Rob is a Lighting Designer and Lighting Programmer and currently Senior Lecturer in Technical Theatre Production. He is also the Editor of On Stage Lighting and teaches stage lighting practice.
A guide to the basics of RDM (Remote Device Management) DMX. How it works and what it does.
What is RDM DMX?
You might know that DMX512 is a one way, serial control signal, the basics of which are explained in DMX Lighting Systems.
RDM is a development built on the DMX512 stage lighting control protocol that enables Remote Device Management of devices such as moving lights, dimmers and other DMX effects. Outlined in ANSI E1.20-2006 and using the existing DMX signal cores (pins 2 and 3), RDM allows two way communication between a lighting controller and the fixtures in rig.
Two way communication allows the controller to interrogate other RDM devices and make changes to their settings. Common uses might be remote setting of DMX start addresses from the console or collecting fault reports from the equipment.
What equipment do you need to use RDM DMX?
Lighting equipment manufacturers are trumpeting the fact that their new Wobbli Buckettes ™ are RDM compatible. This is because, while RDM systems are backwards compatible with “normal” DMX, the main components of RDM DMX need to be able to deal with the new two way system.
So, if you want to use the RDM functions you will need a controller that can do it. This doesn’t have to be the main lighting desk , it could be a laptop. You will also need devices (fixtures, dimmers etc) that have some degree of RDM control. Importantly, you will need to use RDM compatible DMX buffers/splitters in those parts of your DMX system – these gateways must allow the two way communication all the way back to the controller.
You can still use your old 3 core cables – woo hoo – but RDM is not as “easy” as a normal DMX signal. This means that the correct system design, cabling and termination is even more important. Using cheap mic cables won’t be a satisfactory option (not that it is at the moment).
How does RDM work?
Normal DMX values are sent along the line from the controller and “heard” by all the devices in that DMX universe. RDM values are sent back the other way – but not constantly. During an RDM interaction the controller can ask one or more devices for some information, which they then return. The RDM interactions then subside until the next query. In this way, RDM does not eat up 50% of the signal capacity. During normal operation, the RDM part of the signal only accounts for around 10% – 15% of the action.
The controller can also send instructions to the devices, such as parameters setting, without asking for a response.
For the system to work, the individual parts must be indentifiable. RDM uses UID (Unique Indentification Number) fixed within each device, as well as a manufacturer ID. This is an unalterable hardware setting and is not the same as the DMX start address.
To determine the UID of each RDM device on the line, the controller uses a “one off” process called “discovery”. This is a signal and computer hungry process, begun after all devices are connected up. The actual process of UID discovery is a bit like a digital game of “20 Questions” that gradually eliminates all the “players” (devices) until the controller is satisfied that it has the UID of all the RDM equipment.
How do I set DMX addresses remotely?
So far, the RDM controller knows the UID and model of each fixture – but not where it is in your rig. To locate each fixture and set a DMX address the user can cycle through the devices, each one showing themselves. Once a fixture is identified the operator can set the desired DMX address or any other options available. Order is restored.
What else could RDM do?
As well as setting DMX addresses from the floor, two way RDM communication between operator and device brings other uses such as:
- Lamp hours monitoring
- Temperature sensor reporting
- Fault codes
- Fixture mode setting
- Attribute inverting – Pan/Tilt etc.
The benefit of these depend on your rig and situation, but the possibility of more advanced stage lighting control using a simple and robust system like DMX can only be a good thing. Although lighting manufacturers are now building equipment with one eye on RDM, the actual implementation is slow going. Other developments in the integration of networks, media and lighting control might get in the way. We’ll just have to wait and see.
If you have any questions or comments about RDM, put them in the box below.
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