2024-11-28

SCSI Levels

DATARECOVERYUNION.COM016 mins

SCSI Levels SCSI – Small Computer System Interface

An intelligent peripheral interface characterized by its use of high level communication between devices. Communications are defined as being between an “Initiator” and a “Target”. The Initiator is normally a computer, and the Target is normally a peripheral. Data may be transferred in asynchronous (not clocked) or synchronous (clocked) mode. All messages and commands are always transferred in asynchronous mode.

This term is often used to describe the published ANSI standard now called SCSI-1 (X3.131-1986).

SCSI-2

This is a term describing the published ANSI standard (X3.131-1994). SCSI-2 was an upgrade from the original SCSI interface. Changes included faster data rates and mandated message and command structure to improve compatibility. Synchronous data transfer rate for SCSI-2 is 2.5 to 10 Mbytes/sec for an 8-bit data bus (N/ND models), and 5.0 to 20 Mbytes/sec for a 16-bit data bus (W/WC/WD/DC models).

SCSI-3

This term describes a set of related ANSI standards that are currently being developed for the SCSI bus. The SCSI-2 document is very large (400+ pages) and covers the full range of topics. SCSI-3 split this large document into a series of smaller documents, each covering a “layer” of the interface definition.

The basic layers are:

physical (connectors, pin assignments, electrical specifications)

protocol (physical layer activity is organized into bus phases, packets, etc.)

architecture (a description of how command requests are organized, queued, and responded to by any protocol)

primary commands (description of commands that must be supported by all SCSI devices)

device specific commands (commands that are specific to a particular class of devices; CD-ROMs or -WORM drives, for example)

The set of standards needed to do a SCSI-3 parallel interface disc drive implementation are:

SPI (SCSI Parallel Interface) for the physical layer
SIP (SCSI Interlocked Protocol) for the protocol layer
SAM (SCSI Architecture Model) for the architecture
SPC (SCSI Primary Commands) for the primary command set
SBC (SCSI Block Commands) for the disc drive specific command set
The SCSI-3 standards are layered in this manner to allow substitution of parts of the structure as new technology emerges. For example, a comparable set of standards for a SCSI Fiber Channel interface disc drive replaces the physical and protocol layers with new documents but uses the same documents for the other 3 layers. The main point to remember here is that the terms SCSI-2 or SCSI-3 do not imply any particular performance per se, rather they refer to the generation of documents to which a product conforms. Since the newest features are only in SCSI-3 and tend to be higher performing, SCSI-3 devices should demonstrate better performance than SCSI-2 in most cases.

SCSI FAST

This refers to timings defined in SCSI-2 for a 10 MegaTransfer/sec transfer rate. A “MegaTransfer” (MT) is a unit of measure referring to the rate of signals on the interface regardless of the width of the bus. For example, a 10 MT/sec rate on a 1 byte wide (narrow) bus results in a 10 Mbytes/sec transfer rate, but on a 2 byte (wide) bus, it results in a 20 Mbytes/sec transfer rate.

SCSI FAST-20

This refers to timings defined in SCSI-3 SPI for a 20 MT/sec transfer rate, which achieves data rates twice as fast as SCSI FAST rates. For example, a 20 MT/sec rate on a 1 byte wide (narrow) bus results in a 20 Mbytes/sec transfer rate, but on a 2 byte (wide) bus, it results in a 40 Mbytes/sec transfer rate.

SCSI FAST-40

This refers to timings being defined for a future revision of the SCSI-3 SPI that achieve 40 MT/sec, which is twice as fast as SCSI FAST-20 rates. For example, a 40 MT/sec rate on a 1 byte wide (narrow) bus results in a 40 Mbytes/sec transfer rate, but on a 2 byte (wide) bus, it results in an 80 Mbytes/sec transfer rate. For more information, see Ultra2 SCSI.

SCSI FAST-80

This refers to timings being defined for a future revision of the SCSI-3 SPI that achieve 80 MT/sec, which is twice as fast as SCSI FAST-40 rates. For example, an 80 MT/sec rate on a 1 byte wide (narrow) bus results in an 80 Mbytes/sec transfer rate, but on a 2 byte (wide) bus, it results in a 160 Mbytes/sec transfer rate.

Ultra SCSI

This is a term describing the latest published ANSI standard (X3T10/1071D rev. 6), commonly known as Fast-20. Ultra SCSI, like all synchronous transfers, is a negotiated clock rate. For more information, see The UltraSCSI Buzz.

Ultra2 SCSI

This is a term describing the latest published ANSI standard (X3T10/1071D rev. 6), commonly known as Fast-40. Ultra2 SCSI, like all synchronous transfers, is a negotiated clock rate.

SCSI Narrow

This term refers to the 1 byte wide data bus on a 50-pin parallel interface that is defined in the ANSI standard SCSI-1 (X3.131-1986).  The narrow bus consists of 8 data lines with parity, a series of control lines and the matching ground lines.  Seagate designates a narrow SCSI interface with an “n” in the model number.

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SCSI WIDE

This term usually refers to the 2 byte wide data bus on a 68 pin parallel interface that is defined in the SCSI-3 SPI document. The term can be generically applied to any implementation wider than 1 byte, but at the time of this writing, there are no implementations wider than 2 bytes. Future implementations may include more data bytes because FAST transfer rates are giving plenty of life to 2 byte transfers until serial interfaces (like Fibre Channel or FireWire) become more popular. Seagate designates a wide SCSI interface with a “w” in the model number.

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SCSI FAST-WIDE

This refers to a combination of a FAST transfer rate with a 2 byte wide connector, resulting in an increased data transfer rate. Wide FAST-20 (40 Mbytes/sec) products will be available in the year of this writing. Wide FAST-40 (80 Mbytes/sec) and FAST-80 (160 Mbytes/sec) products will be available in the near future.

Differential or High Voltage Differential (HVD)

Differential (D, ND, WD, WDC) is a logic signal system used in some SCSI drives. It uses a paired plus and minus signal level to reduce the effects of noise on the SCSI bus. Any noise injected into the signal would be present in both a plus and minus state, thereby being canceled. Seagate designates a differential (high voltage) SCSI interface with a “d” in the model number.

Due to changing definitions, Differential is now often referred to as High Voltage Differential (HVD).

Low Voltage Differential (LVD)

Low Voltage Differential is a differential logic scheme using lower voltage levels than HVD. For more information, see Ultra2 SCSI. Seagate designates low voltage differential SCSI interface with an “l” in the model number.

Fibre Channel Arbitrated Loop (FCAL)

This is the formal name for the Fibre Channel system used by SCSI. It is more commonly known as Fibre Channel SCSI. The loop part of the name refers to the way the system is connected as one large ring. Because of the loop characteristics, this interface has more in common with local area networks than with parallel SCSI.

Fibre Channel SCSI

This refers to products with fibre channel physical and protocol layers using the SCSI command set. The Fibre Channel interface is completely different from parallel SCSI in that it is a serial interface, meaning command and data information is transmitted on one signal stream organized into packets. The fibre may be either a copper coaxial cable or a fiber optic cable. The signal on the first implementation of fibre channel uses a 1 GHz rate, thereby achieving 100 Mbytes/sec over the cable. Fibre channel also implements increased software control of configuration and pushes the total device count on the bus to 126 IDs, as opposed to only 8 or 16 on a parallel bus. For more information, see our Technology Paper on Fibre Channel. Seagate designates a fiber channel SCSI interface with an “fc” in the model number.

ASA-2 SCSI

This is a Seagate specific term describing the basic structure of the SCSI firmware included with a Seagate disc drive and standing for Advanced SCSI Architecture, generation 2. It provides better performance than ASA-1 code in certain user environments (such as sequential 1 block data transfers). The labels ASA-1 and ASA-2 can apply to code shipped on a wide range of products, meaning different products have firmware originating from the same base firmware, but such firmware is individually adapted to the particular servo and read/write channel hardware on that product. The performance of a disc drive is still primarily determined by the seek times and data transfer rates, but the code base label provides a level of commonality and lack of bugs assurance in SCSI features for all drives with that label.

The most attractive feature of a common code architecture is that most enhancements made on one drive (such as a Barracuda 4) can be quickly migrated to other drives (such as the Barracuda 2 or Hawk 4) developed from the same code base. All new products since January 1995 use the ASA-2 code base, and many older products developed with ASA-1 code have been updated to the new structure.

SCA-2

This is a miniature D style, 80 pin connector used on SCSI drives to plug to backplane connections. The SCA-2 connector provides grounds, voltage, and control lines needed to allow hot-plugging of parallel interface SCSI drives. Seagate designates an SCA-2 SCSI interface with a “c” in the model number.

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Free Disk Catalogue Programs

DATARECOVERYUNION.COM11 mins

Catfish
Small, free and automatic disk inventory program for Windows. For people who can’t remember what they put on their ZIP disks, floppies or CD-roms! We use it at Freebyte.

Cathy
A similar program (not only in name) to Catfish. Has searching capabilities based on file name, date and size. Customizable date format. Found files can be opened directly, if they are present. Only contains a small single executable file, meaning that no installation is needed. Runs on Windows.

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What are the common Reasons of Hard Disk Drive failure?

DATARECOVERYUNION.COM19 mins

What are the common Reasons of Hard Disk Drive failure?

1. Electronic Component Failure
2. Motor Failure
3. Read / Write Head Failure
4. Media Damage
5. Firmware Corruption
6. Logical Failure

One or all of the above primary causes may be evident when diagnosing a failed hard disk drive.

Electronic Component Failure

Electronic components may fail due to voltage transients, heat or poor handling. Substitution, repair and re-programming is generally required in order to recover data stored on the hard disk. PCB assemblies are however hyper tuned at the manufacture stage and specialist re-programming and calibration is subsequently required to restore the hard disk to a working condition.

Motor Failure

Hard disk motor spindles have fluid bearings; sometimes this fluid leaks or becomes overheated and in-effective. The motor will then seize and the hard disk platters fail to rotate. Platter and component re-location to another hard disk assembly is required to effect a repair and restore data.

Read / Write Head Failure
Read / write heads are aerodynamically designed to “fly” at nanometer distances above the surface of the platters. Ceramic thin film sensors at their tip detect magnetic information (data) stored on the surface of the platter. Occasionally the atmosphere in the hard disk enclosure will become contaminated or vibration will cause the dynamic of the head to be disturbed. This disturbance will cause the read /write process to malfunction resulting in bad data read write cycles and eventual failure.
This type of failure usually manifests itself as a distinct clicking noise as the head actuator makes failed repeat attempts to locate data at the same platter track location.

Media Damage

Amazingly all computer hard disk magnetic storage media is manufactured imperfect but to acceptable and controllable levels. During normal operations imperfections will sometimes increase above the predefined acceptable level. This can be due to heat, vibration, head crash, shock or other factors. The operating system will flag errors or fail to boot and data files will then become in-accessible. Read/.Write head replacement and file repair will allow data file structures to be examined and assessed as to their validity.

Firmware Corruption

Hard disk firmware holds precise parameters relevant to the configuration of the assembly at the time of manufacture. Occasionally the firmware becomes corrupt or will “roll back” to an incorrect set of parameters. Under these conditions the location of the stored data as reported to the operating system will be lost. Simple restoration of the correct parameters will allow the hard disk to function correctly. What causes this corruption? Operating system to drive software bugs, control bus protocol failure, it is difficult to determine but failures do occur.

Logical Failure

Data files are stored at logical locations that relate to a number of physical locations on the surface of the hard disk platters. These logical locations are held in tables by the operating system and indexed when running specific software applications. Operating system errors, reloads or incorrect upgrade applications will sometimes corrupt these tables and data will become in-accessible. This is generally referred to as a logical failure. Logical errors can be repaired with software tools available from the internet. Be cautious however – if you are intending to run a fix utility on your disk you can inadvertently damage these tables irreparably and your data will be unrecoverable. This is especially true when running ScanDisk and Chkdsk on a damaged hard drive.

Hard Drive Failure Signs

  • Your computer “freezes” too often (the picture on the screen is still and does not react to mouse or keyboard manipulations)
  • Regular booting problems. That may be a sign of bad sectors on the disk with corrupted booting data.
  • You computer is terribly slow while accessing, saving and opening files.
  • The usual sound produced by your hard drive is louder than before
  • Regular appearance of BSOD (Blue Screen of Death), “Operating system not found or Missing Operating System” or “your hard drive is not formatted” messages at startup.

Even if you haven’t backed up your files yet, these signs give you a chance and some time to copy the data before the drive crashes.

Far more ominous signs are:

  • Your computer is still running normally, but you can hear unusual metallic sounds (grinding, clicking, whirring, scratching, buzzing). That’s a very bad sign that may imply mechanical damage.
  • You cannot hear any hard drive sounds at all. When the information is written to or read from the disk, it spins and produces sounds, you must be accustomed to these normal sounds. As an example, hard drive becomes silent when it’s inside components expand and get stuck because of overheating.
  • Your hard drive is clicking or producing grinding metallic sounds, your computer won’t recognize the hard disk. This is a sign that hard drive failure have happened.

If the above occurs, shut down immediately and contact a disk recovery service! If you keep your computer running the platters may be damaged and your files will be unrecoverable. Also, if your hard drive has undergone mechanical damage or was exposed to water, fire, smoke or high temperatures, don’t try to power it up. Contact a disk recovery service.

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