Oki JOB60851, User Manual, Page: 43 / 67

Chapter 4 Software Development
page 4-8
7 SET_DESCRIPTOR Update or add a descriptor
8 GET_CONFIGURATION Return the current device configuration value
9 SET_CONFIGURATION Set the device configuration value
10 GET_INTERFACE Return the selected alternate setting for the specified interface
11 SET_INTERFACE Set the alternate setting for the specified interface
12 SYNCH_FRAME Set and report an endpoint's synchronization frame
4-1-12. Device Descriptors
When a USB device first connects to the bus, it reports its attributes to the host with a device
descriptor. Figure 4.1.3 illustrates the hierarchical structure of this data structure with the device
descriptor for a bidirectional printer.
Endpoint descriptors give the physical capabilities of the endpoints already discussed in Section
4.1.7 above.
Groups of endpoint descriptors form logical communications functions called interfaces. These
interfaces support alternate settings for changing the interface and thus device characteristics. Note
that these interfaces are mutually exclusive in that only one can be in use at any given time.
n
Device
o
Configuration #1
u
Interface #1
alternate setting #0 ; output-only printer
¡
Endpoint #1
BulkOut
u
Interface #1
alternate setting #1 ; bidirectional printer
¡
Endpoint #1
BulkOut
¡
Endpoint #2
BulkIn
u
Interface #1
alternate setting #2 ; bidirectional printer
¡
Endpoint #1
BulkOut
¡
Endpoint #2
BulkIn
¡
Endpoint #3
InterruptIn ; Inform of data volumes by interrupt transfer
Figure 4.1.3. Device Descriptor for Bidirectional Printer
Higher up the hierarchy are the configuration and device layers. The same device can have multiple
configurations--drawing its power from the bus or its own local power supply, for example. It can
even start as a power-saving device and later switch to a high-consumption one.
The following are brief listings of descriptor contents. For detailed descriptions, refer to Section 9.4
of the USB version 1.1 specifications.
(1) Device descriptor
USB Specification release number in binary coded decimal
Device class code
Device subclass code
Protocol code
Maximum packet size for endpoint 0 (EP0)
Vendor and product IDs