Null Device

The Device Table MAY include addresses that do not represent a functional Device. Possible cases are:

• An address reserved for a device currently not present or active on the system

• A Detachable Device lacking a required transducer element

• A non-functional device due to an error or issue

• An address intentionally left empty by the Hub developer

In summary, a representation of a non-present or non-functional device.

This is done by filling the address’ Device Descriptor with a Null Device. This descriptor is represented by a Device ID of 0.0.0.0. All other fields on the descriptor MUST be 0.

A Null Device:

• MUST NOT consume of produce any kind of data.

• MUST NOT implement any register interface.

• Any attempt to access a Null Device MUST return the same error as accessing a non-existing device or register.

Note

The last two points imply an exception to the requirement for a Register Interface. This exception is justified because a Null Device is not an actual device but a representation of the absence of one.