Lecture Notes: SCSI (Small Computer Systems Interface)

Overview

• SCSI, pronounced "scuzzy," is a standard interface for connecting storage devices, scanners, printers, and other network devices.
• Allows multiple devices to be daisy-chained and connected to a single interface.
• Supports up to 16 devices in one chain, depending on the standard.

SCSI Standards

• Many SCSI standards exist with various interfaces and cabling.
• Latest standard: iSCSI or SCSI over IP.
• Widely implemented in virtual networks.

Connectivity

• SCSI supports both parallel and serial connectivity (e.g., SATA and PATA).
• Historically used before SATA and USB for connecting peripherals like scanners and CD-ROM drives.
• Can daisy-chain up to 8 devices on a narrow bus or 16 on a wide bus.

Intelligent Protocol

• SCSI identifies and accesses connected devices automatically.
• Uses SCSI ID numbers for each device to differentiate them on the same cable.
• Logical Unit Numbers (LUNs) identify individual drives in a multi-drive system.
• Requires a SCSI terminator at the end of the cable for proper communication.

Serial Attached SCSI (SAS)

• SAS is a newer standard simplifying connections without SCSI IDs or terminators.
• Point-to-point connections between motherboard and drives, removing daisy chains.
• Provides higher throughput and simplified implementation.

Example: SCSI Connections

• Motherboards may have SCSI connectors among other legacy interfaces.
• 68-pin connectors often used on motherboards and cables.
• Different SCSI connectors exist, evolving over time.

Practical Application

• External SCSI storage devices have in/out interfaces for daisy chaining.
• Devices can have adjustable SCSI IDs to avoid conflicts.
• Internal SCSI setups involve controllers with SCSI IDs and terminators on cables.
• Serial Attached SCSI drives resemble SATA drives but with different connectors.

Conclusion

• Transition from PATA to SATA mirrors SCSI’s evolution from parallel to serial communication.
• SAS provides the benefits of SCSI protocols with modern serial speed and simplicity.