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ST43401N/ND and ST43402ND Reference Manual, Rev. C
SCSI command execution
Figure 13 illustrates the basic flow of a command sequence. The sequence
illustrated cannot and does not represent all variations. Refer to the SCSI
Interface Specification for the Elite Product Family (document 64721702) for
command descriptions, execution details, and timing constraints. The follow-
ing information appears at the end of this topic:
• Table 9 describes the command set for the drive
• Table 10 lists the message codes and shows the direction of flow for each
• Table 11 describes the status codes
As shown in Figure 13, the communication sequence starts with the SCSI bus
in the Bus Free phase. This phase indicates that no other SCSI devices are
using the bus and it is free for use by other devices. Each device detects the
Bus Free phase when the Select and Busy lines are both false.
The Bus Free phase is followed by an Arbitration phase where the initiator
attempts to gain access to the bus. Access to the bus is based on the device
priority ID bit. Arbitration occurs when the device asserts Busy and its ID on
the data bus. This ID bit is a single bit on the data bus that corresponds to the
unique SCSI address assigned to each device when it was installed. The other
15 bits are released by the SCSI device. The SCSI device examines the data
bus. If a higher priority SCSI ID bit is true (data bus bit 15 is the highest) the
SCSI device loses arbitration and the device releases its signals. If no higher
priority bit is true, the device wins arbitration.
After winning the Arbitration, the initiator selects the target (Selection phase).
The initiator places the SCSI ID of the target on the bus (asserts the data bus
bit), as well as its own ID. After a delay, the initiator asserts the Select line. The
target determines it is selected when its SCSI bus ID bit and the Select line are
true, and the Busy and I/O lines are false. Selection with Attention informs the
target that the initiator has a message ready.
Note. For dual port drives, a SCSI hard reset causes a unit attention condition
for all initiators on the port receiving the reset. Commands, reserva-
tions, wide negotiations, and synchronous negotiation agreements
associated with the alternate port are not affected.
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