Small Computer System Interface (SCSI, play /ˈskʌzi/ skuz-ee) is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it can connect a wide range of other devices, including scanners and CD drives. The SCSI standard defines command sets for specific peripheral device types; the presence of “unknown” as one of these types means that in theory it can be used as an interface to almost any device, but the standard is highly pragmatic and addressed toward commercial requirements.
SCSI is an intelligent, peripheral, buffered, peer to peer interface. It hides the complexity of physical format. Every device attaches to the SCSI bus in a similar manner. Up to 8 or 16 devices can be attached to a single bus. There can be any number of hosts and peripheral devices but there should be at least one host.
With the advent of SAS and SATA drives, provision for SCSI on motherboards is being discontinued.
Parallel SCSI
| Interface | Alternative names |
Specification document |
Connector | Width (bits) |
Clock | Maximum | Electrical | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Bandwidth (MB/s) | Bandwidth (Mbit/s) | Length (single ended) |
Length LVD | Length HVD | Devices | Impedance [Ω] | Voltage [V] | ||||||
| SCSI-1 | Narrow SCSI | SCSI-1 (1986) | IDC50; Centronics C50 | 8 | 5 MHz | 5 MB/s | 40 Mbit/s | 6 m | NA | 25m | 8 | SE 90 ± 6 Ω | 5 |
| Fast SCSI | SCSI-2 (1994) | IDC50; Centronics C50 | 8 | 10 MHz | 10 MB/s | 80 Mbit/s | 3 m | NA | 25m | 8 | SE 90 ± 6 Ω | 5 | |
| Fast-Wide SCSI | SCSI-2; SCSI-3 SPI (1996) |
2 x 50-pin (SCSI-2); 1 x 68-pin (SCSI-3) |
16 | 10 MHz | 20 MB/s | 160 Mbit/s | 3 m | NA | 25m | 16 | SE 90 ± 6 Ω | 5 | |
| Ultra SCSI | Fast-20 | SCSI-3 SPI | IDC50 | 8 | 20 MHz | 20 MB/s | 160 Mbit/s | 1.5 m | NA | 25m | 8 | SE 90 ± 6 Ω | 5 |
| 3 m | NA | NA | 4 | ||||||||||
| Ultra Wide SCSI | SCSI-3 SPI | 68-pin | 16 | 20 MHz | 40 MB/s | 320 Mbit/s | NA | NA | 25m | 16 | SE 90 ± 6 Ω | 5 | |
| 1.5 m | NA | NA | 8 | ||||||||||
| 3 m | NA | NA | 4 | ||||||||||
| Ultra2 SCSI | Fast-40 | SCSI-3 SPI-2 (1997) | 50-pin | 8 | 40 MHz | 40 MB/s | 320 Mbit/s | NA | 12m | 25m | 8 | LVD 125 ± 10 Ω | |
| Ultra2 Wide SCSI | SCSI-3 SPI-2 | 68-pin; 80-pin (SCA/SCA-2) | 16 | 40 MHz | 80 MB/s | 640 Mbit/s | NA | 12m | 25m | 16 | LVD 125 ± 10 Ω | ||
| Ultra3 SCSI | Ultra-160; Fast-80 wide | SCSI-3 SPI-3 (1999) | 68-pin; 80-pin (SCA/SCA-2) | 16 | 40 MHz DDR | 160 MB/s | 1280 Mbit/s | NA | 12m | NA | 16 | LVD 125 ± 10 Ω | |
| Ultra-320 SCSI | Ultra-4; Fast-160 | SCSI-3 SPI-4 (2002) | 68-pin; 80-pin (SCA/SCA-2) | 16 | 80 MHz DDR | 320 MB/s | 2560 Mbit/s | NA | 12m | NA | 16 | LVD 125 ± 10 Ω | |
| Ultra-640 SCSI | Ultra-5 | SCSI-3 SPI-5 (2003) | 68-pin; 80-pin | 16 | 160 MHz DDR | 640 MB/s | 5120 Mbit/s | 16 | |||||
Other SCSI interfaces
| Interface | Alternative names |
Specification document |
Connector | Width (bits) |
Clock | Maximum | ||||
|---|---|---|---|---|---|---|---|---|---|---|
| Throughput (MB/s) | Throughput (Mbit/s) | Length | Devices | |||||||
| SSA | 1 | 200 MHz | 40 MB/s | 320 Mbit/s | 25 m | 96 | ||||
| SSA 40 | 1 | 400 MHz | 80 MB/s | 640 Mbit/s | 25 m | 96 | ||||
| FC-AL 1Gb | 1 | 1 GHz | 100 MB/s | 800 Mbit/s | 500m/3 km | 127 | ||||
| FC-AL 2Gb | 1 | 2 GHz | 200 MB/s | 1600 Mbit/s | 500m/3 km | 127 | ||||
| FC-AL 4Gb | 1 | 4 GHz | 400 MB/s | 3200 Mbit/s | 500m/3 km | 127 | ||||
| SAS 1.1 | 1 | 3 GHz | 300 MB/s | 2400 Mbit/s | 6 m | 16,256 | ||||
| SAS 2.0 | 1 | 6 GHz | 600 MB/s | 4800 Mbit/s | 6 m | 16,256 | ||||
| Implementation- and network-dependent | ||||||||||
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