Information Technology Reference
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INQUIRY response with PERIPHERAL DEVICE TYPE = 00h (direct
access block device) and VERSION = 05h (SPC-3) should implement all
commands defined as mandatory in the SPC-3 specification and all manda-
tory commands in a SCSI block commands specification such as SBC-2 or
SBC-3. Table 6-1 shows the mandatory commands and some optional com-
mands for devices that use SCSI block commands. In practice, however,
many devices don't implement every mandatory command.
The USB-IF is developing a USB Mass Storage Class Compliance Test Spec-
ification, which names required and optional SCSI and multimedia com-
mands for different peripheral device types. At this writing, a preliminary
version of the specification is available from the USB-IF. Also available is a
preliminary version of the USB-IF's Command Verifier software (USBCV)
with mass-storage compliance tests. Check usb.org for the latest versions.
It's also useful to learn what commands your device's host(s) typically use
and to be sure to implement those commands in your device. Some develop-
ers concentrate on supporting these commands rather than implementing
every command a specification dictates. To learn what commands a host
issues, use a bus analyzer to observe bus traffic when your device or similar
devices are attached to a host.
In practice, to enable communications, a device should implement at mini-
mum these SCSI commands:
INQUIRY
READ CAPACITY(10)
READ(10)
REQUEST SENSE
TEST UNIT READY
WRITE(10) (for writable devices)
A specific host or device is likely to require commands in addition to these,
however. When a device claims to implement a command set, it's reasonable
for a host to assume that the device supports all commands declared as man-
datory in that command set.
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