Oki JOB60851 user manual Device Descriptors, Device Descriptor for Bidirectional Printer

Chapter 4 Software Development

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

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