SubDevice Required PIDs
 

What PID if any, is required to be supported by an SubDevice. The spec says that Supported Parameters is only required if the root has supported parameters. I would interpret the spec to mean "Required" for the root and all sub-devices. How can the controller determine which SubDevices are valid since they do not have to be consecutively numbered between 1 and Sub-device_Count? To me the would Required means each device not just the root.

This thread covers the required parameters for a Sub-Device.
http://www.rdmprotocol.org/forums/showthread.php?t=1034

To find the subdevices that exist, really all you can do is send a GET DEVICE_INFO (or other similar PID) to every sub-device starting with 1 and work your way up until you find all of the sub-devices that the responder reports. If the responder returns a NACK with NR_SUB_DEVICE_OUT_OF_RANGE, you can conclude that subdevice does not exist and move on to try the next one.

There's been some discussion of adding a PID that will retrieve a packed list of sub-devices in the future, but it doesn't exist yet.