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Appendix A-8
A.3 Bus Phases
The SCSI architecture includes the following eight distinct phases:
The SCSI bus can never be in more than one phase at any given time.
The following diagram shows how each phase transits to another.
The signal delay times for each bus phase are defined as follows:
Figure 4.2 Phase sequence
•
BUS FREE phase
• ARBITRATION phase
• SELECTION phase
• RESELECTION phase
• COMMAND phase
• DATA phase
• STATUS phase
• MESSAGE phase
INFORMATION TRANSFER phase
Table A.3 Method of driving the interface signal
OR connection
No signal is driven by any SCSI device.
Signal status is made false by the
termination resistor
circuits.
NON-OR connection
The signal is driven false by a certain
SCSI device (initiator or target), or is
not driven by any SCSI
device.
False
True
A SCSI device drives the signal true.
Reset
BUS FREE
SELECTION
ARBITRATION
RESELECTION
MESSAGE OUT
COMMAND
DATA IN or
DATA OUT
STATUS
MESSAGE IN