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SCSI BUS
7-14 C156-E228-02EN
(3) Signal sources
Table 7.6 lists signal sources for each interface phase.
Table 7.6 Bus phases and signal sources
Signal
Bus phase
BSY SEL I/O
C/D
MSG
REQ ACK
DB7-0,
P
ATN TRS
BUS FREE N N N N N N N N A
ARBIRATION A W N N N N ID N A
SELECTION I&T I N N N N I I A
RESELECTION I&T T T N N N T I A
COMMAND T N T T T I I I A
DATA IN T N T T T I T I A
DATA OUT T N T T T I I I A
STATUS T N T T T I T I A
MESSAGE IN T N T T T I T I A
MESSAGE OUT T N T T T I I I A
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.