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4.2.3 Bus signal drive conditions
SCSI devices drive signals of the SCSI bus. The types of SCSI devices are summarized in the following table, showing the
signals that they can drive for each operating phase of the interface.
There are two kinds of signal driving methods, OR tied and NON-OR tied, as shown in Table 4.2. During an interface
operating sequence. the BSY signal could be driven simultaneously by two or more SCSI units when the data bus is in the
ARBITRATION or RESELECTION phase. This situation also occurs with the RST signs (Reset). These two signals must be
ORtied. For other signals, either of the two methods may be used: further more, different drive methods may coexist for a
signal on the bus.
Bus phase
BUSFREE
ARBITRATION
SELECTION
RESELECTION
COMMAND
DATAIN
DATAOUT
STATUS
MESSAGE IN
MESSAGE OUT
N:
A:
I:
T:
W:
ID:
I&T:
The signal shall be driven by the initiator, target, or both, as specified in the SELECTION phase and RESELECTION phase.
Table 4-3
Bus
Signal
BSY
SEL
N
N
A
W
I&T
I
l&T
T
T
N
T
N
T
N
T
N
T
N
T
N
The signal shall be released, since it is not being driven by any SCSI device.
The signal shall be driven by all SCSI devices that are actively arbitrating.
If driven, this signal be driven only by the active initiation
If the signal is driven, it shall be driven only by the active target.
The signal shall be driven by the one SCSI device that wins arbitration.
A unique data bit (the SCSI ID) shall be driven by each SCSI device that is actively arbitrating. The
other seven data bits shall be released (shall not be driven) by this SCSI device. The parity bit (DB(P))
may be released or driven to the true state, but shall never be driven to the false state during this
phase.
The initiator and target drive the signal according to the interface operating sequence. The
RESELECTION phase includes a sequence in which the initiator and target simultaneously drive the
signal.
vs. signal drive sources (1/2)
phases
C/D
'10
MSG
REQ
ACK
N
N
N
N
N
N
N
N
N
N
N
I
T
T
T
I
T
T
T
I
T
T
T
I
T
T
T
I
T
T
T
I
T
T
T
I
T
T
T
I
4-8
DB7 to 0
DBP
ATN
RST
N
N
A
ID
N
A
I
I
A
T
I
A
I
I
A
T
I
A
I
I
A
T
I
A
T
I
A
I
I
A
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