4-9
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
ARBITRATION
BUS FREE
STATUS
MESSAGE IN