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Small Computer Systems Interface , abbreviated as SCSI and pronounced “skuzzy” .
SCSI is a much more advanced interface than its chief competitor, IDE, and has several advantages over IDE that make it preferable for many situations, usually in higher-end machines.
However, it is far less commonly used than IDE due to its higher cost and the fact that its advantages are not useful for the typical home or business desktop user .
SCSI was first developed for hard disks. However, SCSI is not an interface tied specifically to hard disks.
SCSI was to be a high-level, expandable, high-performance interface.
For this reason, it is frequently the choice of high-end computer users. It includes many commands and special features, and also supports the highest-performance storage devices.
Limitation Of SCSI
Most PC systems do not provide a built-in support for SCSI the way they do for IDE
It has limited system BIOS support, and all the different SCSI types have different speeds, bus widths and connectors.
That is why implementing SCSI on a PC typically involves the purchase of not only the storage device, but also a special card called a host adapter .
Special cables and terminators may also be required.
Advantages of SCSI
SCSI is very reliable and bug free.
SCSI can support multiple platforms ( PC, Apple Macintosh and others).
SCSI can be very fast (faster than any other methods currently used to connect components to your computer including IDE & USB).
SCSI devices can be daisy chained (up to seven devices from the one connector).
Some SCSI adapters support both internal and external devices.
SCSI SCSI Card
Each SCSI device must have a unique identifier (ID) in order for it to work properly.
Internal devices connect to a SCSI controller with a ribbon cable.
External SCSI devices attach to the controller in a daisy chain using a thick, round cable
Daisy Chain Connection
Major Types of SCSI
SCSI-1 (also known as Regular SCSI ) defines the basics of the first SCSI buses, including cable length, signaling characteristics, commands and transfer modes.
It was quite limited, especially by today's standards, and defined only the most fundamental of SCSI features and transfer modes.
Devices corresponding to the SCSI-1 standard use only a narrow (8-bit) bus, with a 5 MBps maximum transfer rate.
There could be only seven devices besides the host adapter. Only single-ended transmission was supported, with passive termination.
the SCSI-2 (also known as fast Fast SCSI ) specification.
SCSI-2 is an extensive enhancement of the very limited original SCSI
Important goals of this evolution of the SCSI standard were to improve performance, enhance reliability, and add features to the interface. However, the most important objective was to formalize and properly standardize SCSI commands.
SCSI-3 (also known as Ultra SCSI ) began in 1993. It has grown to be a set of standards rather than a single standard. It supports both parallel and serial data transmission, supports FireWire connections and increases the possible rate of data transfer to 320 MBps and higher.
The most implemented form of SCSI, is the SCSI-3 Parallel Interface (SPI )
The latest revision of SPI will be called the SCSI-3 Parallel Interface 4 or SPI-4 . It is still in development. The one feature that we now know will be included in SPI-4 is yet another doubling of maximum throughput on the SCSI bus. It is expected to soon be rated at 640 MBps to 1280 MBps.
Signaling Methods Used on SCSI Cables
A SCSI cables are built in two different ways, depending on the method by which the electrical signal is placed on the cable:
Both types of cables send a signal on a pair of twisted wires.
In single-sided (SE) cables, one of the wires carries voltage and the other is a ground .
While in differential cables, both wires carry voltage and the signal is calculated to be the difference between the two voltages.
Signaling Methods Used on SCSI Cables
A single-sided cable is less expensive than a differential cable, but the cable length is limited to sustain the data integrity.
Termination prevents an echo effect from the electrical noise and reflected data at the end of the SCSI daisy chain, which can cause interference with data transmission. Each end of a SCSI chain must be terminated.
Types of terminator are:
Forced Perfect Terminators (FPTs)
Passive terminators, active terminators and FPTs are all used with single-ended SCSI cables. Differential cables use either HVD or LVD terminators.
SETTING UP A SCSI SUBSYSTEM
Now, you will learn how to add some external SCSI devices to the SCSI subsystem.
First of all, shut down the PC.
Connect the SCSI cable to the SCSI port on the SCSI device (printer, scanner, external storage, etc). Each end of the cable may be different. Securely tighten hand screws.
Connecting SCSI Cable to SCSI Device
Connect the other end of the cable to the SCSI port on your computer. Securely tighten hand screws or fasten clips.
Connecting SCSI Cable to the SCSI Port
Connect the cable for a second SCSI device to the output SCSI port on the first SCSI device. These steps can be repeated for up to seven devices.
Put a SCSI terminator on the last device in the SCSI chain (unless the device has built-in termination).
Connect the power cable for the SCSI devices to a power supply.
Connecting the Power Cable
Turn on the devices.
Turn on the computer. Your devices are ready for use.
File Allocation Table
File Allocation Table (FAT)
File Allocation Table (FAT) is a partially patented file system developed by Microsoft for MS-DOS and was the primary file system for consumer versions of Microsoft Windows.
FAT as it applies to floppy and optical disk has been standardized as ECMA-107 and ISO/IEC 9293.
The FAT file system is relatively uncomplicated, and is supported by virtually all existing operating systems for personal computers
The FAT file system was created by Bill Gates and Marc McDonald in 1977 for managing disks in Microsoft Disk BASIC.
The name originates from the usage of a table which centralizes the information about which areas belong to files, are free or possibly unusable, and where each file is stored on the disk.
To reduce the management complexity, disk space is allocated to files in contiguous groups of hardware sectors called clusters
The most common implementations have a serious drawback in that when files are deleted and new files written to the media, their fragments tend to become scattered over the entire media, making reading and writing a slow process.
Defragmentation is the process or reorganizing and rewriting files so that they occupy one large continuous area on your hard disk rather than several smaller areas scattered all over the surface
Defragmentation The process or reorganizing and rewriting files so that they occupy one large continuous area on your hard disk rather than several smaller areas scattered all over the surface
HOW FAT WORKS
A disk is divided into partitions. Under the FAT file system each partition is divided into clusters, each of which can be one or more sectors, depending on the size of the partition. Each cluster is either allocated to a file or directory or it is free (unused). A directory lists the name, size, modification time and starting cluster of each file or subdirectory it contains.
At the start of the partition is a table (the FAT) with one entry for each cluster. Each entry gives the number of the next cluster in the same file or a special value for “not allocated” or a special value for “this is the last cluster in the chain”.
FAT32 is a derivative of the FAT file system that supports drives over 2GB. FAT32 drives can contain more than 65,526 clusters, smaller clusters are used than on large FAT16 drives, resulting in more efficient allocation of space on the drive.
The 32-bit FAT includes four bytes per cluster within the File Allocation Table. This differs from FAT16 and FAT12 in that those files systems contain 2 bytes and 1.5 bytes within the File Allocation Table, respectively.
File System Cluster Limits
NT File System (NTFS) is the standard file system of Windows NT and its descendants Windows 2000, Windows XP, Windows Server 2003, and Windows Vista.
NTFS replaced Microsoft’s previous FAT file system, used in MS-DOS and early versions of Windows.
NTFS has several improvements over FAT in terms of performance, reliability, and disk space utilization .
It is designed to quickly perform standard file operations such as read, write, and search, and even advanced operations such as file-system recovery on a very large hard disks.
The NTFS file system includes security features required for file servers and high-end personal computers in a corporate environment.
The NTFS file system also supports data access control and ownership privileges that are important for the integrity of critical data.
While folders shared on a Windows NT computer are assigned particular permissions, NTFS files and folders can have permissions assigned whether they are shared or not.
NTFS is the only file system on Windows NT that allows you to assign permissions to individual files.
Formatting a volume with the NTFS file system results in the creation of several System Files and the Master File Table (MFT) , which contains information about all the files and folders on the NTFS volume.
The first information on an NTFS volume is the Partition Boot Sector . Figure below illustrates the layout of an NTF volume when formatting has finished.
Master File Table
Each file on an NTFS volume is represented by a record in a special file called the MFT
It is possible to compress individual files or directories on a hard disk, or even compress an entire disk volume
many PC users needed disk compression just to allow them enough room for the latest operating system, and for their applications and data files
Advantages of Compression
Compression not only saves disk space, it can help you organize and archive older files that you don't use regularly.
It has also become a standard for allowing the easy downloading of large numbers of files from the Internet.
Compression allows them to be packed into a single file that takes less time to transmit than if the files were sent in their regular format.
These are the simple steps on how to do file compression using the utilities available in Windows XP.
Select the folder which you want to compress.
Right click the folder and select Properties .
Under the general tab, click the Advanced button.
Select (or Check) the Compress contents to save disk space option.