Design Overview of Drives, Controllers and Interfaces

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Design Overview of Drives, Controllers and Interfaces


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copyright (c) 1992 -1998
Nicholas Majors & ActionFront Data Recovery Labs Inc.
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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.)

A hard drive subsystem is comprised of the following components:

The Hard Disk, with one or more boards (PCB) attached.
A Controller Mechanism, either on the hard disk PCB or on the bus adapter within the PC.
Bus Adapter for interfacing the controller to the host PC.
Cables and Connectors to link it all together.

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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 to the disk assembly through varied 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:

Disk Platter(s), separated by spacers and held together by a clamp.
Spindle shaft onto which platters are mounted.
Spindle motor for rotating the platters.
Electromagnetic read/write heads (one per surface).
Access arms or armatures from which the heads are suspended.
Actuator for moving the arms (with heads attached).
Preamplifier circuitry to maximize read/write signals.
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 this represents a major design change 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.


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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 smallest unit that a CPU can work with is a byte (8 bits). 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 problems 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 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.

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