Evolution of Parallel SCSI

•Developed by Shugart Associates & named as SASI

•ANSI acknowledged SCSI as an industry standard

•Prior to the development of SCSI,the interface used to communicate with devices varied with each device.
For example,an HDD interface could only be used with a hard disk drive.

•SCSI versions:

–SCSI–1

•Defined cable length, signaling characteristics, commands, & transfer modes

•Used 8-bit narrow bus with maximum data transfer rate of 5 MB/s

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–SCSI–2

•Defined Common command Set (CCS)

•Improved performance, reliability, and added additional features

–SCSI–3

•Latest version of SCSI,

•Comprised different but related standards, rather than one large document.

SCSI–3 Architecture

•SCSI command protocol

–Primary commands common to all devices

•Transport layer protocol

–Standard rules for device communication and information sharing.

•Physical layer interconnect

–Interface details such as electrical signaling methods and data transfer modes

SCSI Device Model

SCSI communication involves:

•SCSI initiator device

–Issues commands to SCSI target devices

–Example: SCSI host adaptor

•SCSI target device

–Executes commands issued by initiators

–Examples: SCSI peripheral devices

SCSI Device Model

•Device requests uses Command Descriptor Block (CDB)

–8 bit structure

–Contain operation code, command specific parameter and control parameter

•SCSI Ports

–SCSI device may contain initiator port, target port, target/initiator port

–Based on port combination device is classified

–For example

•Target/initiator device contain target/initiator port and can switch orientations depending on the role it plays while participating in an I/O operation

–To cater to service requests from multiple devices, a SCSI device may also have multiple ports 

SCSI Addressing Example