Quantcast Magnetic disk drive preventive maintenance disk drives require less maintenance than magnetic tape drives. The following paragraphs describe the preventive maintenance requirements for both floppy and hard disk drives.">

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MAGNETIC DISK DRIVE PREVENTIVE MAINTENANCE

Like magnetic tape recorders, if you want a magnetic disk drive to continue storing and retrieving data without errors, you must periodically perform preventive maintenance. Fortunately, disk drives require less maintenance than magnetic tape drives. The following paragraphs describe the preventive maintenance requirements for both floppy and hard disk drives.

FLOPPY DISK DRIVE PREVENTIVE MAINTENANCE

Of all of the magnetic disk drives in use today, floppy disk drives require the most maintenance. This is because they are not sealed units like most hard drives and because they use flimsy plastic disks that are coated with the same type of oxide as magnetic tape.

It's this oxide that causes most of the problems you'll have with floppy disk drives. Just as with magnetic tape, the oxide coating wears off of the plastic backing and sticks (mainly) to the magnetic heads. This contamination causes dropout errors which have much graver consequences than with magnetic tape. It can cause a program to crash, or even worse - it can destroy your valuable data.

To prevent this, you must periodically clean the floppy disk drive's magnetic heads. There are many kits available to do the job. A kit has a cleaning disk and a bottle of cleaning solution.

A cleaning disk looks just like a regular disk, except that instead of an oxide-coated disk, it has a cloth or fiber cleaning disk inside the protective jacket. The instructions that come with the cleaning kit will lead you through the cleaning process. Here is an example of the cleaning procedures for a floppy disk drive's magnetic heads:

  • Pour some of the cleaning solution onto the cleaning disk through the access hole in the protective jacket.
  • Insert the cleaning disk into the disk drive.
  • Exercise the disk drive for at least 30 seconds.
  • Remove the disk from the disk drive.

There are also some cleaning kits that use disposable cleaning disks. These kits will instruct you to clean the heads as follows:

  • Open the scaled envelope that contains a cleaning disk soaked in cleaning solution.
  • Insert the cleaning disk into the protective jacket provided.
  • Insert the cleaning disk (with protective jacket) into the disk drive.
  • Exercise the disk drive for 30 seconds.
  • Remove the disk from the disk drive.
  • Remove the cleaning disk from the protective jacket and throw it away.

Now comes the question "How often must I clean the heads?"

That's hard to say. It depends on the type of disk drive, the quality of the floppy disks you use, and how much you use the disk drive. On the average, you should clean a floppy disk drive once a month if it gets heavy use, once every 6 months if it gets moderate use, or once a year if it gets very little use.

HARD DISK DRIVE PREVENTIVE MAINTENANCE

Hard disk drives need little or no preventive maintenance. If it's a fixed hard disk drive, it doesn't need preventive maintenance because it's a sealed unit that you must not open for any reason.

If it's a cartridge disk drive, the manufacturer will have a special cleaning disk with instructions for doing the preventive maintenance. The Navy uses some larger cartridge disk drives, such as the 14" disk pack drives, that require some other preventive maintenance. This could include the following:

  • Cleaning air filters.
  • Cleaning spindles, rails, and slides.
  • Cleaning and buffing read and write heads.

The technical manual for the disk drive will guide you through this type of preventive maintenance.

Q.28 Why do floppy disk drives require more preventive maintenance than hard disk drives? answer.gif (214 bytes)
Q.29 A kit for cleaning floppy disk drives contains what two items? answer.gif (214 bytes)
Q.30 Approximately how often should you clean a floppy disk drive that gets heavy use? answer.gif (214 bytes)
Q.31 Cartridge hard disk drives with 14" disk packs may require what additional types of preventive maintenance?answer.gif (214 bytes)

MAGNETIC DISK DRIVE ELECTRONICS

Magnetic disk drive electronics consist of three main parts:

  • Control electronics to control the electromechanical parts of a disk drive.
  • Write/read electronics to write data to and read data rom a disk drive.
  • Interface electronics to interface the disk drive to the host computer.

Some disk drives require a separate controller card. When this is true, some of the drive electronics are part of the disk drive itself, and some are part of the host computer's controller card.

As different as disk drives can be (floppy, fixed, cartridge, etc.), their electronics is surprisingly similar. That's why the following paragraphs will only very basically describe these three main parts.

CONTROL ELECTRONICS

The main functions of a disk drive's control electronics are to:

  • Spin the disk at the proper speed.
  • Move the magnetic heads across the recording surface.
  • Tell write/read heads when to write data and when to read data.

WRITE/READ ELECTRONICS

The write/read electronics consists of the write part and the read part. The write part takes incoming data from the interface electronics, formats it as needed, and writes it onto the disk. The read part reads the data off of the disk, formats it as needed, and sends it to the interface electronics for output to the host computer.

The write/read electronics also performs the initial disk formatting function.

INTERFACE ELECTRONICS

Interface electronics do two things:

  • Receive control signals from the host computer that tells them to spin the disk, move the magnetic heads, write/read data, format a disk, etc.
  • Convert the incoming and outgoing data as needed.

A disk drive is a serial device.

This means the data stored on the disk is stored in a serial pulse-train format.

But the data coming from the disk drive and going to the host computer needs to be in a parallel data format. The interface electronics converts the data from parallel to serial, and vice versa, as needed.

There are many types of disk drive interfaces in use today. The five most common ones are the:

  • Naval Tactical Data System (NTDS) interface.
  • ST-506/412 interface.
  • Enhanced small device interface (ESDI).
  • Small computer systems interface (SCSI).
  • Integrated drive electronics (IDE).

The following paragraphs describe each of these interfaces.

  • Naval Tactical Data System (NTDS) Interface
  • The NTDS interface is used by many naval electronic warfare systems.
  • There are three versions of this interface:

NTDS FAST: A parallel interface that can transfer data at a rate of 250,000 32-bit words per second.

NTDS SLOW: A parallel interface that can transfer data at a rate of 41,667 32-bit words per second.

NTDS SERIAL: A serial interface that can transfer data at a rate of 10 million bits (Mbits) per second.

ST-506/412 Interface

The ST-506/412 interface was developed by Seagate Technology, Inc.

It's often used in the hard disk drives installed in older IBM-compatible desktop computers that have a maximum capacity of 125 MB. It's also the interface used to control most floppy disk drives in use today.

This is one of the interfaces where most of the electronics is actually on a controller card mounted in the host computer. With this interface, the controller card does most of the work (moving the magnetic head, spinning the disk, etc.). The controller card also cleans any data coming from the disk drive by stripping off the formatting and control signals that were used to store the data onto the hard disk.

A hard disk drive is connected to the controller card in the host computer via two ribbon cables (a 34-pin control cable and a 20-pin data cable).

Floppy drives use only the 34-pin control cable to transfer both data and control signals.

When this interface was originally developed in 1981, it's 5-Mbits per second transfer rate was considered too fast.

They actually slowed it down by using a 6:1 interleave factor (we'll define this later) so it could operate with the computers being built at that time.

With today's transfer rates pushing the envelope at 24 Mbits per second, you can see that it's now one of the slowest interfaces.

Enhanced Small Device Interface (ESDI)

The ESDI is an optimized version of the ST-506/412 interface. The main difference is that with ESDI, most of the disk drive's interface electronics is located in the disk drive itself, rather than on a controller card in the host computer. The result is a much faster transfer rate and more hard disk capacity. ESDIs have a transfer rate of up to 24 MB per second. And, they can handle disk drives with a maximum capacity of 1.2 GB (gigabytes).

The ESDI uses the same interface cables as the ST-506/412 interface, but that's where the similarity ends. With ESDI drives, only the clean data is sent to the controller card in the host computer. All formatting and control signals are stripped off at the hard disk drive.

Small Computer Systems Interface (SCSI)

The SCSI (pronounced skuzzy) is very different from both the ST-506/412 and the ESDI. The SCSI is an 8-bit, parallel, high-level interface. High-level means that instead of a host computer asking for data by specifying a track, cylinder, and sector number, all it asks for is a logical sector number. The SCSI then translates the logical sector number into the actual disk location.

The SCSI also has other improvements over the previous disk drive interfaces. For example, it can:

  • Transfer data at rates of up to 4 MB per second.
  • Handle hard disk drives of almost any size.
  • Disconnect itself from a host computer's bus while it processes requests. This frees-up the host computer to do other things.
  • Daisy-chain up to eight units off of one controller.

The SCSI interface uses one 50-pin ribbon cable to connect the hard disk drive(s) to the controller card mounted in the host computer. Some computer manufacturers include the SCSI electronics in their motherboards and do away with a separate controller card altogether. This interface got its big break when Apple Computer Corporation used the SCSI as its hard disk drive interface in its MacIntosh computers.

Integrated Drive Electronics (IDE)

The IDE is the newest interface available. It was developed as a result of trying, to find a cheaper way to build computer systems. It includes all of the controller card electronics in the hard disk drive itself, thus, the hard drive does all the work.

The hard disk drive connects to the host computer's bus with a

40-pin ribbon cable. The ribbon cable connects directly to either a 40-pin connector on the host computer's motherboard or a 40 pin connector on a small interface card that plugs into the host computer's motherboard. This interface offers a transfer rate of up to 1 MB and can handle hard drives with a maximum capacity of 300 MB.

MAGNETIC DISK RECORDING SPECIFICATIONS

Think back to the chapter 6 on "Magnetic Tape Recording Specifications."

Do you remember how to measure and adjust them if needed? Well, magnetic disk recording specifications are a little different.

They're set by the manufacturer and you can't change them. All you can do is measure them. The following paragraphs describe six of the most common specifications.

SEEK TIME

The seek time is the amount of time it takes for the magnetic head to position itself over a specific track of a magnetic disk. It's usually stated in milliseconds.

LATENCY PERIOD

The latency period is the amount of time it takes for a specific sector of a specific track to position itself under the magnetic head. It too, is usually stated in ms.

ACCESS TIME

The access time is the sum of the seek time and the latency period. It's the total amount of time in ms that it takes a disk drive to retrieve a sector of data from the magnetic disk.

Access time is stated in one of the following three ways:

  • Track-to-track seek time: This is the amount of time it takes a disk drive to access data from a track next to the track it's presently over.
  • Average seek time: This is the amount of time it takes a disk drive to access data that's located one-third of the way across the magnetic disk.
  • Maximum seek time: This is the amount of time it takes a disk drive to access data from the last track of a magnetic disk when it's presently on the first track of the magnetic disk.

INTERLEAVE FACTOR

The interleave factor applies only to hard disk drives. They spin at 3600 RPM, a very fast speed compared to floppy disk drives which only spin at 300-600 RPM. Interleave indicates how many physical sectors are between sequentially numbered logical sectors on a hard disk. It's used when the magnetic heads and the control circuitry can't process the data fast enough to sequentially number the sectors on a hard disk platter. With interleave, the magnetic head is told to skip X number of sectors to get to the next one.

For example, a hard disk with 17 sectors per track and no interleave is numbered 1, 2, 3, 4.... 17. The same hard disk with an interleave factor of 3 is numbered 1, 7, 13, 2, 8, 14, 3, 9, 15, 4, 10, 16, 5, 11, 17, 6, 12, and then back to 1. If you count every third sector, they're sequential. The most efficient hard disk drives have no interleave.

TRANSFER RATE

The transfer rate states how fast a disk drive and a disk drive controller (working together) can transfer data to the host computer. An example of a transfer rate specification is "2 Mbits/sec," or two million bits per second. The higher the number, the faster the data transfer rate.

RECORDING DENSITY

The recording density states how close together bits can be stored on the recording surface of a magnetic disk.

It determines two things: (1) How close together the tracks on the disk will be, and (2) how close together the bits on each track will be. An example of a recording density specification is "12 Mbits/in2," or 12 million bits per square inch.

Q.32 The control electronics component of a floppy or hard disk drive performs what three main functions?answer.gif (214 bytes)
Q.33 The write/read electronics of a disk drive performs what three functions? answer.gif (214 bytes)
Q.34 The interface electronics of a disk drive performs what three functions? answer.gif (214 bytes)
Q.35 What type of interface electronics is used in many naval electronic warfare systems? answer.gif (214 bytes)
Q.36 What type of disk drive interface has most of the electronics on a controller card mounted in the host computer? answer.gif (214 bytes)
Q.37 The SCSI is a high level disk drive interface. What does this mean? answer.gif (214 bytes)
Q.38 What type of hard disk drive interface has all of the controller card electronics included in the disk drive itself? answer.gif (214 bytes)




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