Quantum 10K II - Manual (Page 362)

Appendix B. SCSI Bus Signal Timing
Quantum Atlas 10K II Ultra 160/m SCSI Hard Disk Drives B–41
B.8.24 PARALLEL PROTOCOL REQUEST Message (04h) (continued)
Table B–18 PARALLEL PROTOCOL REQUEST Message — Field Descriptions
(continued)
Field Description
Transfer Width Exponent The value within this field defines the width of the transfer used during
data-in buffer transfers and data-out buffer transfers. The transfer width
that is established applies to all logical units on both SCSI devices. Valid
transfer widths are 8 bits (m = 00h) and 16 bits (m = 01h), if the
Protocol Options field value is 0h; for any other value in the Protocol
Options field, the only valid transfer width is 16 bits.
For best performance for an Atlas 10K IIdisk drive, the value of this field
is 01h.
Protocol Option Bits
(IU_REQ, DT_REQ, and
QAS_REQ)
This bits are used by the originating SCSI device to indicate which
protocol options are to be enabled. The SCSI device responding uses
these bits to indicate which of the requested options have been
enabled.
An Information Units Enable Request (IU_REQ) = 0 indicates that
information units are to be disabled when received from the originator
and that information units received from the responding SCSI device
are not supported. An IU_REQ bit = 1 indicates that information units
are to be enabled when received from the originator and that
information units received from the responding device are supported.
A Dual Transition Request (DT_REQ) = 0 indicates DT DATA phases are
to be disabled when received from the originator and that information
units received from the responding SCSI device are not supported. An
DT_REQ bit = 1 indicates that DT DATA phases are to be enabled when
received from the originator and that DT DATA phases received from
the responding device are supported.
A QAS Enable Request (QAS_REQ) = 0 indicates that QAS is to be
disabled when received from the originator and QAS received from the
responding SCSI device is not supported. An QAS_REQ bit = 1 indicates
that QAS is to be enabled when received from the originator and that
QAS received from the responding device are supported.
The Atlas 10K II disk drive supports two operating modes selected via
the Protocol Option Bits:
QAS_REQ DT_REQ
IU_REQ Description
0 0 0 Use ST data-in buffer transfers and
ST data-out buffer transfers to
transfer data.
0 1 0 Use DT data-in buffer transfers and
DT data-out buffer transfers to
transfer data.
For best performance for an Atlas 10K II disk drive the value of Byte 7
should be 02h.