Seagate Ultra 320 User Manual download pdf (Page 154)

140 Parallel SCSI Interface Product Manual, Rev. A

7.4 Command examples

7.4.1 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

19.

The initiator has active pointers and a set of stored pointers representing active disconnected SCSI devices (a

SCSI initiator port 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 pro

cess 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

command 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 19. 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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Seagate Ultra 320 User Manual Page 154