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