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SCSI Operating Registers
5-50
SYM53C825A/825AE Data Manual
Register 4E (CE)
SCSI Test Two (ST EST 2)
Read/Write
Bit 7
SCE (SCSI Control E nable)
T his bit, when set, allows all SCSI control and
data lines to be asserted through the SOCL
and SODL registers regardless of whether the
SYM53C825A is configured as a target or ini-
tiator.
Note: T his bit should not be set during normal
operation, since it could cause contention
on the SCSI bus. It is included for
diagnostic purposes only.
Bit 6
ROF (Reset SCSI Offset)
Setting this bit clears any outstanding synchro-
nous SREQ/SACK offset. T his bit should be
set if a SCSI gross error condition occurs, to
clear the offset when a synchronous transfer
does not complete successfully. T he bit auto-
matically clears itself after resetting the syn-
chronous offset.
Bit 5
DIF (SCSI Differential Mode)
Setting this bit allows the SYM53C825A to
interface properly to external differential trans-
ceivers. Its only real effect is to tri-state the
SBSY/, SSEL/, and SRST / pads so that they
can be used as pure inputs. Clearing this bit
enables single-ended mode operation. T his bit
should be set in the initialization routine if the
differential pair interface is used.
Bit 4
SLB (SCSI Loopback Mode)
Setting this bit allows the SYM53C825A to
perform SCSI loopback diagnostics. T hat is, it
enables the SCSI core to simultaneously per-
form as both initiator and target.
Bit 3
SZM (SCSI High-Impedance Mode)
Setting this bit places all the open-drain
48 mA SCSI drivers into a high-impedance
state. T his is to allow internal loopback mode
operation without affecting the SCSI bus.
Bit 2
AWS (Always Wide SCSI)
When this bit is set, all SCSI information
transfers will be done in 16-bit wide mode.
T his includes data, message, command, status
and reserved phases. T his bit should normally
be deasserted since 16-bit wide message, com-
mand, and status phases are not supported by
the SCSI specifications.
Bit 1
E X T (E xtend SRE Q/SACK Filter-
ing)
Symbios TolerANT SCSI receiver technology
includes a special digital filter on the SREQ/
and SACK / pins which will cause glitches on
deasserting edges to be disregarded. Setting
this bit will increase the filtering period from
30ns to 60ns on the deasserting edge of the
SREQ/ and SACK / signals.
Note: T his bit must never be set during fast SCSI
(greater than 5M transfers per second)
operations, because a valid assertion could
be treated as a glitch.
Bit 0
LOW (SCSI Low level Mode)
Setting this bit places the SYM53C825A in
low level mode. In this mode, no DMA opera-
tions occur, and no SCRIPT S execute. Arbi-
tration and selection may be performed by
setting the start sequence bit as described in
the SCNT L0 register. SCSI bus transfers are
performed by manually asserting and polling
SCSI signals. Clearing this bit allows instruc-
tions to be executed in SCSI SCRIPT S mode.
Note: It is not necessary to set this bit for access
to the SCSI bit-level registers (SODL,
SBCL, and input registers).
SCE
7
ROF
6
DIF
5
SLB
4
SZM
3
AWS
2
EXT
1
LOW
0
Default>>>
0
0
0
0
0
0
0
0