Understanding SCSI - CompTIA A+ 220-801: 1.5

The SCSI standard (Small Computer Systems Interface)

• Small Computer Systems Interface
• Not really "small" any longer
• Designed to string many peripherals together onto a single cable / controller
• Up to 16 devices in a SCSI "chain"
• Many different formats
• Fast SCSI, Ultra SCSI, Ultra Wide SCSI, Ultra2 SCSI, Ultra3 SCSI, Ultra-320 SCSI, Ultra-640 SCSI, iSCSI (SCSI over IP)

SCSI advantages 

• Not just for hard drives
• Scanners, tape drives, CD-ROM drives
• Many devices on a single bus
• 8 on narrow bus, 16 on wide bus

• Very intelligent interface functionality
• Much of the difficult configuration work is done between the SCSI devices

• Industry longevity
• Tends to be specialized these days

• SCSI is particular about termination
• The recent SCSI standards automatically configure most drive settings

SCSI ID and logical unit (LUN)

• Every SCSI device on a single bus is assigned a separate ID number
• i.e., SCSI ID 0 (boot disk), ID 2 (floppy disk), ID 3 (CD-ROM)
• Logical units (LUNs) are defined within each SCSI ID
• Separate drives in a storage array or virtual machine
• The signal at the "end" of the SCSI bus is terminated
• Can be internal to the device or a separate termination device

• Serial attached SCSI (SAS) devices have no jumpers, terminators, or settings.

Daisy chaining

The SCSI drive