Answer: The term "SCSI" is an acronym for Small Computer System Interface. (It is pronounced "scuzzy"). In the 1970s the name was appropriate. Today, SCSI is used for PCs, Workstations, Servers, Mainframes, Supercomputers. The foundation of the SCSI standard was laid by Shugart Associates (the same guys that gave the world the first mini floppy disks) when they introduced the SASI bus (Shugart Associates Standard Interface). (SCSI provides a high-speed, intelligent interface that allows an easy connection for up to 16 devices (8 devices for Narrow SCSI) on a single bus. These devices may be hard disks, floppy disks, CDs, tape drives, printers and scanners to name a few. Peripherals may be mounted in the computer or in an external enclosure).

 

So what actually is SCSI?...

• There is a large variety of peripheral devices available for SCSI, including hard disk drives, floppy drives, CDs, optical storage devices, tape drives, printers and scanners to name a few. There are many implementations of SCSI starting with SCSI-1 to SCSI-2 to SCSI-3 including, Narrow, Wide, Fast, Ultra, Ultra-2, Ultra-3 and Ultra-320 SCSI. The SCSI specifications are approved and issued by ANSI and are developed by the X3T10 SCSI Committee.

SCSI-1

The original standard that was derived from SASI and formally adopted in 1986 by ANSI. SCSI-1 features an 8-bit parallel bus (with parity), running asynchronously at 3.5 MB/s or 5 MB/s in synchronous mode, and a maximum bus cable length of 6 meters (just under 20 feet—compared to the 18 inch (0.45 meter) limit of the ATA interface). A rarely seen variation on the original standard included a high-voltage differential (HVD) implementation whose maximum cable length was 25 meters.

SCSI-2

This standard was introduced in 1994 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 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.

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 1996. The bus speed doubled again to 20 MB/s for narrow (8 bit) systems and 40 MB/s for wide (16-bit). The maximum cable length stayed at 3 meters but single-ended 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).

Ultra-2

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. LVD's greater immunity to noise allowed a maximum bus cable length of 12 meters. 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.

Ultra-3

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.

Ultra-320

This is the Ultra-160 standard with the data transfer rate doubled to 320 MB/s. The latest working draft for this standard is revision 10 and is dated May 6, 2002. Nearly all new SCSI hard drives being manufactured at the time of this writing (October 2003) are actually Ultra-320 devices.

Ultra-640

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. Ultra-640 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.

SCSI  Connectors

No version of the standard has ever specified what kind of connector should be used. Specific types of connectors for parallel SCSI devices were developed by vendors over time. Connectors for serial SCSI devices have diversified into different families for each type of serial SCSI protocol. This is a brief summary, but see the SCSI connector article for a more detailed description.

Parallel SCSI connectors

Although parallel SCSI-1 devices typically used bulky Blue Ribbon Centronics connectors, and SCSI-2 devices typically used Mini-D connectors, it is not correct to refer to these as "SCSI-1" and "SCSI-2" connectors. One valid rule is that connectors for wide SCSI buses have more pins and wires than those for narrow SCSI buses. A Centronics-50 or HD-50 connector is for narrow SCSI, while a Centronics-68 or HD-68 connector is for wide SCSI. On some early devices, wide parallel SCSI busses used two or four connectors and cables while narrow SCSI busses used only one.

The first parallel SCSI connectors were the Centronics type. They then evolved through two main stages, High-Density (HD) and most recently SCA - 80 pin.

With the HD connectors, a cable normally has male connectors while a SCSI device (e.g. host adapter, disk drive) has female. A female connector on a cable is meant to connect to another cable (for additional length or additional device connections).

50Pin Narrow SCSI -

The most commonly used early SCSI cables were terminated with a Centronics-type 50-pin connector, similar to the 36-pin Centronics connector used for early parallel PC printers. This connector is sometimes referred to as a "CN-50" or "Centronics SCSI" and, confusingly, as a "SCSI-1 connector". Since many connectors have been used for SCSI-1, this is probably a poor name to use.

68Pin Wide SCSI - As time went on, some manufacturers desired connectors even smaller than the SCSI-2 connector. One such in somewhat common use was the VHDCI (Very High Density InterConnect) connector, also known as an "AMP HPCN68M" (a manufacturer part number), and sometimes as "SCSI-5". There are 68 pins on the connector in two rows, pins are 0.8 mm apart. This connector is reputed to suffer fewer bent pins than the 68-pin SCSI-2 connector despite its miniscule pins.

  68pin male connector

 

SCA 80Pin Ultra SCSI - Eventually, there was a desire to combine power and data signals into a single connector. This allows for quick drive replacement, more reliable connections, and is more compact. Most parallel SCSI disk-drives now utilize an 80-pin SCA (Single Connector Attachment) connector. This connector includes a power connection and also has long and short pins which enable hot swapping. Note that this connector is primarily found on disk drive HDA's (and of course the mating enclosure backplane connector).

 

    

                                            80pin SCA female connector                                                             80pin SCA male connector