SCSI

SCSI Overview

SCSI is the standard includes protocol and interface to transfer data among the devices attached to internal and external computer buses. SCSI is the most common standard found in computers. It is used for hard disk especially but also supports number of devices. The devices such as CD ROM drives, DVD drives, tape storage devices, CD recorders, Scanners, and Printers. It was standardized in 1986 and became very popular at that time. The introduction of FireWire, USB and ATAPI brought its graph quite down.

SCSI Protocol

SCSI protocol was first developed for parallel SCSI. But with enhancements in its architecture the protocol was also enhanced for serial SCSI. When two devices communicate one is referred to as initiator and other is target. An initiator sends a desired command to the target. The target responds to the command by an acknowledgement. The acknowledgement is known as sense code. The sense code is ooh indicates success, 08h indicates busy and 02h in case o f errors. in response to sense code for error initiator sends SCSI Request Sense Command. Then asks for KCQ (key code qualifier) from target to check whether the error has been removed.

SCSI Commands

The commands are placed in CDB (command descriptor Block). One byte of the command specifies the operation while other bytes carry information about action to be performed. SCSI allows total of sixty commands, which can be categorized as N- Type, W-Type, R-Type, and B-type. W-Type commands are referred to as writing data from initiator to target, R-Type for reading data from either side, B-type for bi-directional, and N-Type for the commands not related to data such as acknowledgments or signals. The device identification that is either target or initiator is done automatically in serial SCSI.

SCSI Compatibles

SCSI interface and protocol are backward compatibles. It means that it allows previous version to fit in the latest. For example Ultra – 2 standard hard disk works with Ultra – 3 standard but Ultra – 3 does not works on Ultra – 2.

SCSI standards

SCSI – 1 was the firs SCSI standard introduced in 1986. It was 8- bit parallel version. SCSI – 2 followed it in 1989 provided fast and wide SCSI. It was 16-bit standard. Next standard was SCSI – 3 in 1992. It was 32 – bit standard also called Ultra SCSI due o faster speed. Other parallel standards include Ultra – 3, Ultra – 320, Ultra – 640, and iSCSI.

Serial SCSI

Serial SCSI standards come in three versions. Serial Storage Attachment (SSA), FC (Fibre Channel)-AL and, serial attached SCSI (SAS). Serial SCSI is much faster, improved but expensive than parallel SCSI.

SCSI Applications

SCSI standard especially serial SCSI is being used in High performance workstations, high-end peripheral, and servers. Especially in RAID Arrays of servers the hard disk are connected through SCSI.