Small Computer System Interface

(scuzzy)

A high-speed expansion bus used to connect mass storage and other devices via a single cable. Typically each SCSI "chain" can consist of up to seven devices plus the adapter. Transmission on a SCSI bus is in parallel and can have either synchronous or asynchronous modes.

The adaptor is usually a card or chip in the host computer the devices will talk with. It is usually given SCSI ID 7. Not all adapters will support all SCSI cable systems. There are a variety of cable connector configurations. An adapter may even carry a BIOS to allow booting from an attached SCSI hard drive.

SCSI IDs (addresses) range from 0 to 31 (32-bit systems), 0-15 (16-bit systems), and 0-7 for 8-bit systems. Newer devices can have their IDs set via software; early devices usually required setting switches on the device or card.

Cable length in a SCSI system runs from a few feet to a few yards depending on the drivers (single ended versus differential).

Shugart Associates developed SCSI. Originally, before becoming a standard, the interface was called the Shugart Associates System Interface or SASI. Seagate took over Shugart.

The original SCSI specification (SCSI-1) has been replaced with SCSI-2, an expanded version. Primary competition includes IDE, EIDE, and EPP.

SCSI-2 adds a Fast SCSI mode and Wide SCSI. SCSI-2 also expanded the definition of command sets which allowed many more devices easier access to a SCSI bus. Some SCSI-2 devices can be connected to a SCSI-1 bus.

The next step will be SCSI-3. As usual, goals can be summarized as more, faster, over longer distances. A structured protocol is also desired.

More Information

• Hard drive terms
• Hard drive history
• SCSI FAQ
• SCSI Trade Association