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SCSI Operating Registers
5-8
SYM53C825A/825AE Data Manual
Bit 1
IARB (Immediate Arbitration)
Setting this bit causes the SCSI core to imme-
diately begin arbitration once a Bus Free phase
is detected following an expected SCSI discon-
nect. T his bit is useful for multi-threaded appli-
cations. T he ARB1-0 bits in SCNT L0 should
be set for full arbitration and selection before
setting this bit.
Arbitration will be re-tried until won. At that
point, the SYM53C825A will hold BSY and
SEL asserted, and wait for a select or reselect
sequence to be requested. T he Immediate
Arbitration bit will be reset automatically when
the selection or reselection sequence is com-
pleted, or times out.
An unexpected disconnect condition will clear
IARB without attempting arbitration. See the
SCSI Disconnect Unexpected bit (SCNT L2,
bit 7) for more information on expected versus
unexpected disconnects.
An immediate arbitration sequence can be
aborted. First, the Abort bit in the ISTAT reg-
ister should be set. T hen one of two things will
eventually happen:
1. T he Won Arbitration bit (SSTAT 0 bit 2)
will be set. In this case, the Immediate
Arbitration bit needs to be reset. T his will
complete the abort sequence and
disconnect the SYM53C825A from the
SCSI bus. If it is not acceptable to go to
Bus Free phase immediately following the
arbitration phase, a low level selection may
be performed instead.
2. T he abort will complete because the
SYM53C825A loses arbitration. T his can
be detected by the Immediate Arbitration
bit being cleared. T he Lost Arbitration bit
(SSTAT 0 bit 3) should not be used to
detect this condition. No further action
needs to be taken in this case.
Bit 0
SST (Start SCSI Transfer)
T his bit is automatically set during SCRIPT S
execution, and should not be used. It causes
the SCSI core to begin a SCSI transfer, includ-
ing SREQ/SACK handshaking. T he determi-
nation of whether the transfer is a send or
receive is made according to the value written
to the I/O bit in SOCL. T his bit is self-reset-
ting. It should not be set for low level opera-
tion.
CAUT ION:
Writing to this register while not connected may
cause the loss of a selection/reselection by resetting
the Connected bit.