4 signal sources – Seagate Ultra 160 User Manual
Page 33
Parallel SCSI Interface Product Manual, Rev. A )
19
2.4
Signal sources
Table 3 indicates the type of SCSI device allowed to source each signal. No attempt is made to show if the
source is driving asserted, driving negated, or is released. All SCSI device drivers that are not active sources
shall be in the high-impedance state. The RST signal may be asserted by any SCSI device at any time.
Table 3:
Signal sources
Table abbreviations are defined as follows:
All: The signal shall be driven by all SCSI devices that are actively arbitrating.
S ID: A unique data bit (the SCSI ID) shall be driven by each SCSI device that is actively arbitrating; the other
data bits shall be released (i.e., not driven) by this SCSI device. The P_CRCA and DB(P1) bit(s) may be
released or driven to the true state, but shall not be driven to the false state during this phase.
I&T: The signal shall be driven by the initiator, target, or both, as specified in the SELECTION PHASE and
RESELECTION PHASE.
Init: If driven, this signal shall be driven only by the active initiator.
None: The signal shall be released; that is, not driven by any SCSI device. The bias circuitry of the bus termi-
nators pulls the signal to the false state.
Win: The signal shall be driven by the one SCSI device that wins arbitration.
Targ: If the signal is driven, it shall be driven only by the active target.
PT: Target that initiated the QAS arbitration.
16 data bit (P) cable
8 data bit (A) cable
SCSI bus phase
BSY
SEL
C/D, I/O,
MSG, REQ
ACK, ATN
DB(7-0)
P_CRCA
DB(15-8),
DB(P1)
BUS FREE
None
None
None
None
None
None
None
ARBITRATION
(NORMAL)
All
Win
None
None
S ID
S ID
S ID
QAS ARBITRATION
PT
Win
None
None
S ID
S ID
S ID
SELECTION
I & T
Init
None
Init
Init
Init
Init
RESELECTION
I & T
Targ
Targ
Init
Targ
Targ
Targ
COMMAND
Targ
None
Targ
Init
Init
Init
None
ST DATA IN
Targ
None
Targ
Init
Targ
Targ
Targ
ST DATA OUT
Targ
None
Targ
Init
Init
Init
Init
DT DATA IN
Targ
Targ
Targ
Init
Targ
Targ
Targ
DT DATA OUT
Targ
Targ
Targ
Init
Init
Targ
Init
STATUS
Targ
None
Targ
Init
Targ
Targ
None
MESSAGE IN
Targ
None
Targ
Init
Targ
Targ
None
MESSAGE OUT
Targ
None
Targ
Init
Init
Init
None