Floppy Disk Driver Primer

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A Brief History

Floppy disk drives were originally introduced commercially as a read-only device to hold microcode and diagnostics for large IBM mainframe computer systems in the early 1970s. By changing the diskette inside the floppy drive, service engineers could easily update the microcode to the latest revisions or load diagnostics in an easy and timely manner. These first commercial floppy drives were physically quite large and used 8 inch diskettes recorded on only one side. The storage capacity of these early read-only drives was less than 100 kilobytes. In 1973 a new upgraded 8 inch drive with read/write capability and a capacity of about 250 kilobytes began shipping which IBM used in data entry systems. This drive incorporated many technical improvements and became somewhat of a model for drives still in use today. As time went on, designers learned how to reliably record on both sides of the diskette as well as increase the density of the data recorded on the diskette.

In 1976 floppy drives were introduced in the 5.25 inch size by Shugart Associates. In a cooperative effort, Dysan Corporation manufactured the matching 5.25 inch diskettes. Originally these drives were available in only a single-sided low density format, and like the first 8 inch models, stored less than 100 kilobytes. Later they received many of the same enhancements made to the 8 inch models, and eventually 5.25 inch floppy drives settled at a double-sided, "double density" formatted capacity of about 1.2 megabytes. This drive was used in the IBM 'AT' personal computer. It is also the popular 5.25 inch model still with us today.

Modern floppy drives and diskettes have evolved to a much smaller size with larger capacities as well. In 1980, the 3.5 inch floppy drive and diskette was introduced by Sony. During the early 1980's many competing formats were tried to compete with the 3.5 inch drives. From various companies there were 2.0, 2.5, 2.8, 3.0, 3.25, and 4.0 inch formats! Fortunately for us, over time the industry settled on the 3.5 inch format which is now standardized and manufactured by many companies. Today's standard 3.5 inch diskettes hold a formatted capacity of about 1.5 megabytes while still using the same basic technology of the second generation 8 inch drives.

Although technology has not changed substantially, floppy drives have certainly changed considerably in order to meet the very demanding needs of the marketplace. The primary factor which caused designers to reduce the size and cost of floppies was the introduction and evolution of the personal computer. It was in the personal computer market that the low cost, mass produced floppy drive found its first real home. Very quickly the floppy became the standard method of exchanging data between personal computers. It also became the popular method of storing moderate amounts of information outside of the computer's hard drive. Diskettes are small, inexpensive, readily available, easy to store, and have a good shelf life if stored properly.

This article is a discussion of common conventional floppy drives found in today's personal computers. It is important to note that since the early 1980s through the present, high capacity floppy drives have also been introduced to the marketplace. These have come in all three of the popular sizes previously mentioned. Some have achieved data storage capacities and even access times similar to that of small hard drives. Several of these products have come and gone as have some of the companies which produced them. A few designs continue to be popular to this day, especially for certain applications requiring removability but at the same time more storage capacity than the inexpensive common floppy drives can provide. Because of their inherent higher price, few of these high capacity drives have found their way into the mass produced personal computer. These drives are often sold as an add-on (or add-in) accessory for those who need them.

Floppy Drive Basics

Conventional floppy drives contain the following basic components: 1) A spindle clamping mechanism to hold the diskette in place as it spins; 2) Either one or two magnetic read/write heads mounted on a mechanism that moves the heads radially across the diskette's surface; and 3) A sensor that detects the rotational position of the diskette via an index hole (or magnetic sensor in 3.5 inch drives) on floppy disks.

When the computer system needs to access data on the diskette, the read/write heads are stepped by signals generated by the computer system's floppy controller. These steps are along invisible concentric cylinders, which are usually referred to as "tracks". When the computer system's power is first turned on, the read/write heads of the drive are automatically set to track 0 (the first track and starting position). In most drives, this starting position is located by means of a sensor in the drive which has been adjusted to tell the floppy controller when the heads have reached the first track. If this sensor is not in proper adjustment, then this initial starting calibration is also incorrect and the heads are not properly positioned over track 0. In order to move the heads from this first track to other tracks, the head positioner simply moves in or out one track for each step pulse received from the computer's floppy controller.

The floppy drive blindly accepts these pulses and assumes that it is positioned directly over the proper specified track. It has no accurate feedback mechanism from the diskette concerning whether or not the heads are properly positioned. This differs significantly from many hard drives employing servo systems, which constantly monitor exact head position over each track and make very small and almost instantaneous corrections automatically before performing a read or write operation. This feedback is generated by positioning signals pre-recorded on the hard disk's surface. Since common floppy drives are designed without a positional feedback mechanism, they are referred to as "open-loop" whereas these hard drives are referred to as "closed-loop". It should be noted that there are a few special high capacity floppy drives now available that do employ servo systems, but they are also much more expensive than the common variety and not often found as standard equipment in everyday computers. Since common floppy drives don't have a sophisticated servo system, they must be carefully aligned in order to ensure the very important ability of reliably exchanging data diskettes with other drives. This is important because we all rely on our floppy drives whenever we move any data or programs in or out of our computers using diskettes.

It is quite possible for the head positioner to become out of alignment in such a way that the read/write head is only over a portion of a track. This reduces the strength of the data signal detected by the head and may also cause unwanted interference between adjacent tracks, or incomplete erasure when data fields are re-recorded during a disk "save". Should the relative alignment of the drive that recorded the diskette's data and that of the drive reading the same diskette differ substantially, it will be difficult or impossible to read the recorded data. This undesirable condition is known as radial misalignment.

In addition to radial misalignment, data errors may also be caused by the heads being rotated slightly on their axis (azimuth) or the drive's index sensor being out of position. See the section below, Floppy Drive Alignment Test Parameters for more specific information about these other alignment parameters.

The above scenario can be common for anyone attempting to read software or data files written by another drive. The symptom might be that the drive would function very well as long as it was using a diskette recorded on the same drive, but when trying to read from or write to a diskette from another drive data errors occur. These data errors are of course troublesome for anyone, but in particular can be a real nightmare for software duplicators and publishers who routinely copy software on to diskettes in mass quantities for distribution. Knowing that their customers have computers with drives in various stages of misalignment, the software duplicator must make sure to place the software's data fields as perfectly as possible on the duplicated diskette. If the duplication equipment were to be out of alignment it could be catastrophic, resulting in diskettes that can only be read reliably by the duplication equipment on which it was recorded, but not by properly aligned drives in the marketplace.

Floppy Drive Alignment Test Parameters

The read/write heads of the drive must have three basic alignment parameters checked and maintained:

 

1. Radial head alignment determines the exact position of the drive's read/write head over a given track location. As discussed above, this parameter is by far the most critical and most common alignment problem. For instance, a standard 3.5 inch drive with its read/write head displaced from the nominal standard more than +/- 600 millionths of an inch, would be considered out of specification. This drive would probably have difficulty reading a diskette written on another drive, especially if the other drive was misaligned in the opposite direction.

   
   

2. Azimuth head alignment is the angle, in fractions of a degree (minutes), of the read/write head on its vertical axis. Like the radial parameter, azimuth is very critical for reliable data exchange between drives, but the incidence of occurrence of a read/write head with incorrect azimuth causing an interchange problem is somewhat less. This is because the drive's head positioner mechanism routinely moves the heads radially going from track to track, however the azimuth alignment is fixed at the factory and should never change unless the drive is damaged or worn badly. Like the radial parameter, if the read/write head has poor azimuth then it may have difficulty reading a diskette that was written on a drive with a proper azimuth angle, or worse yet a drive with its azimuth angle incorrectly adjusted in the opposite direction from nominal.

3. Index timing head alignment relates to the starting point on each track where data is stored. The starting point is determined by the placement of an index sensor on the drive which triggers once with each revolution of the diskette and serves as the reference point. Should this sensor be out of adjustment, then the starting point of the data written to the diskette will be altered. Fortunately, most modern soft sectored disk formats used with personal computers today don't rely heavily on this parameter being perfectly adjusted. Therefore it is seldom a serious problem unless severely out of adjustment or the index sensor has failed completely.

Beyond just the read/write heads, there are other parameters to be adjusted and maintained within the drive which also are commonly referred to as "alignment":

 

1. Head Positioner Linearity. This is the ability of the drive's head positioner mechanism to maintain the radial alignment of the read/write heads to be constant throughout all of the available tracks on the diskette. Since the track spacing of the recorded tracks should be constant across the entire surface of the diskette (96 tracks per inch in the case of the HD 5-1/4" for example), it is very important that the drive being used to store data or programs be able to maintain its radial alignment very precisely across this entire area. A defective drive might have perfect radial alignment at one track location, but exhibit considerable error at another area of the diskette's surface, which will cause a failure when read by another drive which has no linearity problem.
   
   
2. Motor Spindle Eccentricity. This is defined as the wobble that a diskette may have as it spins in the drive. Ideally, the diskette should rotate in the drive such that the invisible tracks are a fixed distance from the center of the diskette during the entire rotation. When a drive has an eccentricity problem, this results in radial alignment which varies as the diskette turns each revolution. If this happens, some recorded sectors might have good radial alignment, while others are off-track. This will result in failures very similar to a radial alignment problem except that some sectors of a given track may be readable while others will fail.
   
   
3. Track 0 Sensor Adjustment. This is the physical adjustment of the drive's track 0 sensor. In most drives, this is an optical sensor which must be adjusted such that when the system is first powered on, or recalibrated, (ie, blindly sent to track 0 by the controller), the drive's heads are able to be placed accurately at the starting track 0. The computer's floppy controller relies on this sensor to establish a starting point from which all other tracks are referenced.
   
   
4. Index Timing. Index timing is a standardized fixed angular location between the drive's index sensor which triggers with each revolution of the diskette, and the read/write heads. Once the two heads are adjusted to have very close to the same angular location relative to each other, then relative time between the index sensor and the read/write heads must be set. This is usually a straightforward physical adjustment of the drive's Index sensor, to bring it to within specification.
   
   
5. Index Skew. This is the drive's ability to maintain the index timing to be constant along the complete travel of the read/write heads in order to access all of the tracks recorded on the diskette. If the Index timing is good at track 0, but out of specification at the innermost track, then the drive would be said to have unacceptable index skew. This is not a flaw in the read/write heads, but rather a misalignment of the head positioning mechanism, which is not able to move on the correct line as it should. If the mechanism was out of line enough, it could also have a significant effect on the head's azimuth, which would change from track to track as the heads traveled more and more out of line due to the mechanism being skewed.
   
   
6. Motor Spindle Speed. This parameter simply indicates how fast the drive is spinning the diskette. It is usually expressed in revolutions per minute or sometimes in milliseconds per revolution. The drive should be able to spin the diskette at a correct and constant speed. If the drive spins the diskette too slowly, the recorded data will arrive at the floppy controller at a frequency which is too low. If the drive spins too fast, the frequency will be too high and when writing information, the controller will not have enough time within the time frame of one revolution of the diskette to record the necessary data fields for reliable operation.

Causes of Floppy Drive Misalignment

The causes of a drive going out of alignment will vary somewhat depending on the design of the drive itself, but there are some common contributors. Wear of the drive's head positioning mechanism from normal use can cause the radial alignment to drift out of specification over time. Once again, depending on the design of the drive, this may show up in different ways. Various forms of debris often find their way into the drive's mechanical parts and will often accelerate wear of the drive and in some cases affect alignment directly by changing the way the drive's sensors and mechanisms work. Dirt and normal wear may cause the head mechanism to not slide as easily as it should, which puts an excessive load on the small stepper motor used to position the heads.

In some cases the read/write head assembly itself can become bent or otherwise damaged, often not enough to cause a complete failure, but enough to significantly affect the alignment and cause problems when reading a diskette recorded by another drive. Since these problems can be somewhat subtle, they may be difficult to detect without one of our Floppy Drive Diagnostics products. An immediate failure may not happen, but rather the problem slowly worsens with continued use of the drive. In some cases if a drive is out of alignment, but not grossly so, it may work with a good data diskette, but not with a marginal one. No, all data diskettes are not the same, but that's another subject beyond the scope of this article. Suffice it to say, you should avoid most "super bargain" data diskettes, unless you don't care about your drive or your data. Not many people fall into this category.

Sooner or later, even with proper care, all drives will eventually either wear out, go out of alignment, or fail completely. Wear and failures can be minimized by using good quality drives and data diskettes and keeping your drive clean and properly maintained. This can be achieved through simple preventative maintenance. Cleaning disks are generally OK, but results vary. Even when cleaning diskettes do their job effectively, they clean only the read/write heads. They do nothing to keep the drive's mechanics clean. Unfortunately this problem can be compounded by the fact that many computers have cooling fans which move air (and dust) right through the floppy drive, leaving the drive's mechanics subject to contamination.

There are a number of products available to aid the technician in determining the drive's alignment condition. The most basic and traditional method used by drive manufacturers on their production lines is to use the Analog Alignment Diskette (AAD). These test diskettes are also commonly used by service and repair personnel. They are recorded with special unique patterns which are unlike a formatted data diskette. Each recorded pattern is designed to be used to measure the various alignment parameters discussed above. Typically the technician moves the drive's read/write heads (using a floppy drive exerciser or utility software like Drive Probe to the various tracks where these specially recorded patterns are located. He or she then views and interprets the signals with an oscilloscope. Amplitudes and the timing of various signals are then compared and the drive's alignment condition is determined. In some cases, the oscilloscope can be replaced by a specialized piece of test equipment which has been designed to replace the oscilloscope in this specific application. 

Test your Floppy Drive the Easy Way!

Often the drive's test points are difficult to locate, especially when the drive is installed in a computer, and of course using an AAD requires that one be well versed in the usage of an oscilloscope, etc. It is also very helpful to have some knowledge and experience maintaining diskette drives. This is why computer readable diagnostic diskettes have been developed which do not require any special test equipment or an oscilloscope. When used with the appropriate matching software, such as Drive Probe or Drive Probe Advanced Edition these advanced diagnostic test diskettes, called High Resolution Diagnostic Diskettes (HRD), can perform all of the functions of the AAD without any other test equipment. If you have a PC or Mac, this makes the task of keeping track of the drive's alignment much easier and faster. It also means that very accurate diagnostics can be used by a non-technical user to check the drive's alignment without having to disassemble the computer system to attach test probes, etc. Since the software and the HRD do all the work, there isn't any need to interpret signals on an oscilloscope or to use any special test equipment. If desired, the software can be set up as a "go/no-go" test which just alerts the operator when there is a problem. This greatly eases the task of routinely checking the drives without tying up technical personnel or having to disassemble the computer unless the drive does indeed need to be serviced or replaced.

Floppy Drive Repair and Maintenance

If a drive is out of alignment, it should be relatively straightforward to put it back in shape. If the heads are badly worn out or otherwise damaged, then it will take a bit longer because when replacing the heads you will lose the original alignment setup and have to start from scratch. Most who have done this will be familiar with the Accurite Analog Alignment Diskette (AAD). You can also perform alignments directly with your personal computer by using the High Resolution Diagnostic Diskettes (HRD) included with Drive Probe and Drive Probe Advanced Edition. If you need to service a floppy drive from a system other than a PC or Mac, this requires a little more work because generally you will find that system level floppy diagnostics which include testing for the drive's alignment parameters are not available. This can still be easily accomplished by disconnecting the drive from the host system and reconnecting to the "personality module" of the Drive Probe Advanced Edition which supports many different floppy drive interfaces and drive types. Either way, you get an accurate real-time representation of each of the drive's critical alignment parameters. When adjustments are made, you see almost instantaneous results of your adjustment directly on the computer's screen. Each parameter can then be observed, checked against a tolerance, and adjusted as needed.

In most cases, depending on the specific design of the drive and the parameter needing adjustment, you will need to loosen a screw, move a small lever or rotate a small motor, adjust, then tighten down the screw. When you make an adjustment of one parameter, always go back and check the others. The best place to find information specific to the drive that you are working with is the drive's service manual supplied by the drive manufacturer. This may not always be possible to get, but if you are persistent you can usually find enough information about the drives that you are using to maintain them without much problem. Some general guidance will be supplied with the AAD, or Drive Probe product, but this won't give you specific information about any particular drive. If you are using one of the Drive Probe products, the software will easily get you through each test so you can very simply check the drive's alignment parameters even without any knowledge of the specific drive that you are using. This is very important, because even if you plan to replace rather than repair or adjust a bad drive, you first need to make sure that it is indeed the drive causing problems and not a bad diskette, controller, cable, etc. Also after replacement of the drive, it is wise to re-run the diagnostics to make sure that the problem is solved.

The best way to keep your drives in alignment is to keep them clean and treat them well. As mentioned earlier, this includes using good quality data diskettes. By keeping things clean, you may avoid having dirt get into the drive's mechanisms. It's also a good idea to test your drives periodically in order to catch problems early. This is because the alignment and/or general performance of the drive may slowly deteriorate with usage and should be detected before disaster strikes. Unfortunately, your computer system with all of its built-in "retry on error" schemes, often will give you little warning (if any) that a floppy is failing. Your important data or programs could be lost if the drive misbehaves at an inopportune time. Even worse, the drive could fail completely when it is really needed, perhaps leaving you with no way to complete the task at hand until the drive is repaired. A little care and preventative maintenance using proven methods will go a long way toward eliminating a "crash" and the ensuing recovery problems which no one wants to deal with.

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