Magnetic Disk Recording

8-1

Chapter 8

Magnetic Disk Recording

Learning Objectives

After completing this chapter, you'll be able to do the following: 1. Describe how flexible (floppy) disks are constructed; how data is organized on them; how they are handled, stored, and shipped; and how they are erased. 2. Describe how fixed (hard) disks are constructed; how data is organized on them; how they are handled, stored, and shipped; and how they are erased. 3. Describe each of the following methods for recording (encoding) digital data onto magnetic disks: frequency-modulation encoding, modified frequency-modulation encoding, and run length-limited encoding.

4. Describe the characteristics of floppy disk drive transports and hard disk drive transports and describe the preventive maintenance requirements of each type. 5. Describe the following parts of the electronics component of a magnetic disk drive: control electronics, write/read electronics, and interface electronics. 6. Describe the five most common types of disk drive interface electronics. 7. Define the following magnetic disk recording specifications: seek time, latency period, access time, interleave factor, transfer rate, and recording density.

Introduction

Magnetic disk recording was invented by International Business Machines (IBM) in 1956. It was developed to allow mainframe computers to store large amounts of computer programs and data. This new technology eventually led to what's now known as the computer revolution. This chapter introduces you to the following aspects of magnetic disk recording: • Disk recording mediums • Disk recording methods • Disk drive transports • Disk drive electronics • Disk recording specifications 8-2

Magnetic Disk Recording Mediums

There are two types of disk recording mediums: flexible diskettes and fixed (hard) disks. The following paragraphs describe (1) how flexible and fixed disks are made; (2) how data is organized on them; (3) how to handle, store, and ship them; (4) and how to erase them.

Flexible Magnetic Recording Diskettes

Flexible diskettes, or floppy disks as they're more commonly called, are inexpensive, flexible, and portable magnetic storage mediums. They have the following characteristics. Floppy Disk Construction Floppy disks are made of round plastic disks coated with magnetic oxide particles. The disks are enclosed in a plastic jacket which protects the magnetic recording surface from damage. Floppy disks come in three sizes: 8 inch, 5 1/4 inch, and 3 1/2 inch. Figure 8-1 shows each size. All disk sizes can either be single-sided or double-sided. Single-sided disks store data on only one side of the disk; double-sided disks store data on both sides.

Figure 8-1.—Floppy disk construction. When floppy disks are manufactured, the magnetic oxide coating is applied to both sides. Each disk is then checked for errors. Disks certified as single-sided, are checked on only one side; disks certified as double-sided are checked on both sides. Floppy disks are also classified by how much data they can store. This is called a disk's density. There are three levels of floppy disk density: single-density, double-density, and high-density. Some of the more common types of floppy disks and their storage capacity are listed below: 8-3

Type Of Floppy Disk Storage Capacity

5-1/4" double-sided, double-density 360,000 bytes 5-1/4" double-sided, high-density 1,200,000 bytes 3-1/2" double-sided, double-density 720,000 bytes 3-1/2" double-sided, high-density 1,400,000 bytes Floppy Disk Data Organization Data is stored on a floppy disk in circular tracks. Figure 8-2 shows a circular track on a floppy disk. The total number of tracks on a floppy disk is permanently set by (1) the number of steps the disk drive's magnetic head stepper motor can make, and (2) whether the disk drive has a magnetic head for one or both surfaces of the floppy disk. These two things will also determine the type of floppy disk that's needed. Each type of disk is rated with a number that represents how many tracks per inch (TPI) it can hold. Some common track capacities are 40, 48, 80, and 96 TPI.

Figure 8-2.—Tracks and sectors of a magnetic disk. Each track of a floppy disk is broken up into arcs called sectors. A disk is sectored just as you'd slice an apple pie. Figure 8-2 shows the sectors of a floppy disk. How many slices are made? That depends on who made the disk and in what host computer the disk is used. There are two methods for sectoring a floppy disk: 1. Hard Sectoring : This method sectors the disk physically. The disk itself will have marks or sensor holes on it that the floppy disk drive hardware can detect. This method is seldom used today.

2. Soft sectoring: This method sectors the disk logically. The computer software determines the sector size and placement, and then slices the disk into sectors by writing codes on the disk. This 8-4 is called formatting or initializing a floppy disk. During formatting, if the computer software locates a bad spot on the disk, it locks it out to prevent the bad spot from being used. Soft sectoring is by far the most popular method of sectoring a floppy disk. Once a floppy disk is formatted, the computer uses the disk's side number, a track number, and a sector number (together) as an address. It's this address that locates where on the disk the computer will store the data.

Floppy Disk Handling, Storage, and Shipping Floppy disks hold a lot of data. Even disks with only a 360,000-byte storage capacity can hold 180 pages of data! That's why it's important to handle, store, and ship floppy disks properly. One hundred and eighty pages of data is a lot of data to retype just because of carelessness. Before we get into disk handling and storage procedures, let's first learn about head-to-disk contact. Do you remember reading in chapter 2 that the quality of magnetic tape recording is seriously degraded when dust, dirt, or other contaminates get between the magnetic head and the tape? Well, the same is true for magnetic disk recording. In fact, head-to-disk contact is extremely important with floppy disks. This is because floppy disk drives, unlike magnetic tape drives, spin at very high speeds — 300 to 600 revolutions-per-minute (RPM). If anything gets between the head and the recording surface, you can lose data, or even worse, you can damage the magnetic head and the disk's recording surface. Figure 8-3 shows the size relationship between a disk drive's magnetic head, the disk recording surface, and some common contaminants.

Figure 8-3.—Size relationship of distance between head and disk to contaminants. You must handle, store, and ship floppy disks with great care if you want them to stay in good condition. Here's some specific precautions you should take: • DO always store 8" and 5-1/4" floppy disks in their envelopes when not in use. Dirt, dust, etc., can get on the recording surface through the magnetic head read/write access hole if you leave it exposed for any length of time. 8-5 • DO always write on a floppy disk label first, and then place the label on the disk. NEVER write directly on a floppy disk. If you absolutely must write on a disk, use a felt-tip marker.

• DO hold floppy disks by their outside corners only. DO NOT bend them. And NEVER, NEVER paper clip them to anything, or anything to them. • DO always store floppy disks in an upright position. Laying them on their side can cause them to warp. • DO always keep floppy disks away from food, liquids, and cigarette smoke. All of these can easily damage floppy disks. • DO always ship floppy disks in appropriate shipping containers. When shipping only a few disks, use the specially designed cardboard shipping envelopes. If you must ship a large number of disks, make sure the box you use is sturdy enough to protect the disks from damage. A good rule of thumb is to use a shipping box that allows you to place 2 inches of packing material around the disks.

• DO NOT touch any exposed recording surfaces. Something as simple as a fingerprint can destroy the data on a floppy disk. • DO NOT expose a floppy disk to magnetic fields. Telephones, magnetic copy holders, printers, and other electronic equipment generate magnetic fields that can destroy the data on a floppy disk. • DO NOT expose floppy disks to extreme heat or cold. Floppy disks will last longer if they're stored in an environment that stays around 70-80 degrees Fahrenheit and 30-60 percent relative humidity. Floppy Disk Erasing There are two ways to erase a floppy disk: (1) degauss it and then reformat it, or (2) just reformat it.

The process for degaussing floppy disks is the same as for degaussing magnetic tape. Refer back to chapter 2 for the details on this. If the floppy disks were used to store classified, or unclassified but sensitive information, they can't be de-classified by erasing them. This is because, with the right equipment and software, the data that was on the disk can be reconstructed. Floppy disks are cheap and easy to replace. If you can't re-use the floppy disks to store other classified data, just destroy them, using the procedures in OPNAVINST 5510.1, DON Information and Personnel Security Program Regulation.

Q1. Floppy disks are manufactured in what three sizes? Q2. What type of floppy disk is made to store data on both sides of the disk? Q3. What are the three levels of floppy disk density? Q4. What is the storage capacity of a 5-1/4" double-sided, high-density floppy disk? Q5. The floppy disks you are using have a rating of 96 TPI. What does this mean? Q6. The process of formatting a floppy disk is called what type of sectoring? 8-6 Q7. What three components determine the address that locates where on a floppy disk the computer will store the data?

Q8. Why should you always store floppy disks in their envelopes? Q9. Why should you never place floppy disks near telephones or other electronic equipments that generate magnetic fields? Q10. What are the two ways to erase floppy disks?

Fixed Magnetic Recording Disks

Fixed disks, or hard disks as they're more commonly called, are expensive, rigid, semi-portable, magnetic storage mediums. They have the following characteristics: Hard Disk Construction Most hard disks are made of aluminum platters coated on both sides with either iron oxide or thin-film metal magnetic coatings. The first type, iron oxide, is the most common (you can recognize this coating by its rust color). This is the same oxide coating that's used on magnetic tape. The second type of coating, thin-film metal, is the newer and better of the two. This coating is a microscopic layer of metal that's bonded to the aluminum platter. You can recognize it by its shiny silver color. Thin-film metal-coated hard disks are becoming more and more popular because they allow more data to be stored in less space.

Hard disks can hold a lot of data, the smallest disk being 10,000,000 bytes, and the largest being about 2,500,000,000 bytes (and they're working on larger ones). Hard disk platters come in many sizes, ranging from 14" to 2". The most common sizes are 3-1/2", 5-1/4" and 14". The first two sizes are usually used with smaller personal computers. The 14" size is usually used with the larger mini and mainframe computers. Most hard disk drives use more than one hard disk platter to store data. These are called disk packs.

Some hard disk drives use removable hard disk platters. These can use just one platter, or they can use disk packs containing many platters. Most of the multi-platter removable hard disk drives in use today use 14" hard disk platters. Figure 8-4 shows a hard disk-pack. 8-7 Figure 8-4.—Magnetic hard disk pack. Hard Disk Data Organization Data is stored on a hard disk the same way it's stored on a floppy disk, in circular tracks. The total number of tracks on a hard disk is set, just like floppy disk, by (1) the number of steps the disk drive's magnetic head stepper motor can make, and (2) whether the disk drive has a magnetic head for one or both surfaces of the hard disk platter.

A computer places data on a hard disk using one of two methods, either (1) the cylinder method, or (2) the sector method. The manufacturer of the hard disk drive decides which method to use. THE CYLINDER METHOD.—This method uses a cylinder as the basic reference for placing data on a hard disk. Look at figure 8-5 view A. This is a picture of a disk pack containing six hard disk platters. Notice that this particular disk drive uses only 10 out of the 12 available recording surfaces. If you imagine that you're looking down through the disk pack from above, the tracks with the same number on each of the 10 recording surfaces will line up. Put together, these tracks make up a cylinder. Each of these 10 tracks with the same number, one on each recording surface, can be read from and written to by one of the disk drive's 10 read/write magnetic heads that are positioned by the five access arms.

8-8 Figure 8-5.—Cylinder and sector method of organizing data on a hard disk pack. So, to locate a place to store data using the cylinder method, a computer must specify the cylinder number, the recording surface number, and the record number. Figure 8-5 view A shows record number 1 stored on cylinder 25 of recording surface number 6. Special data is stored on each track to tell the computer where the start of a track is. THE SECTOR METHOD.—Although we talked about this method earlier under the heading "Floppy Disk Data Organization," we need to repeat it here as it also applies to hard disks.

The sector method of organizing data on a hard disk is actually a variation of the cylinder method. As you already know, the sector method slices up a hard disk into pie-shaped slices (just like floppy disks). The total number of slices is set by the hard disk drive manufacturer. Figure 8-5 view B shows an example of the sector method. Unlike a floppy disk drive, which locates a place on the disk using the surface number, track number, and sector number, a hard disk drive locates a place on the disk by using the surface number, cylinder number, and sector number. This is true even if the hard disk has only one platter. That's because both surfaces of that one platter still form a cylinder.

Hard Disk Handling, Storage, and Shipping Hard disks hold a lot more data than floppy disks; even the lowest capacity hard disk can hold 5,000 pages of data! That's why it's important to handle, store, and ship hard disks properly. If you think 180 pages of data is a lot to retype, just think of retyping 5,000 pages! 8-9 Hard disk drives spin at a very high speed of about 3600 RPM. It is extremely important that nothing gets between the head and the recording surface. If it does, you can lose data and you can damage both the magnetic head and the disk's recording surface.

Most hard disk failures involve a head-crash. It's the worst thing that can happen to a hard disk. A head-crash is the result of the disk drive's magnetic heads crashing into the recording surface and grinding into the hard disk platter. Figure 8-6 shows a good hard disk platter and a bad hard disk platter that was the victim of a head-crash. Figure 8-6.—Example of a hard disk crash. You must handle, store, and ship hard disks with extreme care if you want them to stay in good condition. Here are some specific precautions you should take: • DO always store removable hard disks in their storage cases when not in use. Dirt, dust, etc., can get on the recording surface through the magnetic head read/write access hole if you leave it exposed for any length of time.

• DO always handle hard disks with extreme care. DO NOT drop them. Even a small drop of 2" can warp a hard disk platter enough to cause a head crash. • DO always keep removable hard disks away from food, liquids, and cigarette smoke. All of these can easily cause damage. • DO always ship hard disks in their proper shipping containers. If you don't have the original shipping container, make sure the shipping box is sturdy and big enough to allow 2" of packing material around the disk. Save the original packing material for the hard disk just in case you need to ship it somewhere.

• DO NOT touch any exposed recording surfaces. Something as simple as a fingerprint can cause a head crash and destroy a hard disk platter. • DO NOT expose hard disks to extreme heat or cold. Hard disks will last longer if they're stored in an environment that stays around 70-80 degrees Fahrenheit and 30-60 percent relative humidity. Hard Disk Erasing There are two ways to erase a hard disk: (1) degauss it and then reformat it, or (2) just reformat it. As you might guess, the first method can only be used for removable hard disk platters. The second method 8-10 (reformatting) is the most common. If you must degauss a removable hard disk, the process is the same as degaussing magnetic tape. Refer back to chapter 2 for the details on this.

If the hard disks were used to store classified information or unclassified but sensitive information, you can't de-classify the hard disks by erasing them. This is because with the right equipment and software, the data that was on the disk can be reconstructed. If you can't re-use the hard disks to store other classified data, you must sanitize or destroy them, using the procedures in OPNAVINST 5510.1. Q11. What are the three most common sizes of hard disk platters? Q12. Computers use what two methods to place data on a hard disk?

Q13. Which method for placing data on hard disks divides a hard disk into pie shaped slices? Q14. When computers use the cylinder method to store data on a hard disk pack, what three items make up the address that tells the computer where on a specific disk to store the data? Q15. What is the most common type of hard disk failure? Q16. Hard disks should be stored in an environment that stays within what relative humidity and temperature range? Q17. What is the most common method for erasing a hard disk?

Recording Digital Data On Magnetic Disks

Digital data is stored on a magnetic disk using magnetic pulses. These pulses are generated by passing a frequency modulated (FM) current through the disk drive's magnetic head. This FM current generates a magnetic field that magnetizes the particles of the disk's recording surface directly under the magnetic head. The pulse can be one of two polarities, positive or negative. Digital data isn't just recorded onto a magnetic disk as-is. Instead, it's encoded onto the disk. Three of the most popular encoding methods are (1) frequency modulation (FM), (2) modified frequency modulation (MFM), and (3) run length limited (RLL). The following paragraphs describe each of these encoding methods.

Frequency Modulation (Fm) Encoding

The FM method of encoding digital data onto a disk uses two pulse periods to represent each bit of data (a pulse period is the time span of one pulse). The first pulse period always contains a clock pulse. The second pulse-period may, or may not, contain a data pulse. If the digital data is a "1," a data pulse will be present in the second pulse-period. But, if the digital data is a "0," then there's no pulse present. Figure 8-7 shows this. The clock pulse, which is always present, tells the disk drive's interface that the next pulse is a data pulse. It is used to compensate for changes in the disk's rotation speed.

8-11 Figure 8-7.—Frequency-modulation (FM) encoding. The FM method of encoding is old, and isn't used much anymore. You'll only see it in some of the older single-sided, single-density floppy disk drives, and in some of the older military hard disk drives.

Modified Frequency-Modulation (Mfm) Encoding

The MFM method of encoding digital data onto a disk is more popular because it is more efficient and more reliable than straight FM encoding. MFM encoding still uses two pulse periods, but uses a lot fewer pulses to store the digital data onto the disk. It does this in two ways: 1. It does away with the clock pulse that the FM method uses. 2. It stores a digital "1" by generating a no-pulse and a pulse in the two pulse periods. It stores a digital "0" as either a pulse and a no-pulse if the last bit was a "0," or as two no-pulses if the last bit was a "1." Figure 8-8 shows this.

Figure 8-8.—Modified frequency-modulation (MFM) encoding. 8-12

Run Length-Limited (Rll) Encoding

The RLL method of encoding digital data onto a disk is actually a refinement of the MFM encoding method. As its name implies, RLL limits the run length (distance) between pulses (also called flux reversals) on a hard disk. The basic theory of RLL encoding is that you can store more data in less space if you reduce the number of flux reversals (or pulses) that you must record. There are several versions of the RLL encoding method, the most popular version being the 2,7 RLL. This means that no fewer than 2 no-pulses and no more than 7 no-pulses can occur between pulses.

Magnetic Disk Drive Transports

Magnetic disk drive transports, like magnetic tape drive transports, move the magnetic disks across the magnetic heads and protect the disks from damage. The following paragraphs will (1) introduce you to the characteristics of both floppy and hard disk drive transports, and (2) describe their preventive maintenance requirements.

Floppy Disk Drive Transports

Floppy disk drive transports contain the electromechanical parts that (1) rotate the floppy disk, (2) write data to it, and (3) read data from it. Figure 8-9 shows a typical floppy disk drive transport. Four of the drive transport's more important parts are the Figure 8-9.—Typical floppy disk drive transport. 8-13 1. drive motor/spindle assembly, 2. head arm assembly, 3. actuator arm assembly, and 4. drive electronics circuit board. Drive Motor/Spindle Assembly The spindle in this assembly holds the floppy disk in place while it spins. The drive motor spins the spindle at 300 to 600 RPM, depending on the type of floppy disk drive. The following is a list of the types of floppy disk drives and the spinning speeds of their spindles.

Floppy Disk Drive Type Spinning Speed

5-1/4" 360-KB storage 300 RPM 5-1/4" 1.2-MB storage 360 RPM 3-1/2" 720-KB storage 600 RPM 3-1/2" 1.44-MB storage 600 RPM The spindle of a 5-1/4" disk drive is activated and released by a small arm that's mounted on the front of the disk drive. You must turn the small arm to lock and release the floppy disk. The spindle of a 3-1/2" disk drive is activated when the floppy disk is inserted into the disk drive. It's released by a push-button that's located on the front of the disk drive. When you push this button, the floppy disk is released and pops out of the disk drive.

Head Arm Assembly This part of a floppy disk drive transport holds the magnetic read/write heads. There are four heads on a head arm assembly, two write heads and two read heads - one of each for each recording surface. The head arm assembly is attached to the actuator arm assembly. Actuator Arm Assembly The actuator arm assembly positions the magnetic heads over the recording surface of the floppy disk. It does this by using a special type of dc motor called a stepper motor. This motor, which can be moved in very small steps, allows the read/write heads to be moved from track to track as needed to write data onto and read data off of the floppy disk.

Drive Electronics Circuit Board This circuit board contains the circuitry which (1) controls the electromechanical parts of the disk drive transport, (2) writes data to and reads data from the floppy disk, and (3) interfaces the floppy disk drive to the host computer. 8-14

Hard Disk Drive Transports

Hard disk drive transports contain the electromechanical parts that (1) rotate the hard disk platter, (2) write data to it, and (3) read data from it. There are two types of hard disk drive transports, fixed disk and cartridge disk. Fixed disk drive transports use non-removable hard disk platters. Cartridge-disk drive transports use removable hard disk platters that are built into protective cartridges. These two transports serve very different purposes, but they each contain the same basic parts. Figure 8-10 shows a typical hard disk drive transport. Four of the more important parts of a hard disk drive transport are the 1. drive motor/spindle assembly, 2. head arm assembly, 3. actuator arm assembly, and 4. drive electronics circuit board.

Figure 8-10.—Typical hard disk drive transport. Drive Motor/Spindle Assembly This assembly holds and spins the hard disk pack. The spindle assembly holds the hard disk pack in place and the drive motor spins the spindle at 3600 RPM. On cartridge disk drives, the spindle is electronically disengaged to release the disk pack so it can be removed. 8-15 Head Arm Assembly This part of the hard disk drive transport holds the magnetic read/write heads. There is a separate head arm assembly for each of the hard disk platters in the disk pack. Each assembly has four magnetic heads, two write heads and two read heads-one pair of heads for each surface of the hard disk platter. The head arm assembly is attached to the actuator arm assembly.

Actuator Arm Assembly This part of the hard disk drive transport positions the magnetic heads so they can write data to and read data from the correct track of the hard disk. It does this by using either a stepper motor or a voice coil servo. A stepper motor is a special type of dc motor which can be moved in very small steps to accurately position the magnetic heads. A voice coil servo by itself cannot move the magnetic heads from track to track. Instead, it must use special signals called servo signals to make sure it's positioning the heads where they should be. The servo signals are pre-recorded signals which are stored on either the same hard disk platter as the data or on a separate hard disk platter.

The voice coil servo method of moving the magnetic heads to the correct track of the hard disk is also called embedded servo control. This type of control is becoming very popular because voice coil actuator assemblies can position the magnetic heads much quicker and more accurately than dc stepper motors. Drive Electronics Circuit Board This circuit board contains the circuitry which (1) controls the electromechanical parts of the hard disk drive transport, (2) writes data to and reads data from the hard disk, and (3) interfaces the hard disk drive to the host computer.

Q18. What are the three most popular methods for encoding digital data onto magnetic disks? Q19. Older, single-sided, single-density floppy disk drives would probably use what method for encoding digital data onto the floppy disk? Q20. What method for encoding digital data enables you to store more data in less space by limiting the distance between pulses on a hard disk? Q21. What are the four most important parts of a floppy disk drive transport? Q22. The drive motor of a 3-1/2", 1.44-MB floppy disk drive spins the disk at what RPM?

Q23. The head arm assembly of a floppy disk drive transport has how many read heads and how many write heads? Q24. What part of a floppy disk drive transport uses a dc stepper motor to position the magnetic heads over the recording surface of a floppy disk? Q25. What part of a floppy disk drive transport contains the circuitry which controls the electromechanical parts of the transport? Q26. Hard disk drive transports contain the electromechanical parts that perform what three functions? Q27. In the actuator arm assembly of a hard disk drive transport, what device can position the magnetic heads to the correct track of a hard disk more accurately than a dc stepper motor?

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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: 1. Pour some of the cleaning solution onto the cleaning disk through the access hole in the protective jacket.

2. Insert the cleaning disk into the disk drive. 3. Exercise the disk drive for at least 30 seconds. 4. 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: 1. Open the scaled envelope that contains a cleaning disk soaked in cleaning solution. 2. Insert the cleaning disk into the protective jacket provided. 3. Insert the cleaning disk (with protective jacket) into the disk drive. 4. Exercise the disk drive for 30 seconds.

5. Remove the disk from the disk drive. 6. 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 8-17 • 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. Q28. Why do floppy disk drives require more preventive maintenance than hard disk drives? Q29. A kit for cleaning floppy disk drives contains what two items? Q30. Approximately how often should you clean a floppy disk drive that gets heavy use? Q31. Cartridge hard disk drives with 14" disk packs may require what additional types of preventive maintenance?

Magnetic Disk Drive Electronics

Magnetic disk drive electronics consist of three main parts: 1. Control electronics to control the electromechanical parts of a disk drive. 2. Write/read electronics to write data to and read data from a disk drive. 3. 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. 8-18

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: 1. 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. 2. 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: 1. Naval Tactical Data System (NTDS) interface. 2. ST-506/412 interface. 3. Enhanced small device interface (ESDI). 4. Small computer systems interface (SCSI). 5. 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: 1. NTDS FAST: A parallel interface that can transfer data at a rate of 250,000 32-bit words per second.

2. NTDS SLOW: A parallel interface that can transfer data at a rate of 41,667 32-bit words per second. 3. 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. 8-19 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.

8-20 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: 1. 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. 2. 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.

3. 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, 8-21 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. Q32. The control electronics component of a floppy or hard disk drive performs what three main functions? Q33. The write/read electronics of a disk drive performs what three functions?

Q34. The interface electronics of a disk drive performs what three functions? Q35. What type of interface electronics is used in many naval electronic warfare systems? Q36. What type of disk drive interface has most of the electronics on a controller card mounted in the host computer? Q37. The SCSI is a high level disk drive interface. What does this mean? Q38. What type of hard disk drive interface has all of the controller card electronics included in the disk drive itself? SUMMARY Now that you've finished chapter 8, you should be able to describe the (1) characteristics of floppy and hard disks, (2) methods for encoding digital data onto magnetic disks, (3) disk drive transports and their preventive maintenance requirements, (4) parts of a disk drive's electronics component, and (5) common types of disk drive interface electronics. The following is a summary of important points in this chapter: FLOPPY DISKS are single-sided or double-sided plastic disks coated with oxide particles. The disks can be single-density, double-density, or high density.

Data is stored on floppy disks in CIRCULAR TRACKS. The tracks are divided into arcs called SECTORS. HANDLE, SHIP, and STORE floppy disks carefully. Contaminates between the heads and the disk surface can cause serious damage. 8-22 FIXED (HARD) DISKS are aluminum platters coated on both sides with iron oxide or thin-film metal. Most hard disk drives use disk packs which are several disk platters stacked together. Some hard disk drives use removable disk platters. Either the CYLINDER OR SECTOR METHOD is used to place data on hard disks.

HANDLE, STORE, AND SHIP HARD DISKS with extreme care. Contaminates on the heads or the disk surface can cause head-crash. ERASE HARD AND FLOPPY DISKS by reformatting or degaussing. Three popular METHODS FOR ENCODING DIGITAL DATA ONTO DISKS are frequency modulation, modified frequency modulation, and run length limited. FLOPPY DISK DRIVE TRANSPORTS contain the parts that (1) spin the floppy disk, (2) write data to the disk, and (3) read data from it. The DRIVE MOTOR/SPINDLE ASSEMBLY of a floppy disk drive transport holds and spins the floppy disk. The transport's HEAD ARM ASSEMBLY holds the read/write heads and its ACTUATOR ARM ASSEMBLY positions the heads over the disk's recording surface.

HARD DISK DRIVE TRANSPORTS contain the parts that (1) rotate the hard disk platter, (2) write data to the disk, and (3) read data from the disk. The DRIVE MOTOR/SPINDLE ASSEMBLY of a hard disk drive transport holds the disk pack in place while the drive motor spins the spindle at 3600 RPM. The transport's HEAD ARM ASSEMBLY holds the read/write heads and its ACTUATOR ARM ASSEMBLY positions the heads over the correct track of the hard disk. FLOPPY DISK DRIVES REQUIRE PREVENTIVE MAINTENANCE at regular intervals because they are not sealed units and the disks use an oxide coating that wears off and sticks to the heads and other parts.

Hard Disk Drives Require Very Little Preventive Maintenance.

Cartridge disk drives will have a special cleaning kit for doing the preventive maintenance. Magnetic DISK DRIVE ELECTRONICS consist of (1) control electronics to control the electromechanical parts of a disk drive, (2) write/read electronics to write data to and read data from a disk drive, and (3) interface electronics to interface the disk drive to the host computer. MAGNETIC DISK RECORDING SPECIFICATIONS are set by the manufacturer; all you can do is measure them. Six of the most common specifications are seek time, latency period, access time, interleave factor, transfer rate, and recording density.

Answers To Questions Q1. Through Q38.

A1. 8 inch, 5 1/4 inches, 3 1/2 inches. A2. Double-sided. 8-23 A3. a. Single-density, b. double-density, and c. high-density. A4. 1,200,000 bytes or 1.2 megabytes. A5. The disks can hold 96 tracks per inch. A6. Soft sectoring. A7. a. disk side number, b. track number, and c. sector number. A8. Dust and other contaminates can get on the recording surface through the read/write hole. A9. Magnetic fields can destroy the data on a disk. A10. a. Degauss the disk and then reformat it. b. Reformat the disk. A11. 3-1/2 inches, 5-1/4 inches, and 14 inches.

A12. (1) Cylinder method and (2) sector method. A13. Sector method. A14. a. Cylinder number. b. Recording surface number. c. Record number. A15. Head-crash. A16. a. 30-60 percent relative humidity. b. 70-80 degrees Fahrenheit. A17. Reformat the disk. 8-24 A18. Frequency modulation (FM). a. Modified frequency modulation (MFM). b. Run length limited (RLL). A19. Frequency-modulation encoding. A20. Run length-limited (RLL) encoding. A21. a. Drive motor/spindle assembly. b. Head arm assembly. c. Actuator arm assembly. d. Drive electronics circuit board.

A22. 600 Rpm.

A23. Two read heads and two write heads. A24. Actuator arm assembly. A25. Drive electronics circuit board. A26. a. Rotates the hard disk platters. b. Writes data to and reads data from the disk platters. A27. Voice coil servo. A28. They are not sealed units, and they use flimsy plastic disks with an oxide coating that wears off and sticks to the heads. A29. a. A cloth or fiber cleaning disk. b. A bottle of cleaning solution. A30. Once a month. 8-25 A31. a. Cleaning with a special cleaning disk. b. Cleaning air filters.

c. Cleaning spindles, rails, and slides. d. Cleaning and buffing read/write heads. A32. a. Spins the disk at the correct speed. b. Moves the heads across the recording surface. c. Tells the write/read heads when to write data and when to read it. A33. a. Formats and writes incoming data from the interface electronics onto the disk. b. Reads data off the disk, formats it, and sends it to the interface electronics for output. c. Performs the initial disk formatting. A34. a. Receives control signals from the host computer and sends them to the control electronics or write/read electronics.

b. Receives data from the write/read electronics and outputs it to the host computer. c. Converts incoming and outgoing data from parallel to serial, and vice versa, if needed. A35. NTDS interface. A36. ST-506/412 interface. A37. The host computer asks for data by specifying a logical sector number. The SCSI translates the sector number into the actual disk location. A38. Integrated drive electronics (IDE).