LSI53C875 LSI Logic, LSI53C875 Datasheet - Page 190

• Manufacturer Part Number: LSI53C875
• Description: PCI to Ultra SCSI I/O Processor
• Manufacturer: LSI Logic

5-74
SLB
SZM
AWS
EXT
LOW
SCSI Operating Registers
SCSI Loopback Mode
Setting this bit allows the LSI53C875 to perform SCSI
loopback diagnostics. That is, it enables the SCSI core to
simultaneously perform as both the initiator and the
target.
SCSI High Impedance Mode
Setting this bit places all the open drain 48 mA SCSI
drivers into a high impedance state. This is to allow
internal loopback mode operation without affecting the
SCSI bus.
Always Wide SCSI
When this bit is set, all SCSI information transfers are
done in 16-bit wide mode. This includes data, message,
command, status, and reserved phases. Normally,
deassert this bit since 16-bit wide message, command,
and status phases are not supported by the SCSI
specifications.
Extend SREQ/SACK Filtering
TolerANT SCSI receiver technology includes a special
digital filter on the SREQ/ and SACK/ pins which causes
the disregarding of glitches on deasserting edges. Setting
this bit increases the filtering period from 30 ns to 60 ns
on the deasserting edge of the SREQ/ and SACK/
signals. Never set this bit during fast SCSI (greater than
5 Mbytes transfers per second) operations, because a
valid assertion could be treated as a glitch. This bit does
not affect the filtering period when the Ultra Enable bit in
the
the LSI53C875 is executing Ultra SCSI transfers, the
filtering period is automatically set at 15 ns.
SCSI Low level Mode
Setting this bit places the LSI53C875 in low level mode.
In this mode, no DMA operations occur, and no SCRIPTS
execute. Arbitration and selection may be performed by
setting the start sequence bit as described in the
Control Zero (SCNTL0)
performed by manually asserting and polling SCSI
signals. Clearing this bit allows instructions to be
executed in SCSI SCRIPTS mode. It is not necessary to
SCSI Control Three (SCNTL3)
register. SCSI bus transfers are
register is set. When
SCSI
4
3
2
1
0