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 Sequence5

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.

SCSI Registers

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LSI 53C875A technical manual Full Arbitration, Selection/Reselection, Start Start Sequence5