Arbitrating Systems; All Systems; Scsi Pointers; Figure 4. Phase Sequences With Arbitration - Seagate SCSI Interface Product Manual

SCSI Interface Product Manual, Rev. J
3.3.2

Arbitrating systems

For systems in which the Arbitration phase is implemented, the allowable sequences are shown in Figure 4.
The normal progression is from the Bus Free phase to Arbitration, from Arbitration to Selection or Reselection,
and from Selection or Reselection to one or more of the information transfer phases (Command, Data, Status,
or Message). See ANSI document T10/1302D, section 11.1.2.2.1, for a description of Quick Arbitration (QA)
phase.
3.3.3

All systems

There are no restrictions on the sequences between information transfer phases. A phase type may even be
followed by the same phase type (e.g., a Data phase may be followed by another Data phase).
Bus
Free
Phase
Reset Condition
*Data phase must follow a command phase except after reselection phase
Figure 4. Phase sequences with arbitration
Bus Free
Phase
Reset Condition
Figure 5. Phase sequences without arbitration
3.4

SCSI pointers

The drive supports systems that use the pointer philosophy described in the following paragraphs.
Consider the system shown in Figure 6 in which an initiator and target communicate on the SCSI bus in order
to execute a command.
Function
Origin
Initiator
Figure 6. Simplified SCSI system
Arbitration
Phase
Selection
Phase
Initiator
Path
Control
Selection or
Reselection
Phase
Target
Path
SCSI
Control
Bus
Command or
Data*
or Status or
Message Phase
Command or Data
or Status or
Message Phase
Function
Execution
Target
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