What is SCSI

“SCSI” is an acronym for S mall C omputer S ystem I nterface. The usual way to pronounce scsi is scuzzy “). It was early in the 1970s that the name scsi was developed and taken to the appropriate industry. Now, SCSI is used in many industry sectors and can even be used for PCs, aswell as Workstations, Servers, Mainframes, and Supercomputers. SCSI now provides a high-speed, intelligent interface which is easily connected for up to 16 devices (8 devices for Narrow SCSI) on a single bus. The devices that can be used are very common today and come in the form of hard disks, floppy disks, CDs, tape drives, printers and scanners to name a few. These devices can be mounted both internally and by external connection.

Ultra 320 SCSI basically means that the transfer time of data runs at 320Mb/sec. This is the maximum that can be achieved , but often this type of speed is not continually met by the scsi bus.

– Many factors determine the higher data transfer rate as follows:

Spindle speed …. . This is the speed the drives disk will continually spin at. This will range from 5400rpm to 15,000rpm. The higher the speed of the disk, means the greater occurance of data being in the correct place, so that the disk heads can read it. Therefore, the faster the data can be transferred.

Average Access Time ……..The disk heads need to be in the right place to read the data, so the faster this happens the faster data is read.

Cache Size.. … .Means the size of the on board cache, It makes small differences in doubling the size but generally it is not very cost effective, but as ever, it is regarded that more is better.

Internal Transfer Rate ……… This is generally the way that data is passed and read throughout the actual drive itself. Usually this is higher than the actual transfer rate of the disk drive.

So, 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. SCSI comes in many forms from – SCSI-1 to SCSI-2 to SCSI-3 which includes Narrow, Wide, Fast, Ultra, Ultra-2 , Ultra-3 and Ultra-320 SCSI.
SCSI-1 

SCSI-1 Is the original standard which was developed back in 1986, it features an 8-bit parallel bus (with parity ), it runs asynchronously at 3.5 MB/s or 5 MB/s in synchronous mode, it has a maximum bus cable length of 6 metres.

SCSI-2 

Introduced in 1994 it brought along new features, such as Fast SCSI and Wide SCSI variants, and additional supplrt for other devices. Fast SCSI then 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).
Although these improvements increased industry productivity, the limitations were that cable lengths were now reduced to 3 metres.

SCSI-3 

Also known as Ultra SCSI and fast-20 SCSI, these were introduced in 1996. Again the speed of the bus was increased to 20 MB/s for 8 bit and 40MB/s for Wide 16 bit. Maximum cable length remained at 3 metres.

Ultra-2 

By 1997 this standard was introduced. It featured a low-voltage differential (LVD) bus. This is why SCSI 2 is sometimes referred to as LVD SCSI. Key features were – Greater cable lengths of 12 metre due to the immunity to noise. It also increased the the data transfer rate 80 MB/s. Ultra-2 SCSI was soon fased out as it made way for Ultra-3 (U160) SCSI.

Ultra-3 

Introduced in 1999, this version made improvements on the ultra-2 standard, by increasing the transfer rate by double. So now the transfer rate was 160 MB/s by the use of double transition clocking . Other new features of Ultra-160 SCSI were cyclic redundancy check (CRC), an error correcting process, and domain validation.

Ultra-320 

The data transfer rate was again increased to 320MB/s. By now, (2003), most new SCSI disk drives being manufactured, were Ultra 320 devices.

Ultra-640

This was otherwise known as Fast 320 The interface speed is again doubled to 640MB.s..But the Ultra 640 now dramatically limits the LVD signaling; thus causing even shorter cable lengths, which makes it impractical for more than one or two devices. Therefore, most manufacturers decided to swerve around Ultra640 and instead develop Serial Attached SCSI (SAS) instead.

SAS – Serial Attached SCSI

Short for Serial Attached SCSI, is an evolution of parallel SCSI into a point-to-point serial peripheral interface in which controllers are linked directly to disk drives.

SAS allows multiple devices up to 128 of differing types, simultaneously, so is therefore an improvement on performance, against the traditional SCSI. SAS disk drives are also hot plugged. In addition they use alot thinner and longer cable; supporting 3.0Gb/s.

SAS – Serial Attached SCSI – 2

An improvement on its predecessor, SAS-1, it now covers speeds up to 6Gb/s, which allows faster data transfer than previous generations. Offering backward compatibility with SATA devices. Note that you can connect SATA devices to a SAS backplane, but not the other way around.

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

Parallel SCSI-1 devices used to be big and bulky Blue connectors, often called “Centronics” and SCSI-2 devices typically used Mini-D connectors, It is incorrect in the industry to call these connections “SCSI-1” and “SCSI-2”. The standard rule is that, the connections for wide scsi have more pins compared to a narrow scsi bus. For example, An HD 68pin connector is used for wide SCSI, where as a Centronics 50 pin connection is used for narrow SCSI.

Centronics SCSI was the first parallel SCSI connector type. Which then evolved into, High-Density (HD) and most recently SCA – 80 pin .

For HD SCSI, a cable will have male connectors while a SCSI device such as a disk drive would have female.

50Pin Narrow SCSI – 

The most used early SCSI cables were terminated with a Centronics -type 50-pin connector, which is not unlike the 36-pin Centronics connector which was used for the old parallel printers. This connection is often called “CN-50” or “Centronics SCSI” .