need to be routed from the RDM devices back to the correct RDM controller.

It is also difficult for responses from a DMX-RDM device to reach the RDM controller if it has to pass through an interface, such as an Ethernet switch or wireless interface. Attention must be paid to using RDM compatible third party devices when setting up the DMX/RDM communication line. This is important for the future of RDM, because lighting control systems are beginning to implement Ethernet based DMX control signals like ArtNet. Such systems will require that the control signal be converted through an Ethernet-to-DMX output device and then the signal can be passed on to DMX-based devices so that RDM will be available.

What equipment is needed for using RDM?

A typical RDM system requires a controller and one or more RDM devices. All existing DMX512 cable remains usable with RDM, but non-RDM compliant DMX devices like splitters and optos will need to be replaced with RDM enabled versions. Many RDM compliant devices like splitters, data distribution amplifiers, testers and controllers are available from companies like Pathport, Doug Fleenor, and Artistic License. New RDM products will continue to enter the market as awareness of the RDM protocol increases.

RDM is a big advancement in lighting protocol capabilities, and offers the lighting programmer and technician a valuable tool in fixture status and error reporting without have to drag out the ladder or fly in the truss to read the onboard menu. RDM has many advantages, and now with more and more RDM compliant equipment on the market, it has relatively few disadvantages.
If you’re planning a new installation or just want to add this functionality to your existing rig, you can’t go wrong!

If you’re interested in learning more about RDM, please visit the ESTA website at http://www.esta.org

For discussions on applications and other RDM related information, visit http://www.rdmprotocol.org/forums/

RDM was approved by the Technical Standards Program for ESTA on March 31, 2006, and is officially known as “ANSI/ESTA E1.20, Entertainment Technology – Remote Device Management over USITT DMX512”. The RDM standard is currently being developed by the RDM Task Group, a part of the ESTA Control Protocols Working Group and is being designed for interoperability between many manufacturers.