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SCSI (Small Computer System Interface) Print
SCSI interface overview
InterfaceBus widthClock speedBus bandwidthMax. cable lengthMax. number of devices
SCSI8 bits5 MHz5 MB/s6m8
Fast SCSI8 bits10 MHz10 MB/s1.5-3m8
Wide SCSI16 bits10 MHz20 MB/s1.5-3m16
Ultra SCSI8 bits20 MHz20 MB/s1.5-3m5-8
Ultra Wide SCSI16 bits20 MHz40 MB/s1.5-3m5-8
Ultra2 SCSI8 bits40 MHz40 MB/s12m8
Ultra2 Wide SCSI16 bits40 MHz80 MB/s12m16
Ultra3 SCSI16 bits40 MHz DDR160 MB/s12m16
Ultra-320 SCSI16 bits80 MHz DDR320 MB/s12m16
SSA1 bit400 MBit80 MB/s25m96
FC-AL1 bit2GBit200 MB/s
per direction; full duplex
Internet SCSI (iSCSI) Dependent upon IP network??
Serial Attached SCSI   3Gbit1 bitN/A375 MB/s
per direction; full duplex
10m16,256 (128 per expander)

The original standard that was derived from SASI and formally adopted in 1986 by ANSI. SCSI-1 features an 8-bit busSCSI-2
This standard was introduced in 1989 and gave rise to the Fast SCSI and Wide SCSI variants. Fast SCSI doubled the maximum transfer rate to 10 MB/s and Wide SCSI doubled the bus width to 16 bits on top of that (to reach 20 MB/s). However, these improvements came at the minor cost of a reduced maximum cable length to 3 meters. SCSI-2 also specified a 32-bit version of Wide SCSI, which used 2 16-bit cables per bus; this was largely ignored by SCSI device makers because it was expensive and unnecessary, and was officially retired in SCSI-3.

Before Adaptec and later SCSITA codified the terminology, the first parallel SCSI devices that exceeded the SCSI-2 capabilities were simply designated SCSI-3. These devices, also known as Ultra SCSI and fast-20 SCSI, were introduced in 1992. The bus speed doubled again to 20 MB/s for narrow (8 bit) systems and 40 MB/s for wide. The maximum cable length stayed at 3 meters but ultra SCSI developed an undeserved reputation for extreme sensitivity to cable length and condition (faulty cables, connectors or terminators were often to blame for instability problems).

This standard was introduced c. 1997 and featured a low voltage differential (LVD) bus. For this reason ultra-2 is sometimes referred to as LVD SCSI. Using LVD technology, it became possible to allow a maximum bus cable length of 12 meters (almost 40 feet!), with much greater noise immunity. At the same time, the data transfer rate was increased to 80 MB/s. Ultra-2 SCSI actually had a relatively short lifespan, as it was soon superseded by ultra-3 (ultra-160) SCSI.

Also known as Ultra-160 SCSI and introduced toward the end of 1999, this version was basically an improvement on the ultra-2 standard, in that the transfer rate was doubled once more to 160 MB/s by the use of double transition clocking. Ultra-160 SCSI offered new features like cyclic redundancy check (CRC), an error correcting process, and domain validation.

This is the ultra-160 standard with the data transfer rate doubled to 320 MB/s. Nearly all new SCSI hard drives being manufactured at the time of this writing (October 2003) are actually ultra-320 devices.

Ultra-640 (otherwise known as Fast-320) was promulgated as a standard (INCITS 367-2003 or SPI-5) in early 2003. Ultra-640 doubles the interface speed yet again, this time to 640 MB/s. Ultra640 pushes the limits of LVD signaling; the speed limits cable lengths drastically, making it impractical for more than one or two devices. Because of this, most manufacturers have skipped over Ultra640 and are developing for Serial Attached SCSI instead.

iSCSI preserves the basic SCSI paradigm, especially the command set, almost unchanged. iSCSI advocates project the iSCSI standard, an embedding of SCSI-3 over TCP/IP, as displacing Fibre Channel in the long run, arguing that Ethernet data rates are currently increasing faster than data rates for Fibre Channel and similar disk-attachment technologies. iSCSI could thus address both the low-end and high-end markets with a single commodity-based technology.

Serial SCSI
Three recent versions of SCSI SSA, FC-AL and Serial Attached SCSI break from the traditional parallel SCSI standards and perform data transfer via serial communications.

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