Bus Signal Drive Conditions - Fujitsu M3097DG Oem Manual

A.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 A.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 signal (Reset). These two signals must be ORtied. For the other
signals, either of the two methods may be used; further more, different drive methods may coexist
for a signal on the bus.
Signal
Bus phase
BUS FREE
ARBITRATION
SELECTION
RESELECTION
COMMAND
DATA IN
DATA OUT
STATUS
MESSAGE IN
MESSAGE OUT
N: The signal shall be released, since it is not being driven by any SCSI device.
A: The signal shall be driven by all SCSI devices that are actively arbitrating.
I: If driven, this signal shall be driven only the active initiator.
T: If the signal is driven, it shall be driven only by the active target.
W: The signal shall be driven by the one SCSI device that wins arbitration.
ID: 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 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.
I&T: 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.
The signal shall be driven by the initiator, target, or both, as specified in the SELECTION phase and
RESELECTION phase.
Table A.2 Bus phases vs. signal drive sources
BSY SEL I/O
N N N
A W N
I&T I N
I&T T T
T N T
T N T
T N T
T N T
T N T
T N T
C/D
REQ ACK
MSG
N N N
N N N
N N I
T T I
T T I
T T I
T T I
T T I
T T I
T T I
A.2 SCSI Bus
DB7 to 0
ATN RST
DBP
N N
ID N
I I
T I
I I
T I
I I
T I
T I
I I
Appendix A-7
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