Small Computer System Interface

Two SCSI connectors.

A small computer system interface, better known by the English acronym SCSI (Small Computer System Interface) is a standard interface for transferring data between different devices on the computer bus.

After spending an entire day discussing the name, Larry Boucher wanted it to be pronounced "sexy". However, ENDL's Dal Allan pronounced it as "scuzzy", and it stuck.

The small part of SCSI is historical; since the mid-1990s, SCSI has been available on even the largest computer systems. Since its standardization in 1986, SCSI has been in common use on the Commodore Amiga and the Apple Macintosh and Sun Microsystems lines of servers and personal computers. Apple began using Integrated Drive Electronics (IDE) for its low-end machines with the Macintosh Quadra 630 in 1994, and included it in its high-end machines with the Power Macintosh G3 in 1997. I completed the inclusion of SCSI (in favor of IDE and FireWire) with the blue and white G3 in 1999. Sun has moved its low end to Serial ATA (SATA). SCSI has never been popular on low-end IBM PC compatibles, due to the lower price and good performance of ATA drives. SCSI hard drives and even SCSI RAID systems are common in PC workstations dedicated to video and/or audio production, but the advent of high-capacity, low-cost SATA drives pushed them out of that market niche.

SCSI is popular today in high performance workstations and servers. RAID systems on servers almost always use SCSI hard drives, although several manufacturers offer SATA-based RAID systems as a lower cost option. Desktop and notebooks computers commonly use ATA/IDE and now SATA for hard drives, and USB, e-SATA and FireWire connections for external devices.

SCSI Types

1. SCSI 1.: with 8-bit bus. Data transmission speed to 5 MB/s. Its generic connector is 50 pins (Centrenics connector) and low density. The maximum cable length is six metres. Allows up to 8 devices (including the controller), identified by addresses 0 to 7.
2. SCSI 2.
   • Fast.: with a bus of 8, double transmission speed (from 5 MB/s to 10 MB/s). Its generic connector is 50 pins and high density. The maximum length of the cable is three meters. Allows up to 8 devices (including the controller), identified by addresses 0 to 7.
   • Wide.: fold the bus (pass from 8 to 16 bits). Its generic connector is 68 pins and high density. The maximum length of the cable is three meters. Allows up to 16 devices (including the controller), identified by addresses 0 to 15.
3. SCSI 3.
   • .1 SPI (Parallel Interface or Ultra SCSI).
     • Ultra: 16-bit devices with execution speed of 20 MBps. Its generic connector is 34 pins of high density. The maximum length of the cable is 1.5 m. Supports up to 15 devices. Also known as Fast 20 or SCSI-3.
     • Ultra Wide: 16-bit devices with execution speed of 40 MBps. Its generic connector is 68 pins and high density. The maximum length of the cable is 1.5 meters. Supports up to 15 devices. Also known as Fast SCSI-3.
     • Ultra 2: 16-bit devices with running speed of 80 MBps. Its generic connector is 68 pins and high density. The maximum length of the cable is 12 meters. Supports up to 15 devices. Also known as Fast 40.
   • .2 FireWire (IEEE 1394).
   • .3 SSA (Serial Storage Architecture): IBM. Use full-duplex with separate channels.
   • .4 FC-AL (Fibre Channel Arbitrated Loop): use fiber optic cables (up to 10 km) or coaxial (up to 24 m). With a maximum speed of 100 MBps.

• They use CCS (Command Common Set): is a set of commands to access the devices that make them more or less compatible.

• SCSI 1, SCSI 2 and SCSI 3.1 (SPI) connect devices in parallel.

• SCSI 3.2 (FireWire), SCSI 3.3 (SSA) and SCSI 3.4 (FC-AL) connect serial devices.

• You need to finish (jumpers, books of the BIOS, FÍSICOS) in the beginning and end of the chain.

• Maximum number of devices: the controller counts as a device (identifier 7, 15).
  • Bus; devices: identifiers; connector:
    • 8-bit bus; 7 devices: identified from 0 to 6; 50-pin connector.
    • 16-bit bus: 15 devices: identified from 0 to 14; 68-pin connector.