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Signal sources
Table 7.5 lists signal sources for each interface phase.
Signal
BSY
Bus phase
BUS FREE
N
ARBIRATION
A
SELECTION
I&T
RESELECTION
I&T
COMMAND
T
DATA IN
T
DATA OUT
T
STATUS
T
MESSAGE IN
T
MESSAGE OUT
T
A:
Any SCSI device can drive the signal. Two or more SCSI devices may drive the signal
at the same time.
I:
Only the SCSI device which operates as an INIT can drive the signal.
I&T:
INIT, TARG, or both can drive this signal according to the interface sequence.
In the RESELECTION phase, there is a sequence to be driven simultaneously by both.
ID:
Each SCSI device which is actively arbitrating the bus drives a unique data bit (SCSI
ID). The parity bit may be undriven or driven to the true state, but must never be driven
to the false state.
N:
Not to be driven by any SCSI device. The terminator pulls the signal to the false state.
T:
Only the SCSI device which operates as an TARG can drive the signal.
W:
Only one SCSI device which wins ARBITRATION can drive the signal.
Table 7.5
Bus phases and signal sources
C/D
SEL
I/O
MSG
N
N
N
W
N
N
I
N
N
T
T
N
N
T
T
N
T
T
N
T
T
N
T
T
N
T
T
N
T
T
C156-E097-01EN
DB7-0,
REQ
ACK
P
N
N
N
N
N
ID
N
N
I
N
N
T
T
I
I
T
I
T
T
I
I
T
I
T
T
I
T
T
I
I
ATN
RST
N
A
N
A
I
A
I
A
I
A
I
A
I
A
I
A
I
A
I
A
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