Command Examples; Single Command Example - Seagate SCSI Interface Product Manual

SCSI Interface Product Manual, Rev. J
4.4

Command examples

4.4.1

Single command example

A typical operation on the SCSI bus is likely to include a single Read command to a peripheral device such as
the drive. This operation is described in detail starting with a request from the initiator. This example assumes
that no linked commands and no malfunctions or errors occur and is illustrated in Figure 8.
The initiator has active pointers and a set of stored pointers representing active disconnected SCSI devices (an
initiator without disconnect capability does not require stored pointers). The initiator sets up the active pointers
for the operation requested, arbitrates for the SCSI bus, and selects the drive. Once this process is completed,
the drive assumes control of the operation.
The drive obtains the command from the initiator (in this case a Read command). The drive interprets the com-
mand and executes it. For this command, the drive reads the requested data from the Disc Media and sends
this data to the initiator. After sending the read data to the initiator, the drive sends a status byte to the initiator.
To end the operation, the drive sends a Command Complete message to the initiator and then goes to the Bus
Free state.
Figure 8. Single command example
Start
Bus Free
Arbitrate
Select
(no attention)
Command
(ex. read)
Data In
Status In
Message In
(command
complete)
Bus Free
End
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