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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 July 29th, 2009   #1
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Default Sub Device and Personality

Can each sub-device have a different set of personalities ?
This would mean that
- Each sub-device can be set to a different personality
- Each sub-device can have a different footprint

When I read the RDM standard I think the answer is YES, however in the book CONTROL FREAK, Wayne Howell answers the question with NO(page 110).

When the answers is YES, the controller has to find each sub-device and read the device info to get the current personality/start address/footprint(or use the specific pid to get the value). It is however difficult to find the sub-devices because the id's must not be continues. The device info tells only how many sub-devices there are, not there id's. So I would expect that there exists a pid GET_SUPPORTED_SUBDEVICE_IDS (like SUPPORTED_PARAMETERS), but this is not the case.


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Old July 29th, 2009   #2
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My interpretation is that each sub-device *could* have a different set of personalities.

Per section 9.2.3: "All sub-devices shall report an identical list for SUPPORTED_PARAMETERS." Other than that, there are relatively few restrictions on sub-devices.

I see nothing in the document that requires all sub-devices to have the same personalities.

I agree that there should have been a GET_SUPPORTED_SUBDEVICE_IDS PID in the document. As I recall, allowing gaps in the sub-device list came up relatively late in the development of the document as a way of dealing with dimmer racks that have modules removed. This detail slipped through the cracks.

Fortuntely, in most cases the sub-devices will be sequential, or nearly sequential. You can query each sub-device starting at 1 and work up the list until you've found the proper number of devices. You're unlikely to have a responder with 3 sub-devices numbered as 157, 393, and 503.
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