The instrument descriptor is assigned by VISA to each device that the device driver returns. For GPIB this is done when you scan for instruments from Measurement & Automation Explorer (MAX). Every address on the bus is queried and an instrument descriptor is assigned for each device that responds. For VXI/VME devices the driver includes a resman utility that is used to perform the resource manager function of the system. VISA assigns resource descriptors to the devices that resman finds.
The overall process is this:
1) Each individual driver finds its own devices that are attached. (NI-488.2, NI-VXI, NI-Serial, etc.)
2) Using MAX, VISA is able to assign an instrument descriptor to all devices found.
3) The application then uses a VISA resource control to address a device listed
