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RDM Interpretation Questions Discussion and questions relating to interpreting and understanding the E1.20 RDM Standard.

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Old April 22nd, 2013   #1
Join Date: Aug 2011
Posts: 32
Default Real World Sub-Devices

We are scratching our heads here on how we should go about handling sub-devices. I will give you some scenarios and also mention how we intend to handle them. If anyone has a device which supports sub-devices then I would love to hear from you or meet at the plug fest.

Just a bit of background specific to our implementation. Due to the limitations of the RDM personality interrogation we instead resort to matching the fixture to a personality in our library. To identify the correct personality and mode we use the Manufacturer Id, Model Id, Personality/Mode, Software Version. This allows us to auto patch the fixture with a fully fledged personality. We are populating the personalities with the RDM info from the open lighting database.

Scenario 1

A dimmer has 24 channels, each channel could either be configured with a relay module or a dimmer module. Each channel is a separate sub-device in the dimmer fixture.

Problem 1

What data should the lighting console use to determine that the channel is a relay and not a 0-100% dimmer channel.


1) Use a different personality/mode
2) Use a different model id
3) Use the product category
4) Use the personality description

Our Answer

It does not seem correct to use the mode since this is something you can change through RDM. The personality description could be used but it does not scale well as matching the correct personality would be difficult with more complex fixtures. You may also get duplicate matches. The product category whilst promising is too loosely defined to be usable. So we have resorted to using the model id. However this relies on manufacturers giving each sub device with a different personality a different model id. So we have no good solution.

Scenario 2

A media server with the following layer controls. Each layer can be assigned a different DMX address. The layers may span multiple universes.

Master Layer
Layer 1
Layer 2

Layers 1 and 2 all have the same personality but effects and master layer are different.

Problem 2A

What data should the lighting console use to determine whether it is a normal layer, master layer or effects. They must be separate devices as each can have its own DMX address.

Problem 2B

How does the console know that some sub-devices are on one universe and the rest are on a separate universe.

Solutions 2A

1) Use a different personality/mode
2) Use a different model id
3) Use the product category
4) Use the personality description

Solutions 2A

1) a device only reports the devices on that universe.

Our Answer

Each layer must have a seperate model id.

When the device is spilt over several universes then you will get the case where a device id appears on two ports. This will cause problems.

The problem we have is that for this all to work, all implementations of sub-devices must come to the same conclusion that each sub-device needs a different model id. I can not see this happening from people reading the spec. It is only when you see the problem that you will come to this solution.

So our conclusion is that this kind of functionality whilst hinted at through RDM is not practically possible when dealing with sub-devices. The only way it might work is for us to use the very basic personalities provided through RDM. Unfortunately this is not an option for us.

I am not sure where to go from here. I want to implement sub-devices as I think some of the behaviour is desirable. However there seem to be too many holes for it to be usable and compatible in a real world enviroment.
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