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Table 4-4 Method of driving the interface signal
OR connection NON_OR connection
False No signal is driven by any
SCSI device. Signal status is
made false by the termination
resistor circuits.
The signal is driven false by
a certain SCSI device
(initiator or target), or is not
driven by any SCSI device
True A SCSI device drives the signal true.
4.3 Bus Phases
The SCSI architecture includes the following eight distinct phases:
• BUS FREE phase
• ARBITRATION phase
• SELECTION phase
• COMMAND phase
• DATA phase
• STATUS phase
• MESSAGE phase
The SCSI bus can never be in more than one phase at any given time.
The following diagram shows how each phase transmits to another.
Reset
Figure 4-2 Phase sequence
}
INFORMATION TRANSFER phase
MESSAGE OUT
SELECTION
COMMAND
DATA IN or
DATA OUT
ARBITRATIONBUS FREE
STATUS
MESSAGE IN