SCSI




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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