Signal Sources
96122 • Revision: F Chapter 3 SCSI Bus Operations 19
Signal Sources
TABLE 3-3 indicates the source and phase of the SCSI bus signals.
TABLE 3-3 Signal Source
Bus Phase
Bus Signals
BSY SEL
C/D
I/O
MSG
REQ
ACK
ATN DB
Bus Free NNNNN
Arbitration A W N N ID
Selection I & T I N I I
Reselection I & T T T I I
Command T N T I I
Data In T N T I T
Data Out T N T I I
Status T N T I T
Message In T N T I T
Message Out T N T I I
Notes:
A The signal is driven by all SCSI devices arbitrating.
I If this signal is driven, it is driven only by the active initiator.
ID A unique data bit (the SCSI ID) is driven by each SCSI device that is arbitrating. The
other 15 data bits are released (not driven) by this SCSI device.
The parity bit DB(P, P1) may be driven or undriven to the asserted state but will never
be driven to the negated state during this phase.
I&T The signal is driven by the initiator, target, or both, as specified in the Selection and
Reselection phases.
N The signal is released—that is, not driven—by any SCSI device.
The bias circuitry of the bus terminators pulls the signal to the negated state.
T If the signal is driven, it is driven only by the active target.
W The signal is driven by the device that wins during the arbitration phase.
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