Causes of Floppy Drive Failure Symtoms and Troubleshooting

Introduction (FLOPPY)

The ability to interchange programs and data between various compatible computers is a fundamental requirement of almost every computer system. This kind of file-exchange compatibility 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.

A floppy-disk drive (FDD) is one of the least expensive and most reliable forms of massstorage 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 Concepts

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 because magnetic media is non-volatile. Unlike system RAM, no electrical energy is needed to maintain the information once it is stored on magnetic media. Although 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.

Media

For the purposes of this book, media is the physical material that 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, but 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. Coercivity is the strength with which magnetic particles resist change. More highly coercive material has a greater resistance to change, so a stronger external field will be needed to cause changes. High coercivity 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 coercivity 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 largely because of a superior (more coercive) magnetic layer. This coercivity principle also holds true for hard drives. To pack more information onto ever-smaller platters, the media must be more coercive. Coercivity is a common magnetic measurement with units in oersteds (pronounced “or-steds”). The coercivity of a typical floppy disk can range anywhere from 300 to 750 oersteds. By comparison, hard-drive and magneto-optical (MO) media usually offer coercivities 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 actuality, 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 degrades 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 a 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. Little can be done to ensure the integrity of floppy disks, aside from maintaining one or more backups on freshly formatted disks. 

However, some commercial software is available for restoring disk data (especially hard drives).

Troubleshooting Floppy disk Systems

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 might 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 lose the files on a disk, don’t use the disk.

Repair vs. Replace

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 8.89-cm drive is roughly that of two hours 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. If a large number of drives are 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.

Preliminary Testing

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 para meters (Fig. 16-10). 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.

Symptom 16-1. 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. Be 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 the test diskette is not defective. It might 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 cannot get diskettes to insert properly, change the floppy drive. 580 Floppy Drives For cleaning and testing your floppy drive, check out AUTOTEST.ZIP, CHKDRV.ZIP, CLEAN4.ZIP, and DFR.ZIP on the companion CDSymptom 16-2. 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

Troubleshooting Floppy disk Systems 581

Driveprobe Screen Display for Automatic Drive. 

Accurite Technologies, Inc. inspect the head-positioning assembly to be certain that no broken parts or obstructions could jam the read/write heads. You might wish to examine the mechanical system with a disk inserted to be certain that the trouble is not a diskalignment problem, which might be interfering with head movement. Gently remove any obstructions that you might 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 16-3. 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. Be certain that no broken parts or obstructions could jam the spindle. If a belt is between the motor and spindle, be sure that 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 might 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

582 Floppy Drives 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 16-4. 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.

If you cannot 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 cannot 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 no pulse is 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. Ifyou 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 16-5. 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  rotected. 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 writeprotect sensor. For an unprotected disk, the sensor output should be a logic 1; a protected disk should generate a logic 0 (some drives might reverse this convention).

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