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SCSI Registers 4-21
Full Arbitration, Selection/Reselection
1. The LSI53C875A waits for a bus free condition.
2. It asserts SBSY/ and its SCSI ID (the highest priority
ID stored in the SCSI Chip ID (SCID) register) onto the
SCSI bus.
3. If the SSEL/ signal is asserted by another SCSI device
or if the LSI53C875A detects a higher priority ID, the
LSI53C875A deasserts SBSY, deasserts its ID, and
waits until the next bus free state to try arbitration
again.
4. The LSI53C875A repeats arbitration until it wins
control of the SCSI bus. When it wins, the Won
Arbitration bit is set in the SCSI Status Zero (SSTAT0)
register, bit 2.
5. The LSI53C875A performs selection by asserting the
following onto the SCSI bus: SSEL/, the target’s ID
(stored in the SCSI Destination ID (SDID) register),
and the LSI53C875A’s ID (stored in the SCSI Chip ID
(SCID) register).
6. After a selection is complete, the Function Complete
bit is set in the SCSI Interrupt Status Zero (SIST0)
register, bit 6.
7. If a selection time-out occurs, the Selection Time-Out
bit is set in the SCSI Interrupt Status One (SIST1)
register, bit 2.
START Start Sequence 5
When this bit is set, the LSI53C875A starts the arbitration
sequence indicated by the Arbitration Mode bits. The
Start Sequence bit is accessed directly in
low level mode; during SCSI SCRIPTS operations, this bit
is controlled by the SCRIPTS processor. Do not start an
arbitration sequence if the connected (CON) bit in the
SCSI Control One (SCNTL1) register, bit 4, indicates that
the LSI53C875A is already connected to the SCSI bus.
This bit is automatically cleared when the arbitration
sequence is complete. If a sequence is aborted, check
bit 4 in the SCNTL1 register to verify that the
LSI53C875A is not connected to the SCSI bus.