
SCSI Registers
5-85
This bit must never be set during fast SCSI (greater than
5 mega transfers per second) operations, because a valid
assertion could be treated as a glitch.
This bit does not affect the ltering period when the Ultra
Enable bit in the SCNTL3 register is set. When the
SYM53C895 is executing Ultra2 SCSI transfers, the
ltering period is automatically set at 8 ns. When the
SYM53C895 is executing Ultra SCSI transfers, the
ltering period is automatically set at 15 ns.
LOW
(SCSI Low level Mode)
0
Setting this bit places the SYM53C895 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
SCNTL0 register. SCSI bus transfers are performed by
manually asserting and polling SCSI signals. Clearing
this bit allows instructions to be executed in SCSI
SCRIPTS mode.
Note:
It is not necessary to set this bit for access to the SCSI bit-
level registers (SODL, SBCL, and input registers).
Register: 0x4F (0xCF)
SCSI Test Three (STEST3)
Read/Write
TE
TolerANT Enable
7
Setting this bit enables the active negation portion of
LSI Logic Symbios TolerANT technology. Active negation
causes the SCSI Request, Acknowledge, Data, and Par-
ity signals to be actively deasserted, instead of relying on
external pull-ups, when the SYM53C895 is driving these
signals. Active deassertion of these signals will occur
only when the SYM53C895 is in an information transfer
phase. When operating in a differential environment or at
fast SCSI timings, TolerANT Active negation should be
enabled to improve setup and deassertion times. Active
negation is disabled after reset or when this bit is cleared.
76543210
TE
STR
HSC
DSI
S16
TTM
CSF
STW
00000000