SCSI (pronounced 'scuzzy') or (Small Computer System Interface) defines commands, protocols and interfaces (both optical and electrical,) serving as a data transfer for computers and peripherals. SCSI connects hard drives, optical drives, and other devices to computers. The SCSI standard includes an “unknown” device type, which means SCSI, in theory, operates on any possible device. SCSI is a commercial standard appropriate for use in every computer environment.
SCSI has a number of key features:
- Intelligence – SCSI translates between physical devices and the standard SCSI bus. It makes device differences transparent to the computer.
- Peripheral Interface – A single SCSI bus accommodates 16 devices.
- Buffered – it connects devices via a hand shake, an automated ritual that negotiates connection parameters between the computer and the device.
- Error Checking – SCSI checks data and commands with a checksum.
- Peer to Peer – SCSI manages communication from hosts to hosts, hosts to peripherals, and peripherals to peripherals. SCSI treats peripherals as only receivers, they do not initiate SCSI communications. However, some peripherals initiate communications.
The SASI (Shugart Associates System Interface) standard preceded SCSI. Shugart Associates developed the standard in 1981, for communications between computers and hard drives. SASI interfaces sat on top of hard drives and used a 50 pin ribbon connector. These connectors later became the standard SCSI connectors. SCSI-1 maintained compatibility with SASI. Larry Boucher, the father of SCSI, developed SASI while at Shugart Associates. He later expanded the SCSI standard while at Adaptec.
Shugart changed the name after the ANSI (American National Standards Institute) committee decided a company name could not be a standards name. Boucher and others from Shugart negotiated with the ANSI committee, and together they eventually named it SCSI. Boucher originally pronounced it SEXY, but people liked SCUZZY better. SCSI later served even large computer systems, but the “small” part stuck with the overall name.
Amiga, Apple (Macintosh,) Sun Microsystems and PC servers used SCSI. Apple dropped SCSI in favor of IDE and FireWire in 1999. Sun switched to Serial ATA (SATA.) In professional environments, server and multimedia workstation PCs used SCSI RAID drives, but cheaper SATA later replaced SCSI .
High end workstations and servers still use SCSI, while server RAID arrays still rely on SCSI drives. Still, companies offer cheaper SATA RAID alternatives. Home PCs used ATA and IDE instead of SCSI, and skipped right to SATA for internal drives and USB, eSATA or FireWire for external drives.
SCSI includes four types of commands:
- N (non-data)
- W (writing data)
- R (reading data)
- B (bidirectional)
The most common SCSI commands include:
- Test ready – questions the device for media transfer readiness
- Inquiry – delivers basic device information
- Request Sense – returns error codes after other commands return error status
- Start/Stop – spins the drive, loads and unloads media
- Write – writes data
- Read – reads data