Seagate Ultra 320 User Manual download pdf (Page 145)

Parallel SCSI Interface Product Manual, Rev. A 131

7.0 SCSI commands

This section defines the SCSI command structure and describes a typical SCSI bus procedure involving a

command, status return, and message interchange.

The command structure defined herein provides for a contiguous set of logical blocks of data to be transferred

across the interface. The number of logical data blocks to be transferred is defined in the command. Initiator

commands to the drive are structured in accordance with the requirements imposed by the drive physical char

acteristics. These physical characteristics are reported to the initiator in response to an INQUIRY command.

A single command may transfer one or more logical blocks of data. The drive may disconnect, if allowed by the

initiators, from the SCSI bus to allow activity by other SCSI devices while the drive performs operations within

itself.

Upon command completion (which may be executed either successfully or unsuccessfully), the drive returns a

status byte to the initiator. Since most error and exception conditions cannot be adequately described with a

single status byte, one status code that can be sent as the status byte is called Check Condition. It indicates

that additional information is available. The initiator may issue a REQUEST SENSE command to request the

return of the additional information as part of the DATA IN phase of the command.

Future implementations of the SCSI protocol will contain an autosense feature (see Section 7.6.4.2) allowing

the application client to request the automatic return of sense data. Fibre Channel protocol already has this

feature in its packetized command structure.

7.1 Command implementation requirements

The first byte of any SCSI command contains an operation code as defined in this document. Three bits (bits

7-5) of the second byte of each SCSI command have historically been used to specify the logical unit if it is not

specified using the Identify Message (see Section

4). These three bits are now shown as “Reserved” in the

new Command data block format. The last byte of most SCSI commands contains a control field. The excep-

tion to this is the new variable length CDB which has the control field in the second byte, rather than the last.

7.1.1 Reserved

Reserved bits, bytes, fields, and code values are set aside for future standardization. Their use and interpreta-

tion may be specified by future revisions to this specification. A reserved bit, field, or byte shall be set to zero,

or in accordance with a future revision to this specification. A drive that receives a reserved code value shall

terminate the command with a CHECK CONDITION status and the Sense Key shall be set to ILLEGAL

REQUEST. It shall also be acceptable for the drive to interpret the bit, field, byte, or code value in accordance

with a future revision to this specification.

7.2 Command Descriptor Block (CDB)

A request by a SCSI initiator port to a drive is performed by sending a Command Descriptor Block (CDB) to the

drive. For several commands, the request is accompanied by a list of parameters sent during the DATA OUT

phase. The field uses shown in tables

57, 58, 59, 60, and most of those in 61 are used consistently by most

commands. See the specific commands for detailed information.

1 2 ... 140 141 142 143 144 145 146 147 148 149 150 ... 185 186

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Seagate Ultra 320 User Manual Page 145