
SYM53C876/876E Data Manual
4-23
Registers
SCSI Registers
for diagnostic testing and is cleared for normal
operation. It is useful to generate parity errors
to test error handling functions.
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
disconnect. This bit is useful for multi-
threaded applications. The ARB1-0 bits in
SCNTL0 is set for full arbitration and selec-
tion before setting this bit.
Arbitration is re-tried until won. At that point,
the SYM53C876 SCSI function holds BSY
and SEL asserted, and waits for a select or
reselect sequence. The Immediate Arbitration
bit is reset automatically when the selection or
reselection sequence is completed, or times
out. During the time between the assertion of
the IARB bit and the completion of a Perform
Select/Reselect instruction, DMA interrupts
are disabled. Therefore, interrupt instructions
placed between the assertion of the IARB bit
and the Perform Select/Reselect instruction
are not executed.
An unexpected disconnect condition clears
IARB without attempting arbitration. See the
SCSI Disconnect Unexpected bit (SCNTL2,
bit 7) for more information on expected ver-
sus unexpected disconnects.
It is possible to abort an immediate arbitration
sequence. First, set the Abort bit in the
ISTAT register. Then one of two things even-
tually happens:
1. The Won Arbitration bit (SSTAT0 bit 2) is
set. In this case, the Immediate Arbitration bit
needs to be reset. This completes the abort
sequence and disconnects the chip from the
SCSI bus. If it is not acceptable to go to Bus
Free phase immediately following the
arbitration phase, it is possible to perform a
low level selection instead.
2. The abort completes because the
SYM53C876 SCSI function loses arbitration.
This is detected by the clearing of the
Immediate Arbitration bit. Do not use the
Lost Arbitration bit (SSTAT0 bit 3) to detect
this condition. In this case take no further
action.
Bit 0
SST (Start SCSI Transfer)
This bit is automatically set during SCRIPTS
execution. It causes the SCSI core to begin a
SCSI transfer, including SREQ/SACK hand-
shaking. The determination of whether the
transfer is a send or receive is made according
to the value written to the I/O bit in SOCL.
This bit is self-resetting. Do not set it for low
level operation.
CAUTION:
Writing to this register while not connected may
cause the loss of a selection/reselection by reset-
ting the Connected bit.