SCSI
SCSI stands for "Small Computer Systems Interface" though the original name is rarely used in favor of just calling it SCSI (pronounced: scuzzy).
Introduction
SCSI is a collection of standards defining a hardware interface, at the electrical level, an addressing scheme, at the logical level, and the protocol to be used, by software, to communicate with and transfer data between computers and peripheral devices. The term is also used as a prefix for the related hardware attached via the such as "SCSI hard drive" or "SCSI tape drive" or "SCSI controller" and accessories, as in "SCSI cable" or "SCSI port".
It was, and to an extent still is, found as an interface type for a wide variety of hardware. Such hardware commonly included both hard disks and tape drives (especial internal devices), but was also used for a variety of other peripherals including floppy drives, CD-ROM drives (and other optical media), printers, scanners, cameras, and more. A particularly unusual example would be a SCSI network adapter.
Standards
| SCSI-1 | the older, and nominally original, version often found on computers from the late 80s through the 90s. |
| SCSI-2 | a newer more modern version which encompassed two variants referred to as Fast SCSI and Wide SCSI. |
| SCSI-3 | a collection of standards describing various enhancements to SCSI. It was first introduced, as a term, to describe devices which transcended the limitations, particularly speed, of earlier SCSI-2 devices. |
Various later changes are labeled as 'Ultra' and some specifier such as a version of the max speed.
Physical Attachment
Interfaces
SCSI comes in two forms, the older parallel and the newer serial. In each case the commands and protocols are essentially similar above the hardware level.
Parallel SCSI, or just SCSI, refers to use of a parallel data bus (multiple bits sent/received simultaneously) to transmit commands and data. It is very common for such buses to use a large number of separate lines for individual bits, often matching the native word length (or a multiple) used in the system the SCSI hardware is installed in. As such there has historically been both 8-bit SCSI and 16-bit SCSI.
Serial Attached SCSI, aka SAS, transmits data over a serial bus and benefits from the nature of serial communications in the form of speed and increased reliability.
Cabling
A number of different cabling systems have been used, some of which still are, including a manifestation very similar to the PC Parallel Port using a DB-25 (25 pin) connector which was common on Apple Macintosh computers for a long time. Other examples include Serial Attached SCSI (SAS) which passes over a SATA data cable and iSCSI which is "internet enabled" and can be passed over any physical transport supporting IP (internet protocol).