Bus Phases - Fujitsu Scan Partner 300C Instruction Manual

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
BUS FREE
Table 4-4 Method of driving the interface signal
OR connection
False No signal is driven by any
SCSI device. Signal status is
made false by the termination
resistor circuits.
True A SCSI device drives the signal true.
}
INFORMATION TRANSFER phase
SELECTION
ARBITRATION
Figure 4-2 Phase sequence
NON_OR connection
The signal is driven false by
a certain SCSI device
(initiator or target), or is not
driven by any SCSI device
MESSAGE OUT
COMMAND
DATA IN or
DATA OUT
STATUS
MESSAGE IN
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