477 lines
22 KiB
Plaintext
477 lines
22 KiB
Plaintext
From: datarec@inforamp.net (Nicholas Majors)
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Subject: Chapter 2 - Technicians' Guide to Hard Disks
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Date: 25 Mar 1995 18:40:54 GMT
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========================================================================
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Chapter 2 - TECHNICIANS' GUIDE TO PC HARD DISK SUBSYSTEMS
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========================================================================
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copyright (c) 1992, 1995
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Nicholas Majors,
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DATA RECOVERY LABS
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(division of Data Recovery Services Inc) Voice : 1-416-510-6990
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1315 Lawrence Avenue East - Unit 502 FAX : 1-416-510-6992
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Don Mills, Ontario, Canada M3A 3R3 Email : datarec@the-wire.com
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========================================================================
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Before we consider how to install, configure and maintain hard drives,
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we need a basic understanding of drive construction and design concepts.
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This chapter examines in some detail the parts and functional components
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of hard drive subsystems.
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(Note : A number of acronyms are used throughout this chapter and the
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glossary for this booklet is not yet available. Therefore, I have
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attached a brief set of definitions for some of the terminology.)
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HARD DRIVES AND CONTROLLERS:
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A hard drive subsystem is comprised of the following components:
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1. The Hard Disk, with one or more boards (PCB) attached.
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2. A Controller Mechanism, either on the hard disk PCB or on the bus
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adapter within the PC.
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3. Bus Adapter for interfacing the controller to the host PC.
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4. Cables and Connectors to link it all together.
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========================================================================
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THE HARD DISK:
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Within a sealed enclosure (Head Disk Assembly or HDA) are one or more
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rigid platters that are "fixed" or non-removable. These are coated
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with magnetically sensitized material and data can be written to and
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read from the surface by means of electromagnetic read/write heads.
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When powered up, the platters are constantly rotating (except for
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certain pre-programmed sleep modes) and the heads are moved back and
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forth across the surface to access different locations. This is a
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sealed unit which should not be opened, except by qualified personnel in
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a controlled, dust free environment.
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The circuit board(s) attached to the outside of the HDA provide the
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electronics needed for physical control of the motors within the sealed
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unit. They interface the source of electrical power and control signals
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to the disk assembly through various connectors and cables. Most boards
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have some jumpers, dip switches and/or resistors that are used for
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configuration purposes.
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Functionally, these PCB's are separate from the Hard Disk Controller,
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but many of the newer drives (IDE and SCSI) embed the controller chip
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directly onto this board (as opposed to having it on the Bus adapter).
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INSIDE THE HDA - PARTS OF A HARD DISK:
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1. Disk Platter(s), separated by spacers and held together by a clamp.
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2. Spindle shaft onto which platters are mounted.
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3. Spindle motor for rotating the platters.
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4. Electromagnetic read/write heads (usually, one per surface).
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5. Access arms or armatures from which the heads are suspended.
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6. Actuator for moving the arms (with heads attached).
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7. Preamplifier circuitry to maximize read/write signals.
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8. Air filter and pressure vent.
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The Platters:
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Most platters or disks are made of an aluminum alloy, though ceramic or
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glass platters can also be found. The diameter is normally 2 1/2", 3
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1/2" or 5 1/4" with a hole in the center for mounting onto the spindle
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shaft. Thickness of the media can vary from less than 1/32 of an inch
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to about 1/8 of an inch.
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During manufacture the platters are coated with a magnetizable material.
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Older drives used a ferrite compound applied by squirting a solution
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onto the surface and rotating at high speeds to distribute the material
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by centrifugal force. This process left a rust colored ferrite layer
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which was then hardened, polished and coated with a lubricant.
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Newer drives apply the magnetic layer by plating a thin metal film onto
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the surface through galvanization or sputtering. These surfaces have a
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shiny chrome-like appearance.
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Spindle and Spindle Motors:
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Most drives have several platters that are separated by disk spacers and
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clamped to a rotating spindle that turns the platters in unison. A
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direct drive, brushless spindle motor is built into the spindle or
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mounted directly below it. (Sometimes this motor is visible from outside
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of the sealed enclosure.) The spindle, and consequently the platters,
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are rotated at a constant speed, usually 3,600 RPM, though newer models
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have increased that to 4800, 5400, or 7,200.
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The spindle motor receives control signals through a closed loop
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feedback system that stabilizes to a constant rotation speed. Control
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signals come from information written onto the surface(s) during
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manufacture or with older drives, from physical sensors.
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Read/Write Heads:
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Since both sides of each platter are coated to provide separate
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surfaces, there is normally one electromagnetic read/write head for each
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side of each platter. Therefore, a drive with 4 platters would have 8
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sides and 8 heads. Some drives use one side as a dedicated surface for
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control signals leaving an odd number (5,7,etc.) of heads for actual
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use.
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Each head is mounted onto the end of an access arm and these arms (one
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per surface) are moved in unison under the control of a single actuator
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mechanism. When one head is over track 143, all the heads on all other
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sides should be at the same location over their respective surfaces.
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Generally speaking, only one of the heads is active at any given time.
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There are some drives that can read or write from two or more heads at a
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time, but while this has been common with main frame drives, it
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represents a major design change for personal computers and the
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technology is not yet widely used.
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The spinning disk(s) create an air cushion over which the heads float.
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Depending on design, this air buffer ranges from 2 to 15 microns. By
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contrast, a smoke particle or finger print is about 30 microns in size!
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The heads are not supposed to come into contact with the surface during
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rotation. Only when powered off should the heads come to rest on the
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surface, but this should be over a specific area of the surface,
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reserved for that purpose. Most drives built since the late 1980's
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employ an automatic parking feature which moves the heads to this
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designated region and may even lock the heads there until powered up.
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Head Actuators:
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The head actuator is the positioning mechanism used to move the arms and
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consequently the heads, back and forth over the surface. Once again,
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earlier drives used a different method than is now common.
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Originally, head positioning was controlled by a stepper motor that
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rotated in either direction by reacting to stepper pulses and moving the
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head assembly back and forth by means of a "rack and pinion" or by
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spooling and unspooling a band attached to the actuator arms. Each
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pulse moved the assembly over the surface in predefined steps or
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detents. Each step represented a track location and data was expected
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to be under the head. This design, still used for floppy drives, is not
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suitable for current drive densities and is prone to alignment problems
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caused by friction, wear and tear, heat deformation, and lack of
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feedback information needed for correcting positioning error.
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The more common voice coil actuator controls the movement of a coil
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toward or away from a permanent magnet based upon the amount of current
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flowing through it. The armatures are attached to this coil and move in
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and out over the surface with it. This is a very precise method, but
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also very sensitive. Any variation in the current can cause the head
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assembly to change position and there are no pre-defined positions.
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Inherently this is an analog system, with the exact amount of movement
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controlled by the exact amount of current applied.
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The actual position of the coil is determined by servo (or indexing)
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information, which is written to the drive by the manufacturer.
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Location is adjusted to different tracks by reading and reacting to
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these control signals.
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Internal Electronics:
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There is surprisingly little circuitry found within the sealed HDA.
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There are electrical and control wires for the spindle and head actuator
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motors and the head assembly has flex cables with a preamplifier chip
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often built onto it. This chip takes pulses from the heads (as close to
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the source as possible) and cleans up and amplifies these signals before
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transmission to components outside of the housing.
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Air Filtering and Ventilation:
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Minor wear of internal components and occasional contact of the heads
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with the surface can cause microscopic particles to be loosened within
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the HDA. A permanent air filter is mounted within the air stream to
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remove these particles before they can cause damage to delicate
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mechanisms.
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Most drives also have a small vent to allow for minor air exchange from
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outside of the housing. This allows for equalization of air pressure so
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drives can be used in different environments without risk of imploding
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or exploding.
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========================================================================
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CONTROLLERS AND BUS ADAPTERS:
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The hard disk controller provides the logical link between a hard disk
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unit and the program code within the host computer. It reacts to
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requests from the computer by sending seek, read, write, and control
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signals to the drive and must interpret and control the flow of data.
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Data moving to and from the drive includes sector ID's, positioning
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information and timing or clock signals. The controller must encode,
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decode and separate this control information from actual data written to
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or read from the drive.
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Also, data is sent to and from the drive serially, in bit format, but
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the CPU wants to recieve at least a byte (8 bits) at a time. The
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controller must take bits (8 - 16 - or 32 at a time) and assemble them
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into bytes, words, and doublewords that can be transferred to/from the
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computer.
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"OUR INDUSTRY MUST LOVE STANDARDS - WE HAVE THOUSANDS OF THEM!"
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And so it is with hard disk controllers.
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Controllers can be categorized in several different ways, by :
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Basic computer design (PC/XT vs AT-286-386-486,etc)
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- as mentioned in the first chapter, standard AT controllers use
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different I/O addresses, IRQ and employ PIO as opposed to DMA.
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Bus Architecture (8-16 bit ISA, 32 bit MCA/EISA/VLB/PCI, etc.)
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- The adapter must be designed to interface with and use features of
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available expansion spots in the host computer.
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Controller Card vs Adapter
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- The expansion board that plugs into the PC is commonly referred to
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as a controller card, but for many drives (primarily IDE and SCSI)
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the controller mechanism is built directly onto the drive PCB and
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the expansion board in the PC (or built into motherboard) is
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actually a Host/Bus adapter.
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TROUBLESHOOTING TIP - If the BIOS reports "HDD CONTROLLER FAILURE" don't
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assume the problem is with your AT/IO board. It might well be the drive
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PCB that has failed.
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Controller/Drive Interface
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- Both drive and controller must communicate in the same 'language'
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and several different standards for electrical properties and
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logical meaning of signals have been established. These
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include ST506/412, ESDI, SCSI, IDE(ATA/XTA) and EIDE(ATA2).
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Data Encoding Method
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- Determines how densely data can be packed onto a track. MFM
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encoding is sufficient for only 17 x 512 byte sectors per track.
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RLL permits up to 27 and variations of ARLL allow 34 or more
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sectors per track. This recording density is a major determinant
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of storage capacity, and with rotation speed and interleave are
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critical factors for true data transfer capability.
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Support for Translation
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- Some controllers present different logical parameters to the PC
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than the actual physical geometry of the drive.
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Need for ROM Extension or Software Device Driver
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- Additional program code is used to provide support for hard drives
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when none exists (as in PC/XTs), to implement translation schemes
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(as in ST506/RLL and ESDI designs), allow for non-standard devices
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or features (SCSI), or for a combination of these (EIDE).
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Below is a quick list of the major combinations that have been used in
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PCs past and present. While I am sure many others could be added, these
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are the ones I have come across over the years.
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------------------------------------------------------------------------
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Computer Bus Connection Interface Encoding Translate ROM
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------------------------------------------------------------------------
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PC/XT 8 bit ISA Controller ST506/412 MFM NO YES
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PC/XT 8 bit ISA Controller ST506/412 RLL OPTION YES
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AT 16 bit ISA Controller ST506/412 MFM NO NO
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AT 16 bit MCA Controller ST506/412 MFM NO NO
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AT 16 bit ISA Controller ST506/412 RLL OPTION YES
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AT 16 bit MCA Controller ST506/412 RLL YES YES
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AT 16 bit ISA Controller * ESDI (10 Mbps) RLL OPTION YES
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AT 16 bit ISA Controller * ESDI (24 Mbps) ARLL OPTION YES
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AT 16 bit MCA Controller ** ESDI (PS/2) RLL,ARLL YES YES
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PC/XT 8 bit ISA Adapter SCSI RLL YES YES
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AT 16 bit ISA Adapter SCSI RLL,ARLL YES YES
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AT ?? bit MCA Adapter *** SCSI RLL,ARLL YES YES
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AT 32 bit EISA Adapter SCSI RLL,ARLL YES YES
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AT 32 bit VLB Adapter SCSI RLL,ARLL YES YES
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AT 32 bit PCI Adapter SCSI RLL,ARLL YES YES
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PC/XT 8 bit ISA Adapter IDE / XTA RLL OPTION YES
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AT 16 Bit ISA Adapter IDE / ATA RLL,ARLL OPTION NO
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AT 32 Bit VLB Adapter EIDE / ATA2 ARLL OPTION YES
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AT 32 Bit PCI Adapter EIDE / ATA2 ARLL OPTION YES
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* ESDI drives have some of the controller logic built onto
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the hard drive PCB and some on the controller card.
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** PS/2 ESDI uses the same physical interface as other ESDI
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devices, but supports additional features specific to their
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implementation.
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*** 16 bit? 32 bit? Who knows? I have never been sure.
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------------------------------------------------------------------------
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Not to mention hundreds of other combinations to support different
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interleaves, track buffers, hardware caching, bus mastering, error
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correction schemes, SCSI I-II-III, optional floppy control, ESDI to SCSI
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converters, ST506 to SCSI converters (etc., etc., etc.).
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So, what does all this mean to you?
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Specifically, don't be surprised if the drive you have in your left
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hand, does not work correctly with the controller / adapter you have in
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your right hand. Also, if controllers are changed it may affect
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performance as well as the ability to access previously recorded data.
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========================================================================
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END OF CHAPTER 2
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========================================================================
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How to deal with some of this confusion will be addressed in
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CHAPTER 3 - Installing and Configuring Hard Drives.
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========================================================================
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ADDITIONAL READING MATERIAL:
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========================================================================
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I have always had difficulty finding appropriate reading material to
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recommend, but there are a few sources that I consider a must for
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technicians and support personnel.
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First, a number of Internet Newsgroups have exceptional FAQ's
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(Frequently Asked Questions) which are updated and posted on a regular
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basis. These include:
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"YET ANOTHER ATA-2/FAST-ATA/EIDE FAQ"
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by John Wehman and Peter Herweijer
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Newsgroup : comp.sys.ibm.hardware.storage
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"BIOS TYPES"
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by Hale Landis
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Newsgroup : comp.sys.ibm.hardware.storage
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"SCSI FAQ - 2 Parts"
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by Gary A. Field
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Newsgroup : comp.periphs.scsi
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While bookstores are full of titles, most of them simply provide a
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rehash of basics. The following two books are an important part of my
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library because they cover much more than the usual:
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"THE INDISPENSABLE PC HARDWARE BOOK" by Hans-Peter Messmer (1994)
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Addison-Wesley Publishing Company,
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ISBN - 0-201-62424-9
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"THE UNDOCUMENTED PC" by Frank Van Gilluwe (1994)
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Addison-Wesley Publishing Company,
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ISBN - 0-201-62277-7
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Other books well worth the read include:
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"THE HARD DISK SURVIVAL GUIDE" by Mark Minasi (1991)
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Sybex Inc.,
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ISBN - 0-89588-799-1
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A little dated, but full of useful information. Hopefully there is a
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revised and updated version.
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"OFFICIAL SPINRITE II AND HARD DISK COMPANION" by J. M. Goodman, (1990)
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IDG Books Worldwide, Inc.,
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ISBN - 878058-08-8
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Current advances are not covered, but great explanation of drive basics.
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"HARD DRIVE BIBLE" by Martin Bodo (1993)
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Corporate Systems Center (CSC)
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My copy is the sixth edition from April, 1993. The first 50 pages of
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the book should be of interest (though not always clearly organized).
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The balance of the book (150 pages) is a listing of drive types and
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jumper settings. It's quite good, but keeping something like that
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updated is virtually impossible.
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WARNING - DO NOT BUY - "The Data Recovery Bible" by Pamela Kane.
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Poorly organized material, most of which has nothing to do with data
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recovery. Waste of a good title if you ask me!
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========================================================================
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ACRONYM DEFINITIONS:
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========================================================================
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IRQ (Interrupt Request) - Lines on the bus used to signal hardware
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interrupts.
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I/O (Input Output) - Peripherals accessible by the CPU through registers
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at specific I/O addresses (or I/O ports).
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PIO (Programmed Input Output) - Exchange of data between memory and
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peripherals by means of Input Output commands.
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DMA (Dynamic Memory Access) - Transferring data directly between memory
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and peripherals without going through the CPU.
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BUS ARCHITECTURES:
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ISA (Industry Standard Architecture) - 8 bit and 16 bit expansion slots
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used by PC, XT, and AT designs. Often called IBM Standard Architecture.
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EISA (Extended Industry Standard Architecture) - Developed by several
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independent manufacturers (Compaq, AST, Zenith, Tandy, etc.) to
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standardize 32 bit operation and combat IBM's MCA.
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MCA (Micro Channel Architecture) - Expansion bus introduced by IBM in
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1987, used by some (but not all) PS/2 models.
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PCI (Peripheral Component Interconnect) - High speed bus developed by
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Intel to support the demands of Pentium and 486 based computers.
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VLB (VESA Local Bus) - High speed, 32 bit extension to the ISA bus
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promoted by the VESA (Video Electronics Standards Association).
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DRIVE INTERFACES:
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ST506/412 - Standard interface used on XT and AT drives and controllers.
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Originally developed by Seagate Technologies to support their ST506
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(5 MB) and ST412 (10 MB) drives. The entire controller mechanism is
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located on a controller card and communications between the drive and
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controller flow over 2 ribbon cables - one for drive control and one for
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data.
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ESDI (Enhanced Small Device Interface) - Developed by Maxtor in the
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early 1980's as an upgrade and improvement to the ST506 design. While
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the drive does not have an embedded controller, one of the most critical
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functions ,encoding-decoding, is performed on the drive. This allows
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for faster communications and higher drive capacities. Uses the same
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cabling as ST506 interface, but carries different signals on each line.
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SCSI (Small Computer System Interface) - Based on an original design by
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Shugart Associates, SCSI is not specifically a drive interface, but a
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method of allowing different devices to communicate with a PC. For hard
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drives the entire controller is built onto the drive PCB, allowing for
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very high speed transfers to and from the drive. Fully interpreted,
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parallel data is then transferred to and from the PC by way of a single
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cable through a bus interface that has configured the device as a hard
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drive.
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IDE (Integrated Drive Electronics) - A technology pioneered by Compaq
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and Conner that embedded a controller onto the hard disk PCB while
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maintaining compatibility with the register level commands sent by the
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computer's INT 13 routines. IDE drives are configured and appear to the
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computer like standard ST506 drives.
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ATA (AT Attachment) - Implementation of the IDE design with a 16 bit AT
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style controller on board the drive.
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XTA (XT Attachment) - Rarely used implementation of IDE with an
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integrated 8 bit XT controller.
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ATA-2 - Enhancement to the AT Attachment standard to provide for
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considerable performance improvement and more sophisticated drive
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identification.
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EIDE (Enhanced IDE) and FAST-ATA - Various implementations of the ATA-2
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standard as marketed by Western Digital (EIDE) and Seagate/Quantum
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(FAST-ATA).
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DATA ENCODING SCHEMES
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MFM (Modified Frequency Modulation) - Common technique used to encode
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the magnetic fluxes recorded on a drive into data. Still used on floppy
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drives and most original XT and AT systems. Notice that most drive
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types supported by the motherboard BIOS have 17 sectors per track. This
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is the standard density for MFM encoding.
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RLL (Run Length Limited) - Encoding method that allows 50% more
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information to be recorded on a track than MFM. Accomplished by
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recording larger representations for every byte, but able to pack them
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more tightly onto the surface, because of fewer actual flux changes.
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Often called 2,7 RLL because the recording scheme involves patterns with
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no more than 7 successive zeros and no less than two.
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ARLL (Advanced Run Length Limited) - More complex yet powerful
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derivatives of the RLL scheme. Include 1,7 and 3,9 encoding. Most
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every new drive made today uses some form of RLL or ARLL encoding.
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