
5-18
Registers
Full Arbitration, Selection/Reselection
1.
The SYM53C895 waits for a bus free condition.
2.
It asserts SBSY/ and its SCSI ID (the highest priority
ID stored in the SCID register) onto the SCSI bus.
3.
If the SSEL/ signal is asserted by another SCSI
device or if the SYM53C895 detects a higher priority
ID, the SYM53C895 deasserts SBSY, deasserts its
ID, and waits until the next bus free state to try
arbitration again.
4.
The SYM53C895 repeats arbitration until it wins
control of the SCSI bus. When it has won, the Won
Arbitration bit is set in the SSTAT0 register, bit 2.
5.
The SYM53C895 performs selection by asserting
the following onto the SCSI bus: SSEL/, the target ID
(stored in the SDID register), and the SYM53C895
ID (stored in the SCSI Chip ID (SCID) register).
6.
After a selection is complete, the Function Complete
bit is set in the SIST0 register, bit 6.
7.
If a selection timeout occurs, the Selection Timeout
bit is set in the SIST1 register, bit 2.
START
Start Sequence
5
When this bit is set, the SYM53C895 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, the SCRIPTS
processor controls this bit. An arbitration sequence
should not be started if the connected (CON) bit in the
SCSI Control One (SCNTL1) register, bit 4, indicates that
the SYM53C895 is already connected to the SCSI bus.
This bit is automatically cleared when the arbitration
sequence is complete. If a sequence is aborted, bit 4 in
the SCNTL1 register should be checked to verify that the
SYM53C895 did not connect to the SCSI bus.
WATN
Select with SATN/ on a Start Sequence
4
When this bit is set and the SYM53C895 is in initiator
mode, the SATN/ signal is asserted during SYM53C895
selection of a SCSI target device. This is to inform the
target that the SYM53C895 has a message to send. If a