Bus Phases
16 SL500: Interface Reference Manual • July 2008 Revision: F • 96122
The SCSI bus follows a specific sequence to go from one phase to another, shown in
FIGURE 3-1.
The normal progression of the SCSI bus is from:
1. Bus Free phase to the Arbitration phase
2. Arbitration phase to the Selection or Reselection phase
3. Selection or Reselection phase to one or more of the Information Transfer phases
4. Information Transfer phases to the Bus Free phase
Notes:
1. At any time, any phase can be followed by the Bus Free phase.
2. There are no restrictions on the sequences in the Information Transfer phase.
Any Information Transfer phase can be followed by the same phase or any other
Information Transfer phase.
3. A Reset condition can abort any phase and is always followed by the Bus Free
phase.
Bus Free
During the Bus Free phase, the SCSI bus is available for use by any device (initiator
or target) connected to it.
FIGURE 3-1 SCSI Bus Phases
BUS
FREE
ARBITRATION
RESELECTION
WAITING BUS CONTROL INFORMATION
TRANSFER
L204
157
SELECTION
MESSAGE OUT
COMMAND
DATA IN
DATA OUT
STATUS
MESSAGE IN
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