SCSI (Small Computer System Interface)

Overview

The SCSI architecture separates three main concerns: the physical transport (cables, connectors and signaling), the protocol used to carry commands and data, and the command set that targets (devices) implement. Originally developed for parallel buses, SCSI evolved to include serial transports and networked transports while retaining a common command model. For a general introduction to the standards, see SCSI standard overview.

Structure and common components

Key elements of a SCSI environment include initiators (hosts or controllers that issue commands), targets (devices that respond) and logical unit numbers (LUNs) that identify individual device functions. Physical implementations historically used daisy-chained ribbon cables and unique device IDs with termination at each end to preserve signal integrity.

• Transports: Parallel SCSI, Serial Attached SCSI (SAS) and Internet SCSI (iSCSI) are examples of how SCSI commands are carried over different physical layers.
• Connectors and signals: Various connectors (for example 50-pin and 68-pin types on older hardware) and later serial connectors for SAS reflect generational change.
• Command set: Standard SCSI commands (INQUIRY, READ, WRITE, etc.) determine how storage and other devices behave and report status.

History and development

SCSI originated in the 1980s as a standardized method for connecting storage to microcomputers and workstations. Over time the family of standards expanded: new generations increased bus width and clock rates, introduced synchronous transfers, and eventually moved from parallel signaling to serial links and IP-based transports. Standards bodies continued to refine the architecture and command specifications to support higher performance and new device types.

Uses, examples and importance

SCSI saw broad adoption in servers, workstations and storage systems because of its flexibility and robust command set. Typical SCSI-attached devices included hard disk drives, tape drives and optical drives; many earlier CD and DVD drives used SCSI interfaces or adapters. For an example of peripheral evolution and alternatives, see examples of SCSI devices and the rise of consumer interfaces such as USB.

Notable distinctions and legacy

Today, direct-attached consumer devices more commonly use SATA or USB, but the SCSI command set remains influential: SAS uses the SCSI command model for enterprise storage, and iSCSI carries SCSI commands over networks. While physical implementations have changed, the logical model—initiator, target, LUN and a rich command set—continues to underpin many storage systems in data centers and specialized equipment.

The SCSI family is therefore both a historical milestone in peripheral connectivity and a living set of protocols that persists in modern storage technologies.