HARD DISK CONTROLLERS AND INTERFACE

WHAT IS HARD DISK DRIVE IN A NUTSHELL?

The hard disk drive or hard drive is the main location which all data is stored.  Most hard disk drives consist of spinning platters of aluminum, glass or ceramic that, are coated with a magnetic media.

Below is a picture of what the inside of the hard disk drive looks like. The Hard disk drive has four main components. The head actuator controls the head arm which reads the information off of the disk platter. The chassis encases and holds all the hard disk drive components.

The following is a listing of the major components of the disk platter.

Platter - The actual fixed disk within the hard disk drive. Generally there can be several platters within the hard disk drive.
Tracks - Large sections that completely circle the platter.
Sector - Section on the track.
Cylinder - Tracks on each platter.

 

SIZE INFORMATION

When purchasing a hard disk drive Megabytes and Gigabytes may be confusing terms. The following chart gives you an example of each of these terms and how they compare to other sizes.

Bit

 Value of 0 or 1

Nibble

 4 bits

Byte

 8 bits

KB(Kilobit)

 1,024 bytes

MB(Megabyte) 

 1,024 Kilobytes or 1,048,576 Bytes

GB(Gigabyte)

 1,024 Megabytes or 1,073,741, 824 Bytes

TB(Terabyte)

 One Trillion bytes or 1,099,511,627,776

PB (Petabyte)

1,000,000,000,000,000 bytes

EB(Exabyte)

One quintillion bytes or about 1,152,921,504,606,846,976 bytes in decimal.

ZB (Zetabyte)

1,000,000,000,000,000,000,000 bytes

YB (Yottabyte)

1,000,000,000,000,000,000,000,000 bytes

 

 

 

Hard disk drive interfaces/ Hard Disk Drive Controller:

 

Hard disk Drives require Interface circuitry to interface the Drive to the computers bus. Three standard Hard Disk Drive interfaces have been used with DOS Computers over the years. The SCSI Interface, a universal interface, is also used to connect Hard Disk Drives to DOS Computers, but its use has mainly been limited to high capacity File Servers and powerful Work Stations.

 

In earlier hardware, the Hard Disk Drive Interface was provided by an interface card plugged into a bus slot on the system board. Modern DOS computers have the Hard Disk Drive Interface built into the system board, along with the Floppy Drive Interface and the Parallel and Serial I/O Ports.

 

 

The ST506 Hard Disk Drive Interface

 

The first Hard Disk Drive Interface used with DOS Computers was called the ST506. An alternative name was the ST412 Interface. These two names came from the type numbers of the first Hard Disk Drives used with PC Computers. The ST506 interface used two cables between the Drive and Interface card. One cable had 34 conductors, the other 20 conductors, and both used Edge Connectors on the Drive end. ST506 Interface cards were available for both PC/XT (8 Bit bus) and PC/AT (16 Bit bus) Computers. The 8 Bit bus Computers did not have support for Hard Disk Drives built into the BIOS ROM and so a BIOS Extension ROM was required to add Hard Disk Drive support. This was located on the Hard Disk Drive Interface card and was addressed in the memory space, starting at address C8000hex. With ST506 interface Hard Disk Drives, the Disks organization (cylinders/sectors/sides) is important to the Interface card. The cards are either built for one particular Drive organization, or they have jumpers or DIP switches to select the Drive parameters. Some of the last ST506 interface cards had a nonvolatile RAM that stored the Drives parameters and usually had a list of Drives to choose from when entering the low level format routine.

 

Most of the BIOS extension ROMs on 8 bit ST506 Hard Disk Drive interface cards usually provided a routine for performing a low level format on the Drive. This routine was accessed via DEBUG using Debugs "GO" command. The start address of this routine depends on the brand of Interface card. Most common addresses were G = C800:5, C800:CCC and C800:6. (See Segment and Offset addressing for details of what C800:5 means)

 

NOTE: Real IBM Hard Disk Drive Interface cards did not have a low level format routine built in, you had to use IBMs (or someone else) disk based low level format routine.

 

MFM and RLL Recording Methods - The ST 506 Interface can use two alternative data recording methods

MFM provides 17 Sectors per Track.

RLL provides 25 to 27 Sectors per Track.

 

RLL provided a 50% increase in storage capacity at the expense of some error checking. Hard Disk Drives for use with RLL Interface cards had to be of better quality with closer tolerance on rotational speed and long term stability. Using non RLL approved Drives on RLL Interface cards earned the RLL technology a bad name, as the Drives worked for a while but sooner or later they started to produce read errors and eventually may have failed altogether.

The ST506 Interface used two ribbon cables between the Interface and the Drive, with edge connectors on the Drive end. These cables were a 34 conductor cable and a 20 conductor cable.

When two Hard Disk Drives were required with the ST506 interface, the 34 pin cable was Daisy Chained to both Drives, and each Drive had its own 20 Pin cable. If you look at an ST506 Interface card you will see one 34 pin header connector and two 20 pin header connectors. The Drives must be set to be First or Second Drive and jumpers are provided on the Drives, for Drive selection. In a two Hard Disk Drive set up, the Drives could be selected in two ways:

By using a straight 34 Pin cable and selecting one drive as the first Drive and the other as the second Drive. (Select Drives using the jumpers on the Drive) Some drives label the jumpers as 0 and 1, others as 1 and 2.

By using a 34 Pin cable with a Twist in it and selecting both Drives as either first or second Drive. The twist transposes the Drive select lines. Only a few Interface card manufacturers supplied the 34 wire cables with a twist in them, most provided plain cables and expected the installer to set one Drive as the first Drive and the other Drive as the second drive. This technique was left over from the way IBM designed the Floppy Disk Drive cable. The ST506 cable with a twist, is quite different to the Floppy Drive cable

 

Note:- Both Floppy Disk Drives, and ST506 and ESDI Hard Disk Drives use a 34 wire Interface cable. The 34 wire cables used on Floppy Disk Drives and on ST506 Hard Disk Drives are not the same. The Floppy Disk Drive cables used by IBM, and also by most DOS Computer manufacturers, have a twist in the cable between pin 10 and pin 16 that transposes the Drive select lines. A few manufacturers supplied Hard Disk Drive Interface cables with a twist but the twist was on the other side of the cable, between pins 25 and 29. This was not common.

 

ISA Bus(16 bit) ST506 Hard Disk Drive Interface cards

 

The AT type Computer was built with support for Hard Disk Drives built in and did not require a BIOS Extension ROM on the Hard Disk Drive Interface card. Later generations of DOS computers have followed this procedure. The drive organization details are stored in a battery backed-up RAM situated on the System Board. This RAM is called the CMOS and holds all of the computers setup details, how much RAM, what type of Floppy Disk Drives are installed, what type of Hard Disk Drives are installed, what type of Video card is in use and many other scraps of information.

16 BIT Bus interface cards are identified by having an ISA bus connector rather than the old eight bit (PC/XT) bus connector. Remember the ISA bus has an extra 36 pin edge connector on the end of the 62 pin eight bit bus connector. The 36 pin edge connector provides the extra Data lines (D8 to D15), the extra Address lines (A20 to A23) and extra IRQ and DMA lines, required by the ISA bus.

An 8 bit bus interface card could be used in an ISA Bus computer but we had to tell the computers CMOS setup that the computer has no Hard Disk Drive. This is because the BIOS extension ROM on the 8 BIT bus card provided Hard Disk Drive support instead of the computers own built in BIOS.

 

The ESDI Interface

 

This Hard Disk Drive Interface used the same drive cables as the ST506 but achieved more sectors per track by using advanced signal processing techniques. ESDI Drives had 35 or more sectors per track. ESDI has been used for high capacity Drives (200 to 1000 Meg). ESDI Drives were usually supplied paired with a Interface card and with the low level format already in place.

 

Drive terminations

 

Drive terminations were required on ST506 and ESDI Hard Disk Drives and when two Hard Disk Drives were fitted, the terminators usually had to be removed from the second Drive. As no standards are followed in this, you had to consult the manufacturers’ specification sheets for the particular Drives involved. The ST506 and ESDI interfaces had a lot of the signal processing electronics on the Interface card but with the ever increasing scale of circuit integration, it became possible to put this on the Hard Disk Drive itself, and to use a very simple Hard Disk Drive Interface.

 

The IDE interface

 

IDE stands for In-built Drive Electronics. An alternative name for this interface is the AT Attachment (ATA) Interface. The controller circuitry is built into the Hard Disk Drive itself rather than on a Interface card and the Interface card provides simple buffers and address decoding circuits to interface the Drive to the PC Ccomputers bus. The IDE Interface uses a 40 pin ribbon cable and header connectors on the Hard Disk Drive. This cable is daisy chained when two Drives are fitted, and the Drives are selected by jumpers on the Drives as Master and Slave devices.

You must consult the Drive manufactures documentation when installing additional Hard Disk Drives in a system with IDE Interface Drives, not all Drives follow the same standards, and some older Drives will not work with some other brand/model of Drive. Not all Drives use the same jumper configuration also. Some Drives were built without Master/Slave jumpers and cannot be used in a two Hard Disk Drive setup. Most modern Drives have diagrams of the jumper configurations on the top of the Drive. Today an Enhanced IDE (EIDE) standard has been introduced that overcomes the 528 MByte limit imposed by the Int 13 routine, provides much faster data transfer rates and allows for the connection of CDROM, Tape Backup and high capacity Removable Media Drives.

 

I/O Resources

 

. All three types of Interface cards (ST506, ESDI and IDE) use the same I/O address and Interrupt Request line. This has meant only one Primary Interface card can be installed in a PC Computer. The EIDE Interface specifications have made use of Secondary addresses that were assigned to a Secondary Hard Disk Drive Interface but never implemented with the ST506 or ESDI Interfaces. The EIDE standard has also added two more EIDE I/O channels and these are called the Tertiary channel and the Quaternary channel. See the IRQ and I/O notes for details of the resources assigned to these devices.

 

The SCSI (pronounced SCUZZI) Interface

 

SCSI stands for Small Computer System Interface and it can be used to connect Hard Disk Drives to a PC Computer. The SCSI interface is the standard Hard Disk Drive Interface for the APPLE MAC but it is not often used with DOS Computers. The most common use of SCSI Hard Disk Drives on DOS computers is in Network File Servers. Techniques called RAID, Mirroring and Striping, are used to provide very large secure disk storage and a SCSI interface is used to implement these technologies. The SCSI interface can be used to interface to other storage or I/O devices. These include: Tape Backup, CDROM and, high capacity Removable Media Drives, and some times, Optical Scanners and Printers.

 

The eight-bit SCSI Interface uses two types of Interface cable:

 

·        External devices are connected via a special Centronics like 50 pin connector; the computer end of the external cable usually has a DB25P connector that goes into a DB25S connector on the interface card. Two important points to remember when connecting older SCSI Interfaced devices:

 

1.     Check the pin configuration on the connectors because no one standard exists.

2.     The DOS computers parallel printer interface also uses a DB25S connector and if a parallel printer is plugged into the SCSI socket the SCSI I/O port may be destroyed.

 

 

 

·        Internal SCSI devices are connected via a 50 pin ribbon cable and header connectors.

 

Line terminators are important on SCSI cables both ends of a SCSI chain must be terminated. The SCSI Interface will be covered in more detail in PC Servicing two.

 

Enhanced IDE and Fast-ATA

 

To overcome the limits imposed on Hard Disk Drive size by the various factors outlined in this document, Hard Disk Drive and controller manufacturers have adopted a series of new standards. Although you see it advertised by different names, sometimes generally referred to as simply Enhanced IDE, there are actually two standards, Enhanced IDE and Fast AT Attachment. Fortunately for us, the two work effectively the same.

 

Enhanced IDE and ATA are changing all the rules.

 

Maximum Hard Disk Drive size is now 8.4 GByte, the maximum number of EIDE Interface devices is now four, and the maximum transfer rate is in excess of 32 MByte/sec. The ATA Packet Interface (ATAPI) allows for CDROM Drives, and other specifications provide for Tape Backup Drives and high capacity Removable Media Drives. Enhanced IDE has changed four main elements of the old IDE specification:

 

·        BIOS redesign to support mush higher capacity EIDE Hard Disk Drives

·        Increased data transfer rates

·        The implementation of multiple EIDE channels (up to four)

·        Support of non-disk EIDE peripherals

 

What’s Ultra SCSI?

 

Also called Fast-20, Ultra SCSI supports up to 20MBps burst transfers across 8-bit paths and up to 40MBps burst transfers across Wide 16-bit paths. Because it’s pin- and cable-compatible with SCSI-2 Fast and Fast/Wide, there’s no need to change the physical connection, although you are restricted to shorter cables for external attachments. And it’s backward-compatible with SCSI-1 and SCSI-2 devices. Ultra SCSI can protect your investment in existing SCSI devices while giving you access to high-performance applications. One of the common complaints about SCSI was that it was difficult to install and configure. But with the introduction of Ultra SCSI, comes SCAM—SCSI Configuration Automatically. A subset of Plug and Play, SCAM will make the configuration of SCSI adapters and devices automatic—provided, of course, they support the SCAM specification.

 

Ultra SCSI and the IBM IntelliStation

 

Implementing Ultra SCSI technology is even simpler on select IBM IntelliStation M Pro models that have an integrated dual-channel, Ultra-Wide SCSI controller chip. Dual Ultra SCSI channels allow for twice the throughput of a single channel. And, no additional card is required since the controller chip is integrated. RAID can also be implemented with an additional card. A RAID-port connector is also integrated into select IntelliStation M Pro models when populated with an Adaptec ARO-1130 RAIDport card. RAID 0 or 1 can be implemented (mirroring or striping).

 

What’s Ultra 2 SCSI?

 

Ultra 2 SCSI is again a doubling of the bus data rates of Ultra SCSI. This is accomplished through the improved noise immunity and transmission speeds offered by a new low voltage implementation of differential signaling technology, sometimes referred to as LVDS. While Ultra 2 drives are implemented allowing operation on Ultra SCSI data buses, the new performance levels only exist in buses that are Ultra 2 exclusive. Consequently, many new Ultra 2 controllers will be designed to support separate Ultra 2 and Ultra buses to protect existing investments in Ultra technology. The robust nature of LVDS technology also enables the cable length limitation of Ultra 2 SCSI to be extended to 12 meters from the existing limit of 1.5 meters with Ultra SCSI.

 

Serial Storage Architecture (SSA)

 

Today’s huge databases and data intensive applications demand incredible amounts of storage. Transferring massive blocks of information requires technology that is robust, reliable and scalable. Serial Storage Architecture (SSA) is a powerful interface for connecting storage devices, storage subsystems, servers and workstations.

SSA provides data protection for critical applications by helping to ensure that a single cable failure will not prevent access to data. All the components in a typical SSA subsystem are connected by bidirectional cabling. Data sent from the adapter can travel in either direction around the loop to its destination. SSA detects interruptions in the loop and automatically reconfigures the system to help maintain connection while a link is restored.

In addition, SSA is designed with expansion in mind. You can initially configure with only a few disk drives and add more subsystems and disk drives into the loop. SSA makes the disk drives self-configuring, helping to avoid the addressing limitations and complexity of SCSI drive installation.

 

 

 

 

Here are some highlights about SSA:

 

§ High-capacity storage—Adapter supports for up to 192 hot-swappable hard disk drives per system. You can also designate drives for use by an array should a failure occur. Drives are available in 4.51 and 9.1GB capacities.

§ Flexible, simple interconnection between servers and subsystems—You can have up to 25 meters between disk drives, connected by thin, low-cost copper cables. Subsystems can be placed in secure, convenient locations, far from the server.

§ Flexible RAID, non-RAID configurations—Up to 32 separate RAID arrays per adapter—RAID 0, 1, 5 or any mix. RAID 0 arrays or non-RAID disk drives can be mirrored across servers to provide cost-effective protection for critical applications.

§ Outstanding performance—Provides up to 80MBps of maximum throughput. This translates to sustained data rates as high as 60MBps in non-RAID mode and 35MBps in RAID mode.

Other factors to consider

 

There are several other factors to consider when choosing a hard drive. Here’s a brief look at some of the most important.

§ Rotational speed—Rotational speed is one of many factors indicating performance. And, in general, SCSI drives offer higher rotational speeds, though several manufacturers offer SCSI and versions of the same basic drive.

§ S.M.A.R.T. capability—hard disk drives that use Self-Monitoring, Analysis and Reporting Technology (S.M.A.R.T.) can help to protect your data by predicting some types of drive failures. (See the Self-Monitoring, Analysis and Reporting Technology Information Brief for more information.)

§ Predictive Failure Analysis (PFA)—IBM’s robust implementation predating S.M.A.R.T., but for SCSI drives. (See the SMART Reaction Information Brief for additional information.) PFA and S.M.A.R.T. is fully compatible.

 

 

 

 

 

 

 

 

Bibliography:

 

 

*   http://www.us.pc.iba.com/

*   http://members.iweb.net.au/~pstorr/pcbook/book4/hdinter.htm

*   http://www.computerhope.com/help/hdd.htm

*   http://www.tafe.sa.edu.au/institutes/torrens-valley/programs/eit/pcsupport/hardintr.htm

*   http://new.linuxnow.com/docs/content/Hardware-HOWTO-html/Hardware-HOWTO-7.html

*   Computer Organization and Design, second edition by John L. Hennessy & David A. Patterson

 

 

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