ST43401N/ND and ST43402ND Reference Manual, Rev. C
23
I/O signal processing
The following paragraphs describe the basic SCSI bus communication
process. They describe the bus phases, a typical command sequence,
command set, and message codes. The Request Sense command and the
Receive Diagnostic Results command provide information to the initiator
about drive functions. The coding of this information is explained later in this
section.
SCSI bus phases
Communication on the SCSI bus occurs in eight phases depending on the
type of operation or information transfer being performed. The bus phases
pertain to the condition or state of the lines at a given time. The SCSI bus can
never be in more than one phase at any given time.
The information phases are defined by the MSG, C/D, and I/O signals as
shown in Table 7. The SCSI bus phases are listed and described in Table 8.
Table 7. Information transfer phases
MSG
C/D
I/O
Phase Name
Direction of Transfer
0
0
0
Data Out
Initiator------> Target
0
0
1
Data In
Target-------> Initiator
0
1
0
Command
Initiator------> Target
0
1
1
Status
Target-------> Initiator
1
0
0
Reserved
1
0
1
Reserved
1
1
0
Message Out
Initiator------> Target
1
1
1
Message In
Target-------> Initiator
0 = false, 1 = true
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