
SCSI Operating Registers
SYM53C875/875E Data Manual
5-5
Register 00 (80)
SCSI Control Zero (SCNT L0)
Read/Write
Bit 7
ARB1 (Arbitration Mode bit 1)
Bit 6
ARB0 (Arbitration Mode bit 0)
Simple Arbitration
1. T he SYM53C875 waits for a bus free
condition to occur.
2. It asserts SBSY/ and its SCSI ID
(contained in the SCID register) onto the
SCSI bus. If the SSEL/ signal is asserted by
another SCSI device, the SYM53C875 will
deassert SBSY/, deassert its ID, and set the
Lost Arbitration bit (bit 3) in the SSTAT 0
register.
3. After an arbitration delay, the CPU should
read the SBDL register to check if a higher
priority SCSI ID is present. If no higher
priority ID bit is set, and the Lost
Arbitration bit is not set, the SYM53C875
has won arbitration.
4. Once the SYM53C875 has won
arbitration, SSEL/ must be asserted via the
SOCL for a bus clear plus a bus settle delay
(1.2
μ
s) before a low level selection can be
performed.
Full Arbitration, Selection/Reselection
1. T he SYM53C875 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 SYM53C875 detects
a higher priority ID, the SYM53C875 will
deassert BSY, deassert its ID, and wait
until the next bus free state to try
arbitration again.
4. T he SYM53C875 repeats arbitration until
it wins control of the SCSI bus. When it
has won, the Won Arbitration bit is set in
the SSTAT 0 register, bit 2.
5. T he SYM53C875 performs selection by
asserting the following onto the SCSI bus:
SSEL/, the target’s ID (stored in the SDID
register), and the SYM53C875’s ID
(stored in the SCID register).
6. After a selection is complete, the Function
Complete bit is set in the SIST 0 register,
bit 6.
7. If a selection time-out occurs, the Selection
T ime-Out bit is set in the SIST 1 register,
bit 2.
Bit 5
START (Start Sequence)
When this bit is set, the SYM53C875 will start
the arbitration sequence indicated by the Arbi-
tration Mode bits. T he Start Sequence bit is
accessed directly in low-level mode; during
SCSI SCRIPT S operations, this bit is con-
trolled by the SCRIPT S processor. An arbitra-
tion sequence should not be started if the
connected (CON) bit in the SCNT L1 register,
bit 4, indicates that the SYM53C875 is already
connected to the SCSI bus. T his bit is auto-
matically cleared when the arbitration
sequence is complete. If a sequence is aborted,
ARB1
7
ARB0
6
START
5
WATN
4
EPC
3
RES
2
AAP
1
TRG
0
Default>>>
1
1
0
0
0
X
0
0
ARB1
ARB0
Arbitration Mode
0
0
1
1
0
1
0
1
Simple arbitration
Reserved
Reserved
Full arbitration, selection/reselection