The ability to interchange programs and data between various compatible computers is a fundamental requirement of almost every computer system. It is just this kind of file exchange compatibility that helped rocket IBM PC/XTs into everyday use and spur the personal computer industry into the early 1980s. A standardized operating system, file structure, and recording media also breathed life into the fledgling software industry. With the floppy disk, software developers could finally distribute programs and data to a mass-market of compatible computer users. The mechanism that allowed this quantum leap in compatibility is the floppy disk drive (Fig. 20-1).

A floppy disk drive (or FDD) is one of the least expensive and most reliable forms of mass-storage ever used in computer systems. Virtually every one of the millions of personal computers sold each year incorporates at least one floppy drive. Most notebook and laptop computers also offer a single floppy drive. Not only are FDDs useful for transferring files and data between various systems, but the advantage of removable media - the floppy disk itself - make floppy drives an almost intuitive backup system for data files. Although floppy drives have evolved through a number of iterations; from 8" to 5.25" to 3.5", their basic components and operating principles have changed very little.

Magnetic storage media has been attractive to computer designs for many years - long before the personal computer had established itself in homes and offices. This popularity is primarily due to the fact that magnetic media is non-volatile. Unlike system RAM, no electrical energy is needed to maintain the information once it is stored on magnetic media. While electrical energy is used to read and write magnetic data, magnetic fields do not change on their own, so data remains intact until "other forces" act upon it (such as another floppy drive). It is this smooth, straightforward translation from electricity to magnetism and back again that has made magnetic storage such a natural choice. To understand how a floppy drive works and why it fails, you should have an understanding of magnetic storage. This part of the chapter shows you the basic storage concepts used for floppy drives.

For the purposes of this book, media is the physical material which actually holds recorded information. In a floppy disk, the media is a small mylar disk coated on both sides with a precisely formulated magnetic material often referred to as the oxide layer. Every disk manufacturer uses their own particular formula for magnetic coatings, but most coatings are based on a naturally magnetic element (such as iron, nickel, or cobalt) that has been alloyed with non-magnetic materials or rare earth. This magnetic material is then compounded with plastic, bonding chemicals, and lubricant to form the actual disk media.

The fascinating aspect of these magnetic layers is that each and every particle media acts as a microscopic magnet. Each magnetic particle can be aligned in one orientation or another under the influence of an external magnetic field. If you have ever magnetized a screwdriver's steel shaft by running a permanent magnet along its length, you have already seen this magnetizing process in action. For a floppy disk, microscopic points along the disk's surfaces are magnetized in one alignment or another by the precise forces applied by read/write (R/W) heads. The shifting of alignment polarities would indicate a logic 1, while no change in polarity would indicate a logic 0 (you will see more about data recording and organization later in this chapter).

In analog recording (such as audio tapes), the magnetic field generated by read/write heads varies in direct proportion to the signal being recorded. Such linear variations in field strength cause varying amounts of magnetic particles to align as the media moves. On the other hand, digital recordings such as floppy disks save binary 1s and 0s by applying an overwhelming amount of field strength. Very strong magnetic fields saturate the media - that is, so much field strength is applied that any further increase in field strength will NOT cause a better alignment of magnetic particles at that point on the media. The advantage to operating in saturation is that 1s and 0s are remarkably resistant to the degrading effects of noise that can sometimes appear in analog magnetic recordings.

Although the orientation of magnetic particles on a disk's media can be reversed by using an external magnetic field, particles tend to resist the reversal of polarity. Coercitivity is the strength with which magnetic particles resist change. Higher coercitivity material has a greater resistance to change, so a stronger external field will be needed to cause changes. High coercitivity is generally considered to be desirable (up to a point) because signals stand out much better against background noise and signals will resist natural degradation because of age, temperature, and random magnetic influences. As you might expect, a highly coercive media requires a more powerful field to record new information.

Another advantage of increased coercitivity is greater information density for media. The greater strength of each media particle allows more bits to be packed into less area. The move from 5.25" to 3.5" floppy disks was possible due largely to a superior (more coercitive) magnetic layer. This coercitivity principle also holds true for hard drives. In order to pack more information onto ever-smaller platters, the media must be more coercive. Coercitivity is a common magnetic measurement with units in oersteds (pronounced "or-steds"). The coercitivity of a typical floppy disk can range anywhere from 300 to 750 oersteds. By comparison, hard drive and magneto-optical (MO) drive media usually offer coercitivities up to 6000 oersteds or higher.

The main premise of magnetic storage is that it is static (once recorded, information is retained without any electrical energy). Such stored information is presumed to last forever, but in actual practice, magnetic information begins to degrade as soon as it is recorded. A good magnetic media will reliably remember (or retain) the alignment of its particles over a long period of time. The ability of a media to retain its magnetic information is known as retentivity. Even the finest, best-formulated floppy disks degrade eventually (although it could take many years before an actual data error materializes).

Ultimately, the ideal answer to media degradation is to refresh (or write over) the data and sector ID information. Data is re-written normally each time s file is saved, but sector IDs are only written once when the disk is formatted. If a sector ID should fail, you will see the dreaded "Sector Not Found" disk error and any data stored in the sector can not be accessed. This failure mode also occurs in hard drives. There is little that can be done to ensure the integrity of floppy disks other than maintaining one or more backups on freshly formatted disks. However, some commercial software is available for restoring disk data (especially hard drives).

The first step in understanding digital recording is to see how binary data is stored on a disk. Binary 1s and 0s are NOT represented by discrete polarities of magnetic field orientations as you may have thought. Instead, binary digits are represented by the presence or absence of flux transitions as illustrated in Fig. 20-2. By detecting the change from one polarity to another instead of simply detecting a discrete polarity itself, maximum sensitivity can be achieved with very simple circuitry.

In its simplest form, a logic 1 is indicated by the presence of a flux reversal within a fixed time frame, while a logic 0 is indicated by the absence of a flux reversal. Most floppy drive systems insert artificial flux reversals between consecutive 0s to prevent reversals from occurring at great intervals. You can see some example magnetic states recorded on the media of Fig. 20-2. Notice that the direction of reversal does not matter at all - it is the reversal EVENT that defines a 1 or 0. For example, the first 0 uses left-to-right orientation, while the second 0 uses a right-to-left orientation, but BOTH can represent 0s.

The second trace in Fig. 20-2 represents an amplified output signal from a typical read/write head. Notice that the analog signal peaks wherever there is a flux transition - long slopes indicate a 0, and short slopes indicate a 1. When such peaks are encountered, peak detection circuits in the floppy drive cause marking pulses in the ultimate data signal. Each bit is usually encoded in about 4 m S.

Often, the most confusing aspect to flux transitions is the artificial reversals. Why reverse the polarities for consecutive 0s? Artificial reversals are added to guarantee synchronization in the floppy disk circuitry. Remember that data read or written to a floppy disk is serial, and without any clock signal, such serial data is asynchronous of the drive's circuitry. Regular flux reversals (even if added artificially) create reference pulses that help to synchronize the drive and data without use of clocks or other timing signals. This approach is loosely referred to as the modified frequency modulation (MFM) recording technique. Early hard drives (i.e. ST506/412 drives) also employed MFM recording.

The ability of floppy disks to store information depends upon being able to write new magnetic field polarities on top of old or existing orientations. A drive must also be able to sense the existing polarities on a disk during read operations. The mechanism responsible for translating electrical signals into magnetic signals (and vice versa) is the read/write head (R/W head). In principle, a head is little more than a coil of very fine wire wrapped around a soft, highly permeable core material as illustrated in Fig. 20-3.

When the head is energized with current flow from a driver IC, a path of magnetic flux is established in the head core. The direction (or orientation) of flux depends on the direction of energizing current. To reverse a head's magnetic orientation, the direction of energizing current must be reversed. The small head size and low current levels needed to energize a head allow very high-frequency flux reversals. As magnetic flux is generated in a head, the resulting, tightly-focused magnetic field aligns the floppy disk's particles at that point. In general practice, the current signal magnetizes an almost microscopic area on the media. R/W heads actually contact the media while a disk is inserted into a drive.

During a read operation, the heads are left unenergized while the disk spins. Just as varying current produces magnetism in a head, the reverse is also true - varying magnetic influences cause currents to be developed in the head(s). As the spinning media moves across a R/W head, a current is produced in the head coil. The direction of induced current depends on the polarity of each flux orientation. Induced current is proportional to the flux density (how closely each flux transition is placed) and the velocity of the media across each head. In other words, signal strength depends on the rate of change of flux versus time.

Another important aspect of drive troubleshooting is to understand how data is arranged on the disk. You can not place data just anywhere - the drive would have no idea where to look for the data later on, or even if the data is valid. In order for a disk to be of use, information must be sorted and organized into known, standard locations. Standardized organization ensures that a disk written by one drive will be readable by another drive in a different machine. Table 20-1 compares the major specifications of today's popular drive types.

It is important to note that a floppy disk is a two-dimensional entity possessing both height and width (depth is irrelevant here). This two-dimensional characteristic allows disk information to be recorded in concentric circles which creates a random-access type of media. Random-access means that it is possible to move around the disk almost instantly to obtain a desired piece of information. This is a much faster and more convenient approach than a sequential recording medium such as magnetic tape.

Floppy disk organization is not terribly complicated, but there are several important concepts that you must be familiar with. The disk itself is rotated in one direction (usually clockwise) under read/write heads which are perpendicular (at right angles) to the disk's plane. The path of the disk beneath a head describes a circle. As a head steps in and out along a disk's radius, each step describes a circle with a different circumference - rather like lanes on a roadway. Each of these concentric "lanes" is known as a track. A typical 3.5" disk offers 160 tracks - 80 tracks on each side of the media. Tracks have a finite width which is defined largely by the drive size, head size, and media. When a R/W head jumps from track to track, it must jump precisely the correct distance to position itself in the middle of another track. If positioning is not correct, the head may encounter data signals from two adjacent tracks. Faulty positioning almost invariably results in disk errors. Also notice that the circumference of each track drops as the head moves toward the disk's center. With less space and a constant rate of spin, data is densest on the innermost tracks (79 or 159 depending on the disk side) and least dense on the outermost tracks (0 or 80). A track is also known as a cylinder.

Every cylinder is divided into smaller units called sectors. There are 18 sectors on every track of an 3.5" disk. Sectors serve two purposes. First, a sector stores 512 bytes of data. With 18 sectors per track and 160 tracks per disk, an 8.89cm disk holds 2,880 sectors [18 x 160]. At 512 bytes per sector, a formatted disk can handle about [2,880 x 512] 1,474,560 bytes of data. In actual practice, this amount is often slightly less to allow for boot sector and file allocation information. Sectors are referenced in groups called clusters or allocation units. While hard drives can group 16 or more sectors into a cluster, floppy drives only use 1 or 2 sectors in a cluster.

Second, and perhaps more important, a sector provides housekeeping data that identifies the sector, the track, and error checking results from cyclical redundancy check (CRC) calculations. The location of each sector and housekeeping information is set down during the format process. Once formatted, only the sector data and CRC results are updated when a disk is written. Sector ID and synchronization data is never re-written unless the disk is reformatted. This extra information means that each sector actually holds more than 512 bytes, but you only have access to the 512 data bytes in a sector during normal disk read/write operations. If sector ID data is accidentally overwritten or corrupted, the user-data in the afflicted sector becomes unreadable.

The format process also writes a bit of other important information to the disk. The boot record is the first sector on a disk (sector 0). It contains several key parameters that describe the characteristics of the disk. If the disk is "bootable" the boot sector will also run the files (i.e. IO.SYS and MSDOS.SYS) that load DOS. In addition to the boot record, a file allocation table (FAT) is placed on track 00. The FAT acts as a table of contents for the disk. As files are added and erased, the FAT is updated to reflect the contents of each cluster. As you might imagine, a working FAT is critical to the proper operation of a disk. If the FAT is accidentally overwritten or corrupted, the entire disk can become useless. Without a viable FAT, the computer has no other way to determine what files are available or where they are spread throughout the disk. The very first byte in a FAT is the media descriptor byte which allows the drive to recognize the type of disk that is inserted.

Magnetic media has come a long way in the last decade or so. Today's high-quality magnetic materials, combined with the benefits of precise, high-volume production equipment, produces disks that are exceptionally reliable over normal long-term use in a floppy disk drive. However, floppy disks are removable items. The care they receive in physical handling and the storage environment where they are kept will greatly impact a disk's life span.

The most troubling and insidious problem plaguing floppy disk media is the accidental influence of magnetic fields. Any magnetized item in close proximity to a floppy disk poses a potential threat. Permanent magnets such as refrigerator magnets or magnetic paper clips are prime sources of stray fields. Electromagnetic sources like telephone ringers, monitor or TV degaussing coils, and all types of motors will corrupt data if the media is close enough. The best policy is to keep all floppy disks in a dedicated container placed well away from stray magnetic fields.

Disks and magnetic media are also subject to a wide variety of physical damage. Substrates and media are manufactured to very tight tolerances, so anything at all that alters the precise surface features of a floppy disk can cause problems. The introduction of hair, dirt, or dust through the disk's head access aperture, wild temperature variations, fingerprints on the media, or any substantial impact or flexing of the media can cause temporary loss of contact between media and head. When loss of contact occurs, data is lost and a number of disk errors can occur. Head wear and the accumulation of worn oxides also effects head contact. Once again, storing disks in a dedicated container located well out of harm's way is often the best means of protection.

At the core of a floppy drive (Fig. 20-4) is a frame assembly (15). It is the single, main structure for mounting the drive's mechanisms and electronics. Frames are typically made from die-cast aluminum to provide a strong, rigid foundation for the drive. The front bezel (18) attaches to the frame to provide a clean, cosmetic appearance, and to offer a fixed slot for disk insertion or removal. For 3.5" drives, bezels often include a small colored lens, a disk ejection button hole, and a flap to cover the disk slot when the drive is empty. A spindle motor assembly (17) uses an outer-rotor DC motor fabricated onto a small PC board. The motor's shaft is inserted into that large hole in the frame. A disk's metal drive hub automatically interlocks to the spindle. For 5.25" disks, the center hole is clamped between two halves of a spindle assembly. The halves clamp the disk when the drive lever is locked down. Figure 20-5 shows the spindle motor assembly from the underside of the drive. The disk activity LED (20) illuminates through the bezel's colored lens whenever spindle motor activity is in progress.

Just behind the spindle motor is the drive's control electronics (16). It contains the circuitry needed to operate the drive's motors, R/W heads, and sensors. A standardized interface is used to connect the drive to a floppy drive controller. Figure 20-6 shows you a close-up view of a drive's control board (note the optoisolator just below U1. The read/write head assembly (7), also sometimes called a head carriage assembly, holds a set of two R/W heads. Head 0 is the lower head (underside of the disk), and head 1 is on top. A head stepping motor (12) is added to ensure even and consistent movement between tracks. A threaded rod at the motor end is what actually moves the heads. A mechanical damper (5) helps to smooth the disk's travel into or out of the 8.89 cm drive. Figure 20-7 shows a close-up view of the R/W heads and stepping motor.

When a disk is inserted through the bezel, the disk is restrained by a diskette clamp assembly (2). To eject the disk, you would press the ejector button (19) which pushes a slider mechanism (3). When the ejector button is fully depressed, the disk will disengage from the spindle and pop out of the drive. For 13.34 cm drives, the disk is released whenever the drive door is opened. Your particular drive may contain other miscellaneous components. Finally, the entire upper portion of a drive can be covered by a metal shield (1).

Proper drive operation depends on the intimate cooperation between magnetic media, electromechanical devices, and dedicated electronics. Floppy drive electronics is responsible for two major tasks; controlling the drive's physical operations, and managing the flow of data in or out of the drive. These tasks are not nearly as simple as they sound, but the sleek, low-profile drives in today's computer systems are a far cry from the clunky, full-height drives found in early systems. Older drives needed a large number of ICs spanning several boards that had to be fitted to the chassis. However, the drive in your computer right now is probably implemented with only a few highly-integrated ICs that are neatly surface-mounted on two small, opposing PC boards. This part of the chapter discusses the drive's operating circuits. A complete block diagram for a Teac 3.5" floppy drive is illustrated in Fig. 20-8. The figure is shown with a floppy disk inserted.

Write-protect sensors are used to detect the position of a disk's file-protect tab. For 3.5" disks, the write protect notch must be covered to allow both read and write operations. If the notch is open, the disk can only be read. Optoisolators are commonly used as write protect sensors since an open notch will easily allow light through, while a closed notch will cut off the light path.

Before the drive is allowed to operate at all, a disk must be inserted properly and interlocked with the spindle. A disk-in-place sensor detects the presence or absence of a disk. Like the write protect sensor, disk sensors are often mechanical switches that are activated by disk contact. If drive access is attempted without a disk in place, the sensor causes the drive's logic to induce a DOS "Disk Not Ready" error code. It is not unusual to find an optoisolator acting as a disk-in-place sensor.

The electronics of an 3.5" drive must be able to differentiate whether the disk contains normal (double) density or high-density media. A high-density sensor looks for the hole that is found near the top of all high-density disk bodies. A mechanical switch is typically used to detect the high-density hole, but a separate LED/detector pair may also be used. When the hole is absent (a double-density disk), the switch is activated upon disk insertion. If the hole is present (a high-density disk), the switch is not actuated. All switch conditions are translated into logic signals used by the drive electronics.

Before disk data can be read or written, the system must read the disk's boot sector information and FAT. While programs and data can be broken up and scattered all over a disk, however, the FAT must ALWAYS be located at a known location so that the drive knows where to look for it. The FAT is always located on track 00 -- the first track of disk side 0. A track 00 sensor provides a logic signal when the heads are positioned over track 00. Each time a read or write is ordered, the head assembly is stepped to track 00. Although a drive "remembers" how many steps should be needed to position the heads precisely over track 00, an optoisolator or switch senses the head carriage assembly position. At track 00, the head carriage should interrupt the optoisolator or actuate the switch. If the drive supposedly steps to track 00 and there is no sensor signal to confirm the position (or the signal occurs before the drive has finished stepping), the drive assumes that a head positioning error has occurred. Head step counts and sensor outputs virtually always agree unless the sensor has failed or the drive has been physically damaged.

Spindle speed is a critically important drive parameter. Once the disk has reached its running velocity (300 or 360 RPM), the drive MUST maintain that velocity for the duration of the disk access process. Unfortunately, simply telling the spindle motor to move is no guarantee that the motor is turning - a sensor is required to measure the motor's speed. This is the index sensor. Signals from an index sensor are fed back to the drive electronics which adjusts spindle speed in order to maintain a constant rotation. Most drives use optoisolators as index sensors which detect the motion of small slots cut in a template or the spindle rotor itself. When a disk is spinning, the output from an index sensor is a fast logic pulse sent along to the drive electronics. Keep in mind that some index sensors are magnetic. A magnetic sensor typically operates by detecting the proximity of small slots in a template or the spindle rotor, but the pulse output is essentially identical to that of the optoisolator.

The drive must receive control and data signals from the computer, and deliver status and data signals back to the computer as required. The series of connections between a floppy disk PC board and the floppy disk controller circuit is known as the physical interface. The advantage to using a standard interface is that various drives made by different manufacturers can be "mixed and matched" by computer designers. A floppy drive working in one computer will operate properly in another computer regardless of the manufacturer as long as the same physical interface scheme is being used.

Floppy drives use a physical interface that includes two cables; a power cable and a signal cable. Both cable pinouts are illustrated in Fig. 20-9. The classical power connector is a 4 pin Molex connector, although many low-profile drives used in mobile computers (i.e. laptops or notebooks) may use much smaller connector designs. Floppy drives require two voltage levels; +5.0 Vdc for logic, and +12.0 Vdc for motors. The return (ground) for each supply is also provided at the connector. The signal connector is typically a 34-pin insulation displacement connector (IDC) cable. Notice that all odd-numbered pins are ground lines, while the even-numbered pins carry active signals. Logic signals are all TTL-level signals.

In a system with more than one floppy drive, the particular destination drive must be selected before any read or write is attempted. A drive is selected using the appropriate Drive Select line (Drive Select 0 to 3) on pins 10, 12, 14, and 6 respectively. For notebook or sub-notebook systems where only one floppy drive is used, only Drive Select 0 is used - the remaining select inputs may simply be disconnected. The spindle motor servo circuit is controlled through the Motor ON signal (pin 16). When pin 16 is logic 0, the spindle motor should spinup (approach a stable operating speed). The media must be spinning at the proper rate before reading or writing can take place.

To move the R/W heads, the host computer must specify the number of steps a head carriage assembly must move, and the direction in which steps must occur. A Direction Select signal (pin 18) tells the coil driver circuit whether the heads should be moved inward (toward the spindle) or outward (away from the spindle). The Step signal (pin 20) provides the pulse sequence that actually steps the head motor in the desired direction. The combination of Step and Direction Select controls can position the R/W heads over the disk very precisely. The Side Select control pin (pin 32) determines whether head 0 or head 1 is active for reading or writing - only one side of the disk can be manipulated at a time.

There are two signals needed to write data to a disk. The Write Gate signal (pin 24) is logic 0 when writing is to occur, and logic 1 when writing is inhibited (or reading). After the Write Gate is asserted, data can be written to the disk over the Write Data line (pin 22). When reading, the data that is extracted from the disk is delivered from the Read Data line (pin 30).

Each of the drive's sensor conditions are sent over the physical interface. The Track 00 signal (pin 26) is logic 0 whenever the head carriage assembly is positioned over track 00. The Write Protect line (pin 28) is logic 0 whenever the disk's write protect notch is in place. Writing is inhibited whenever the Write Protect signal is asserted. The Index signal (pin 8) supplies a chain of pulses from the index sensor. Media type is indicated by the Normal/High-Density sensor (pin 2). The status of the disk-in-place sensor is indicated over the Disk Change Ready line (pin 34).

This section of the chapter is concerned with drive problems that cannot be corrected with cleaning or mechanical adjustments. To perform some of the following tests, you should have a known-good diskette that has been properly formatted. The disk may contain files, but be certain that any such files are backed up properly on a hard drive or another floppy disk - if you can't afford to loose the files on a disk, don't use the disk.

As with so many other PC assemblies, the price of floppy drives has dropped tremendously over the last few years. Now that the price of a standard 3.5" drive is roughly equal to 1 hour of labor, most technicians ask whether it is better to simply replace a floppy drive outright rather than attempt a repair. Ultimately, the decision should depend on volume. Clearly, it makes little sense for a anyone to invest valuable time in repairing a single drive. When there are a large number of drives to be repaired, however, an enterprising technician who chooses to deal in floppy drive service can effectively provide rebuilt or refurbished drives to their customers (see Chapter 62 for floppy drive testing and alignment information).

Proper testing is essential for any type of drive repair. Most drive alignment packages such as DriveProbe by Accurite Technologies or AlignIt by Landmark Research measure and display a drive's parameters (Fig. 20-10). When floppy drive trouble occurs, running a diagnostic can help determine whether the drive mechanics or electronics are at fault. Although you can swap a drive symptomatically, thorough testing is an inexpensive means to verify your suspicions before spending money to replace sub-assemblies.

Companion CD: For cleaning and testing your floppy drive, check out AUTOTEST.ZIP, CHKDRV.ZIP, CLEAN4.ZIP, and DFR.ZIP on the Companion CD.

Symptom 20-1. The floppy drive is completely dead. The disk does not even initialize when inserted. Begin troubleshooting by inspecting the diskette itself. When a 3.5" disk is inserted into a drive, a mechanism should pull the disk's metal shroud away and briefly rotate the spindle motor to ensure positive engagement. Make sure that the disk is properly inserted into the floppy drive assembly. If the diskette does not enter and seat just right within the drive, disk access will be impossible. Try several different diskettes to ensure that test diskette is not defective. It may be necessary to partially disassemble the computer to access the drive and allow you to see the overall assembly. Free or adjust any jammed assemblies or linkages to correct disk insertion. If you can not get diskettes to insert properly, change the floppy drive.

If the diskette inserts properly but fails to initialize, carefully inspect the drive's physical interface cabling. Loose connectors or faulty cable wiring can easily disable a floppy drive. Use your multimeter to measure DC voltages at the power connector. Place your meter's ground lead on pin 2 and measure +12 Vdc at pin 1. Ground your meter on pin 3 and measure +5 Vdc at pin 4. If either of both of these voltages is low or missing, troubleshoot your computer power supply.

Before disk activity can begin, the drive must sense a disk in the drive. Locate the disk-in-place sensor and use your multimeter to measure voltage across the sensor. When a disk is out of the drive, you should read a logic 1 voltage across the sensor output. When a disk is in place, you should read a logic 0 voltage across the sensor (this convention may be reversed in some drive designs). If the sensor does not register the presence of a disk, replace the sensor. If the sensor does seem to register the presence of a disk, use your logic probe to check the Disk Change/Ready signal (pin 34) of the physical interface. If the interface signal does not agree with the sensor signal, replace the control circuit IC on the drive PC board. You can also replace the entire drive control PC board, or replace the entire drive outright.

At this point, the trouble is probably in the floppy drive PC board, or the floppy drive controller board. Try replacing the floppy drive PC board assembly. This is not the least expensive avenue in terms of materials, but it is fast and simple. If a new floppy drive PC board corrects the problem, re-assemble the computer and return it to service. You could retain the old floppy drive board for parts. If a new drive PC board does not correct the problem (or is not available), replace the entire drive. You could retain the old floppy drive for parts. If a new floppy drive assembly fails to correct the problem, replace the floppy controller board. You will have to disassemble your computer to expose the motherboard and expansion boards.

Symptom 20-2. The floppy drive rotates a disk, but will not seek to the desired track. This type of symptom generally suggests that the head positioning stepping motor is inhibited or defective, but all other floppy drive functions are working properly. Begin by disassembling your computer and removing the floppy drive. Carefully inspect the head positioning assembly to be certain that there are no broken parts or obstructions that could jam the read/write heads. You may wish to examine the mechanical system with a disk inserted to be certain that the trouble is not a disk alignment problem which may be interfering with head movement. Gently remove any obstructions that you may find. Be careful not to accidentally misalign any linkages or mechanical components in the process of clearing an obstruction.

Remove any diskette from the drive and re-connect the drive's signal and power cables. Apply power to the computer and measure drive voltages with your multimeter. Ground your multimeter on pin 2 of the power connector and measure +12 Vdc at pin 1. Move the meter ground to pin 3 and measure +5 Vdc on pin 4. If either voltage is low or absent, troubleshoot your computer power supply.

Once confident that the drive's mechanics are intact and appropriate power is available, you must determine whether the trouble is in your floppy drive PC board or floppy drive controller IC on the motherboard. Use your logic probe to measure the STEP signal in the physical interface (pin 20). When drive access is requested, you should find a pulse signal as the floppy controller attempts to position the R/W heads. If STEP pulses are missing, the floppy drive controller board is probably defective and should be replaced.

If STEP pulses are present at the interface, check the pulses into the coil driver circuit. An absence of pulses into the coil driver circuit indicates a faulty control circuit IC. If pulses reach the coil driver, measure pulses to the stepping motor. If no pulses leave the coil driver, replace the coil driver IC. When pulses are correct to the stepping motor but no motion is taking place, replace the defective stepping motor. If you do not have the tools or inclination to replace surface-mount ICs, you can replace the drive PC board. You can also replace the entire drive outright.

Symptom 20-3. The floppy drive heads seek properly, but the spindle does not turn. This symptom suggests that the spindle motor is inhibited or defective, but all other functions are working properly. Remove all power from the computer. Disassemble the system enough to remove the floppy drive. Carefully inspect the spindle motor, drive belt (if used), and spindle assembly. Make certain that there are no broken parts or obstructions that could jam the spindle. If there is a belt between the motor and spindle, make sure the belt is reasonably tight - it should not slip. You should also examine the floppy drive with a diskette inserted to be certain that the disk's insertion or alignment is not causing the problem. You can double-check your observations using several different diskettes. Gently remove any obstruction(s) that you may find. Be careful not to cause any accidental damage in the process of clearing an obstruction. Do NOT add any lubricating agents to the assembly, but gently vacuum or wipe away any significant accumulations of dust or dirt.

Remove any diskette from the drive and re-connect the floppy drive's signal and power cables. Restore power to the computer and measure drive voltages with your multimeter. Ground your multimeter on pin 2 and measure +12 Vdc on pin 1. Move the meter ground to pin 3 and measure +5 Vdc on pin 4. If either voltage is low or absent, troubleshoot your computer power supply.

Once you are confident that the floppy drive is mechanically sound and appropriate power is available, you must determine whether the trouble is in the floppy drive PC board or the floppy drive controller board. Use your logic probe to measure the MOTOR ON signal in the physical interface (pin 16). When drive access is requested, the MOTOR ON signal should become true (in most cases an active low). If the MOTOR ON signal is missing, the floppy drive controller board is probably defective and should be replaced.

If the MOTOR ON signal is present at the interface, check the signal driving the servo circuit. A missing MOTOR ON signal at the servo circuit suggests a faulty control circuit IC. If the signal reaches the servo circuit , the servo IC is probably defective. You can replace the servo IC, but your best course is usually to replace the spindle motor/PC board assembly as a unit. If you are unable to replace the spindle motor PC board, you can replace the floppy drive outright.

Symptom 20-4. The floppy drive will not read from/write to the diskette. All other operations appear normal. This type of problem can manifest itself in several ways, but your computer's operating system will usually inform you when a disk read or write error has occurred. Begin by trying a known-good, properly formatted diskette in the drive. A faulty diskette can generate some very perplexing read/write problems. If a known-good diskette does not resolve the problem, try cleaning the read/write heads as described in the previous section. Do NOT run the drive with a head cleaning disk inserted for more than 30 seconds at a time, or you risk damaging the heads with excessive friction.

When a fresh diskette and clean R/W heads do not correct the problem, you must determine whether the trouble exists in the floppy drive assembly or the floppy controller IC. When you can not read data from the floppy drive, use your logic probe to measure the READ DATA signal (pin 30). When the disk is idle, the READ DATA line should read as a constant logic 1 or logic 0. During a read cycle, you should measure a pulse signal as data moves from the drive to the floppy controller board. If no pulse signal appears on the READ DATA line during a read cycle, use your oscilloscope to measure analog signals from the R/W heads. If there are no signals from the R/W heads, replace the head or head carriage assembly. When signals are available from the R/W heads, the control circuit IC is probably defective and should be replaced. If you are unable to replace the IC, you can replace the drive's control PC board. You can also replace the entire drive outright. If a pulse signal does exist during a read cycle, the floppy disk controller board is probably defective and should be replaced.

When you can not write data to the floppy drive, use your logic probe to measure the WRITE GATE and WRITE DATA lines (pins 24 and 22 respectively). During a write cycle, the WRITE GATE should be logic 0 and you should read a pulse signal as data flows from the floppy controller IC to the drive. If the WRITE GATE remains logic 1 or there is no pulse on the WRITE DATA line, replace the defective floppy controller board. When the two WRITE signals appear as expected, check the analog signal to the R/W heads with your oscilloscope. If you do not find analog write signals, replace the defective control circuit IC. If analog signals are present to the heads, try replacing the heads or the entire head carriage assembly. You can also replace the entire drive outright.

Symptom 20-5. The drive is able to write to a write-protected disk. Before concluding that there is a drive problem, remove and examine the disk itself to ensure that it is actually write protected. If the disk is not write protected, write protect it appropriately and try the disk again. If the disk is already protected, use your multimeter to check the drive's write protect sensor. For an unprotected disk, the sensor output should be a logic 1, while a protected disk should generate a logic 0 (some drives may reverse this convention). If there is no change in logic level across the sensor for a protected or unprotected disk, try a new write protect sensor.

If the sensor itself appears to function properly, check the WRITE PROTECT signal at the physical interface (pin 28). A write protected disk should cause a logic 0 on the WRITE PROTECT line. If the signal remains logic 1 regardless of whether the disk is write protected or not, the control circuit IC in the drive is probably defective. If you are unable to replace the IC, change the drive PC board or replace the entire floppy drive outright.

Symptom 20-6. The drive can only recognize either high or double-density media, but not both. This problem usually appears in 3.5" drives during the disk format process when the drive must check the media type. In most cases, the Normal/High-Density sensor is jammed or defective. Remove the disk and use your multimeter to measure across the sensor. You should be able to actuate the sensor by hand (either by pressing a switch or interrupting a light path) and watch the output display change accordingly on your multimeter. If the sensor does not respond, it is probably defective and should be replaced.

If the sensor itself responds as expected, check the Normal/High-Density signal at the physical interface (pin 2). A double-density disk should cause a logic 1 output, while a high-density disk should cause a logic 0 signal. If the signal at the physical interface does not respond to changes in the density sensor, the control circuit IC on the drive PC board is probably defective. If you are unable to replace the control circuit IC, you can replace the drive PC board or the entire floppy drive outright.

Symptom 20-7. Double-density (720KB) 3.5" disks are not working properly when formatted as high-density (1.44MB) disks. This is a common problem when double-density diskettes are pressed into service as high-density disks. In actual practice, double-density disks use a lower grade media than high-density disks - this makes double-density disks unreliable when used in high-density mode. Some good quality diskettes will tolerate this misuse better than other lower quality diskettes. As a general rule, do NOT use double-density diskettes as high-density disks.

Symptom 20-8. DOS reports an error such as "Can Not Read From Drive A:". A diskette is fully inserted in the drive, and the drive LED indicates that access is being attempted. Start by trying a known-good diskette in the drive (a faulty diskette can cause some perplexing R/W problems). If the diskette is working properly, take a few minutes to clean the drive. Oxides and debris on the R/W heads can interfere with head contact. Do NOT run the drive with a head cleaning disk inserted for more than 30 seconds at a time, or you risk damaging the heads with excessive friction.

Next, remove the floppy drive and check the assembly for visible damage or obstructions. Insert a diskette and see that the disk is clamped properly. Clear any obstructions which may be preventing the disk from seating properly. Also inspect the 34-pin signal cable for obvious damage, and see that it is connected properly at both the drive and the drive controller. Try a new signal cable. If problems persist, the drive itself is probably defective. Try replacing the floppy drive. In most cases, this should correct the problem. If not, replace the floppy drive controller.

Symptom 20-9. When a new diskette is inserted in the drive, a directory from a previous diskette appears. You may have to reset the system in order to get the new diskette to be recognized. This is the classic "phantom directory" problem, and is usually due to a drive or cable fault. Check the 34-pin signal cable first. In most cases, the cable is damaged, or is not inserted properly at either end. Try a new signal cable. If this is a new drive installation, check the floppy drive jumpers. Some floppy drives allow the DISK CHANGE signal to be enabled or disabled. Make sure that the DISK CHANGE signal is enabled. If problems persist, the floppy drive itself is probably defective, so try replacing the floppy drive. In the unlikely event that problems remain, try replacing the drive controller board (phantom directory problems are rare in the drive controller itself).

NOTE: If you suspect a phantom directory, DO NOT initiate any writing to the diskette - its FAT table and directories could be overwritten rendering the disk's contents inaccessible without careful data recovery procedures.

Symptom 20-10. Your 3.5" high-density floppy disk cannot format high-density diskettes. You can read and write to them just fine. This is a problem that plagues older computers (i286 and i386 systems) where after-market high-density drives were added. The problem is a lack of BIOS support for high-density formatting - the system is just too old. In such a case, you have a choice. First, you can upgrade your motherboard BIOS to a version that directly supports 3.5" high-density diskettes. You could also use the DRIVER.SYS utility - a DOS driver which allows an existing 3.5" to be "redefined" as a new logical drive providing high-density support. A typical DRIVER.SYS command line would appear in CONFIG.SYS such as:

device = c:\dos\driver.sys /D:1

Symptom 20-11. You cannot upgrade an XT-class PC with a 3.5" floppy disk. XT systems support up to four double-density 5.25" floppy disk drives. It will not support 3.5" floppy diskettes at all. To install 3.5" floppy disks, you should check your DOS version (you need to have DOS 3.3 or later installed). Next, you’ll need to install an 8-bit floppy drive controller board (remember to disable any existing floppy controller in the system first). The floppy controller will have its own on-board BIOS to support floppy disk operations. Finally, take a look at the XT configuration switches and see that any entries for your floppy drives are set correctly. If you’re using a stand-alone floppy controller, you may need to set the motherboard jumpers to "no floppy drives".

Symptom 20-12. You are unable to "swap" floppy drives so that A: becomes B:, and B: becomes A:. This often happens on older systems when users want to make their 3.5" after-market B: drive into their A: drive, and relegate their aging 5.25" drive to B: instead. First, check your signal cable. For floppy cables with a wire twist, the end-most connector is A:, and the connector prior to the twist is B:. Reverse the connectors at each floppy drive to reverse their identities. If the cable has NO twist (this is rare), reset the jumper ID on each drive so that your desired A: drive is set to DS0 (Drive Select 0), and your desired B: drive is jumpered to DS1. If you accomplish this exchange, but one drive is not recognized, try a new floppy signal cable. Also remember to check your CMOS settings - you’ll need to reverse the floppy drive entries for your A: and B: drives, then reboot the system.

Symptom 20-13. When using a combination floppy drive (called a "combo drive"), one of the drives does not work, while the other works fine. This problem is often caused by a drive fault. First, be sure to check the power connector - make sure that both +5volts and +12 volts are adequately provided to the drive through the 4-pin "mate-n-lock" connector. If the drive is receiving the proper power, the drive itself has almost certainly failed - try a new drive.

Symptom 20-14. There are no jumpers available on the floppy disk, so it is impossible to change settings. This is not a problem - as much as it is an inconvenience. Typically, you can expect "un-jumpered" floppy disks to be set to the following specifications; Drive Select 1, Disk Change (pin 34) enabled, and Frame Ground enabled. This configuration supports dual drive systems with twisted floppy cables.

Symptom 20-15. The floppy drive activity LED stays on as soon as the computer is powered up. This is a classic signaling problem which occurs after changing or upgrading a drive system. In virtually all cases, one end of the drive cable has been inserted backwards. Make sure that pin 1 on the 34-pin cable is aligned properly with the connector on both the drive and controller. If problems remain, the drive controller may have failed. This is rare, but try a new drive controller.

That finishes Chapter 20. Be sure to review the glossary and chapter questions on the accompanying CD. If you have access to the Internet, take a look at some of the floppy drive manufacturers listed below:

Mitsumi: http://www.mitsumi.com

Teac: http://www.teac.com

Sony: http://www.ita.sel.sony.com/products/storage/