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Upgrading & Repairing PCs, Eighth Edition

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Author: Scott Mueller
Retail Price: $49.99
Publisher: Que
ISBN: 0789712954
Publication Date: 9/16/97
Pages: 1168


Chapter 13 - Floppy Disk Drives

Explore how floppy disk drives and disks function, types available, how DOS uses a disk, and how to properly install and service drives and disks.

 
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This chapter examines in detail floppy disk drives and disks. It explores how floppy disk drives and disks function, how DOS uses a disk, what types of disk drives and disks are available, and how to properly install and service drives and disks.

Development of the Floppy Disk Drive

Alan Shugart is generally credited with inventing the floppy drive while working for IBM in the late 1960s. In 1967, he headed the disk drive development team at IBM's San Jose lab, when and where the floppy drive was created. One of Shugart's senior engineers, David Noble, actually proposed the flexible media (then 8 inches in diameter) and the protective jacket with the fabric lining. Shugart left IBM in 1969, and took more than 100 IBM engineers with him to Memorex. He was nicknamed "The Pied Piper" because of the loyalty exhibited by the many staff members who followed him. In 1973, he left Memorex, again taking with him a number of associates, and started Shugart Associates to develop and manufacture floppy drives. The floppy interface developed by Shugart is still the basis of all PC floppy drives. IBM used this interface in the PC, enabling them to use off-the-shelf third-party drives instead of custom building their own solutions.

Shugart wanted to incorporate processors and floppy drives into complete microcomputer systems at that time, but the financial backers of the new Shugart Associates wanted him to concentrate on floppy drives only. He quit (or was forced to quit) Shugart Associates in 1974, right before they introduced the mini-floppy (5 1/4-inch) diskette drive, which of course became the standard eventually used by personal computers, rapidly replacing the 8-inch drives. Shugart Associates also introduced the Shugart Associates System Interface (SASI), which was later renamed Small Computer Systems Interface (SCSI) when it was formally approved by the ANSI committee in 1986. After being forced to leave, Shugart attempted to legally force Shugart Associates to remove his name from the company, but failed. The remnants of Shugart Associates still operates today as Shugart Corporation.

For the next few years, Shugart took time off, ran a bar, and even dabbled in commercial fishing. In 1979, Finis Conner approached Shugart to create and market 5 1/4-inch hard disk drives. Together they founded Seagate Technology and by the end of 1979 had announced the ST-506 (6M unformatted, 5M formatted capacity) drive and interface. This drive is known as the father of all PC hard disk drives. Seagate then introduced the ST-412 (12M unformatted, 10M formatted capacity) drive, which was adopted by IBM for the original XT in 1983. IBM was Seagate's largest customer for many years. Today, Seagate Technology is the largest disk drive manufacturer in the world.

When you stop to think about it, Alan Shugart has had a tremendous effect on the PC industry. He (or his companies) has created the floppy, hard disk, and SCSI drive and controller interfaces still used today. All PC floppy drives are still based on (and compatible with) the original Shugart designs. The ST-506/412 interface was the de facto hard disk interface standard for many years and served as the basis for the ESDI and IDE interfaces as well. Shugart also created the SCSI interface, used in both IBM and Apple systems today.

As a side note, in the late 80s Finis Conner left Seagate and founded Conner Peripherals, originally wholly owned and funded by Compaq. Conner became Compaq's exclusive drive supplier, and gradually began selling drives to other system manufacturers as well. Compaq eventually cut Conner Peripherals free, selling off most (if not all) of their ownership of the company. In late 1996, Seagate bought Conner Peripherals, and has fully incorporated all of the Conner products into the Seagate line.

Drive Components

This section describes the components that make up a typical floppy drive and examines how these components operate together to read and write data--the physical operation of the drive. All floppy drives, regardless of type, consist of several basic common components. To properly install and service a disk drive, you must be able to identify these components and understand their function (see Figure 13.1).

Read/Write Heads

A floppy disk drive normally has two read/write heads, making the modern floppy disk drive a double-sided drive. A head exists for each side of the disk, and both heads are used for reading and writing on their respective disk sides. At one time, single-sided drives were available for PC systems (the original PC had such drives), but today single-sided drives are a fading memory (see Figure 13.2).


NOTE: Many people do not realize that the first head is the bottom one. Single-sided drives, in fact, use only the bottom head; the top head is replaced by a felt pressure pad (refer to Figure 13.2). Another bit of disk trivia is that the top head (Head 1) is not directly over the bottom head--the top head is located either four or eight tracks inward from the bottom head, depending on the drive type.

FIG. 13.1  A typical full-height disk drive.

The head mechanism is moved by a motor called a head actuator. The heads can move in and out over the surface of the disk in a straight line to position themselves over various tracks. The heads move in and out tangentially to the tracks that they record on the disk. Because the top and bottom heads are mounted on the same rack, or mechanism, they move in unison and cannot move independently of each other. The heads are made of soft ferrous (iron) compounds with electromagnetic coils. Each head is a composite design, with a read/write head centered within two tunnel-erase heads in the same physical assembly (see Figure 13.3).

The recording method is called tunnel erasure; as the track is laid down, the trailing tunnel erase heads erase the outer bands of the track, trimming it cleanly on the disk. The heads force the data to be present only within a specified narrow "tunnel" on each track. This process prevents the signal from one track from being confused with the signals from adjacent tracks. If the signal were allowed to "taper off" to each side, problems would occur. The forcibly trimmed track prevents this problem.

FIG. 13.2  Single- and double-sided drive head assemblies.

Alignment is the placement of the heads with respect to the tracks they must read and write. Head alignment can be checked only against some sort of reference-standard disk recorded by a perfectly aligned machine. These types of disks are available, and you can use one to check your drive's alignment. Unfortunately, that is not practical because one calibrated analog alignment disk costs more than three new drives today!

The two heads are spring-loaded and physically grip the disk with a small amount of pressure, which means that they are in direct contact with the disk surface while reading and writing to the disk. Because PC-compatible floppy disk drives spin at only 300 or 360 RPM, this pressure does not present an excessive friction problem. Some newer disks are specially coated with Teflon or other compounds to further reduce friction and enable the disk to slide more easily under the heads. Because of the contact between the heads and the disk, a buildup of the oxide material from the disk eventually forms on the heads. The buildup periodically can be cleaned off the heads as part of a preventive-maintenance or normal service program.

FIG. 13.3  Composite construction of a typical floppy drive head.

To read and write to the disk properly, the heads must be in direct contact with the media. Very small particles of loose oxide, dust, dirt, smoke, fingerprints, or hair can cause problems with reading and writing the disk. Disk and drive manufacturer's tests have found that a spacing as little as .000032 inches (32 millionths of an inch) between the heads and the media can cause read/write errors. You now can understand why it is important to handle disks carefully and avoid touching or contaminating the surface of the disk media in any way. The rigid jacket and protective shutter for the head access aperture on the 3 1/2-inch disks is excellent for preventing problems with media contamination. 5 1/4-inch disks do not have the same protective elements; therefore, more care must be exercised in their handling.

The Head Actuator

The head actuator is a mechanical motor device that causes the heads to move in and out over the surface of a disk. These mechanisms for floppy disk drives universally use a special kind of motor, a stepper motor, that moves in both directions in an increment called a step. This type of motor does not spin around continuously; rather, the motor turns a precise specified distance and stops. Stepper motors move in fixed increments, or detents, and must stop at a particular detent position. Stepper motors are not infinitely variable in their positioning. Each increment of motion, or a multiple thereof, defines each track on the disk. The motor can be commanded by the disk controller to position itself according to any relative increment within the range of its travel. To position the heads at track 25, for example, the motor is commanded to go to the 25th detent position.

The stepper motor usually is linked to the head rack by a coiled, split steel band. The band winds and unwinds around the spindle of the stepper motor, translating the rotary motion into linear motion. Some drives use a worm gear arrangement rather than a band. With this type, the head assembly rests on a worm gear driven directly off the stepper motor shaft. Because this arrangement is more compact, you normally find worm gear actuators on the smaller 3 1/2-inch drives.

Most stepper motors used in floppy drives can step in specific increments that relate to the track spacing on the disk. Most 48 Track Per Inch (TPI) drives have a motor that steps in increments of 3.6° (degrees). This means that each 3.6° of stepper motor rotation moves the heads from one track (or cylinder) to the next. Most 96 or 135 TPI drives have a stepper motor that moves in 1.8° increments, which is exactly half of what the 48 TPI drives use. Sometimes you see this information actually printed or stamped right on the stepper motor itself, which is useful if you are trying to figure out what type of drive you have. 5 1/4-inch 360K drives are the only 48 TPI drives available and use the 3.6° increment stepper motor. All other drive types normally use the 1.8° stepper motor. On most drives, the stepper motor is a small cylindrical object near one corner of the drive.

A stepper motor usually has a full travel time of about 1/5 of a second--about 200ms. On average, a half-stroke is 100ms, and a one-third stroke is 66ms. The timing of a one-half or one-third stroke of the head-actuator mechanism often is used to determine the reported average-access time for a disk drive. Average-access time is the normal amount of time the heads spend moving at random from one track to another.

The Spindle Motor

The spindle motor spins the disk. The normal speed of rotation is either 300 or 360 RPM, depending on the type of drive. The 5 1/4-inch high-density (HD) drive is the only drive that spins at 360 RPM; all others, including the 5 1/4-inch double-density (DD), 3 1/2-inch DD, 3 1/2-inch HD, and 3 1/2-inch extra-high density (ED) drives, spin at 300 RPM.

Most earlier drives used a mechanism on which the spindle motor physically turned the disk spindle with a belt, but all modern drives use a direct-drive system with no belts. The direct-drive systems are more reliable and less expensive to manufacture, as well as smaller in size. The earlier belt-driven systems did have more rotational torque available to turn a sticky disk because of the torque multiplication factor of the belt system. Most newer direct-drive systems use an automatic torque-compensation capability that automatically sets the disk-rotation speed to a fixed 300 or 360 RPM, and compensates with additional torque for sticky disks or less torque for slippery ones. This type of drive eliminates the need to adjust the rotational speed of the drive.

Most newer direct-drive systems use this automatic-speed feature, but many earlier systems require that you periodically adjust the speed. Looking at the spindle provides you with one clue to the type of drive you have. If the spindle contains strobe marks for 50Hz and 60Hz strobe lights (fluorescent lights), the drive probably has an adjustment for speed somewhere on the drive. Drives without the strobe marks almost always include an automatic tachometer-control circuit that eliminates the need for adjustment. The technique for setting the speed involves operating the drive under fluorescent lighting and adjusting the rotational speed until the strobe marks appear motionless, much like the "wagon wheel effect" you see in old Western movies. The procedure is described later in this chapter in the "Setting the Floppy Drive Speed Adjustment" section.

Circuit Boards

A disk drive always incorporates one or more logic boards, which are circuit boards that contain the circuitry used to control the head actuator, read/write heads, spindle motor, disk sensors, and any other components on the drive. The logic board represents the drive's interface to the controller board in the system unit.

The standard interface used by all PC types of floppy disk drives is the Shugart Associates SA-400 interface, which is based on the NEC 765 controller chip. The interface, invented by Shugart in the 1970s, has been the basis of most floppy disk interfacing. The selection of this industry-standard interface is the reason that you can purchase "off-the-shelf" drives (raw, or bare, drives) that can plug directly into your controller.


TIP: Logic boards for a drive can fail and usually are difficult to obtain as a spare part. One board often costs more than replacing the entire drive. I recommend keeping failed or misaligned drives that might otherwise be discarded so that they can be used for their remaining good parts--such as logic boards. The parts can be used to restore a failing drive very cost-effectively.

The Faceplate

The faceplate, or bezel, is the plastic piece that comprises the front of the drive. These pieces, usually removable, come in different colors and configurations.

Most drives use a bezel slightly wider than the drive. These types of drives must be installed from the front of a system because the faceplate is slightly wider than the hole in the system-unit case. Other drive faceplates are the same width as the drive's chassis; these drives can be installed from the rear--an advantage in some cases. In the later-version XT systems, for example, IBM uses this design in its drives so that two half-height drives can be bolted together as a unit and then slid in from the rear to clear the mounting-bracket and screw hardware. On occasion, I have filed the edges of a drive faceplate to install the drive from the rear of a system--which sometimes can make installation much easier.

Connectors

Nearly all disk drives have at least two connectors--one for power to run the drive, and the other to carry the control and data signals to and from the drive. These connectors are fairly standardized in the computer industry; a four-pin in-line connector (called Mate-N-Lock, by AMP), in both a large and small style is used for power (see Figure 13.4); and a 34-pin connector in both edge and pin header designs is used for the data and control signals. 5 1/4-inch drives normally use the large style power connector and the 34-pin edge type connector, whereas most 3 1/2-inch drives use the smaller version of the power connector and the 34-pin header type logic connector. The drive controller and logic connectors and pinouts are detailed later in this chapter as well as in Appen-dix A, "Vendor List."

FIG. 13.4  A disk drive female power supply cable connector.

Both the large and small power connectors from the power supply are female plugs. They plug into the male portion, which is attached to the drive itself. One common problem with upgrading an older system with 3 1/2-inch drives is that your power supply only has the large style connectors, whereas the drive has the small style. An adapter cable is available from Radio Shack (Cat. No. 278-765) and other sources that converts the large style power connector to the proper small style used on most 3 1/2-inch drives.

The following chart shows the definition of the pins on the drive power-cable con-nectors:

Large Power Connector Small Power Signal Wire Color Connector
Pin 1 Pin 4 +12 Vdc Yellow
Pin 2 Pin 3 Ground Black
Pin 3 Pin 2 Ground Black
Pin 4 Pin 1 +5 Vdc Red


NOTE: Note that the pin designations are reversed between the large- and small-style power connectors. Also, it is important to know that not all manufacturers follow the wire color coding properly. I have seen instances in which all the wires are a single color (for example, black), or the wire colors are actually reversed from normal! For example, I once purchased the Radio Shack power connector adapter cables just mentioned that had all the wire colors backwards. This was not really a problem as the adapter cable was wired correctly from end to end, but it was disconcerting to see the red wire in the power supply connector attach to a yellow wire in the adapter (and vice versa)!

Not all drives use the standard separate power and signal connectors. IBM, for example, uses either a single 34-pin or single 40-pin header connector for both power and floppy controller connections in most of the PS/2 systems. In some older PS/2 systems, for example, IBM used a special version of a Mitsubishi 3 1/2-inch 1.44M drive called the MF-355W-99, which has a single 40-pin power/signal connector. Other PS/2 systems use a Mitsubishi 3 1/2-inch 2.88M drive called the MF356C-799MA, which uses a single 34-pin header connector for both power and signal connections.

Most standard PC compatible systems use 3 1/2-inch drives with a 34-pin signal connector and a separate small style power connector. For older systems, many drive manufacturers also sell 3 1/2-inch drives installed in a 5 1/4-inch frame assembly and have a special adapter built in that allows the larger power connector and standard edge type signal connectors to be used. These drives included an adapter that enables the standard large style power connector, 34-pin edge type control, and data connector to be used. Because no cable adapters are required and they install in a 5 1/4-inch half-height bay, these types of drives are ideal for upgrading earlier systems. Most 3 1/2-inch drive- upgrade kits sold today are similar and include the drive, appropriate adapters for the power and control and data cables, a 5 1/4-inch frame adapter and faceplate, and rails for AT installations. The frame adapter and faceplate enable the drive to be installed where a 5 1/4-inch half-height drive normally would go.

Drive-Configuration Devices

Most floppy drives come properly configured for PC installation. In some cases, if the drive is used or not properly configured to begin with, you will have to check or change the configuration yourself. Most drives have a stable of jumpers and switches, and many drives are different from each other. You will find no standards for what these jumpers and switches are called, where they should be located, or how they should be implemented. There are some general guidelines to follow, but in order to set up a specific drive correctly and know all the options available, you must have information from the drive's manufacturer, normally found in the original equipment manufacturer's (OEM) manual. The manual is a "must-have" item when you purchase a disk drive.

Many drives have the following configuration settings:

  • Drive select jumper

  • Disk changeline jumper

  • Terminating resistor

  • Media sensor jumper

Drive Select

Floppy drives are connected by a cabling arrangement called a daisy chain. The name is descriptive because the cable is strung from controller to drive to drive in a single chain. All drives have a drive select (sometimes called DS) jumper that must be set to indicate a certain drive's physical drive number. Some drives allow four settings, as that was what the original SA-400 floppy interface called for, but the controllers used in PC systems support only two drives on a single daisy-chain cable. Some controllers support four drives but only on two separate cables--each one a daisy chain with a maximum of two drives.

Every drive on a particular cable must be set to have unique drive select settings. In a normal configuration, the drive you want to respond as the first drive (A:) is set to the first drive select position, and the drive you want to respond as the second drive (B:) is set to the second drive-select position. On some drives, the usable DS jumper positions are labeled DS0 and DS1; other drives use the numbers DS1 and DS2 for the same settings. For some drives then, a setting of DS0 is drive A:. For others, however, DS1 indicates drive A:.


NOTE: If you have incorrect DS settings, both drives respond simultaneously (both lights come on at the same time) or neither drive responds at all.

The type of cable you use controls the drive select configuration. Most systems have a special twist in the floppy cable that electrically changes the DS configuration of the drive plugged in after the twist. This twist causes a drive physically set to the second DS position (B:) to appear to the controller to be set to the first DS position (A:) and vice versa. With such a cable, both drives have to be set to the same DS setting for them to work. Normally, both drives should be set to the second DS position. The drive plugged into the connector farthest from the controller, which is after the twist in the cable, then would have the physical second-DS-position setting appear to be changed to a first-DS-position setting. Then the system would see this drive as A:, and the drive plugged into the middle cable connector still would appear as B:. A typical daisy-chain drive cable with this included "twist" is connected as shown in Figure 13.5.

FIG. 13.5  A floppy controller cable showing the location of the twist in lines 10-16.

An IBM-style floppy cable is a 34-pin cable with lines 10-16 sliced out and cross-wired (twisted) between the drive connectors (refer to Figure 13.5). This twisting "cross-wires" the first and second drive-select and motor-enable signals, and therefore inverts the DS setting of the drive following the twist. All the drives in a system using this type of cable, therefore--whether you want them to be A: or B:--are physically jumpered the same way; installation and configuration are simplified because both floppies can be preset to the second DS position. Some drives used by IBM, in fact, have had the DS "jumper" setting permanently soldered into the drive logic board.

Most drives you purchase have the DS jumper already set to the second position, which is correct for the majority of systems that use a cable with the twisted lines. Although this setting is correct for the majority of systems, if you are using a cable with only a single floppy drive and no provisions for adding a second one (in other words, with only one drive connector attached, and no twist in lines 10-16), then the DS setting you make on the drive is exactly what the controller sees. You can attach only one drive, and it should appear to the system as A:--therefore, set the drive to the first DS position.

Terminating Resistors

Any signal carrying electronic media or cable with multiple connections can be thought of as an electrical bus. In almost all cases, a bus must be terminated properly at each end with terminating resistors to allow signals to travel along the bus error free. Terminating resistors are designed to absorb any signals that reach the end of a cabling system or bus so that no reflection of the signal echoes, or bounces, back down the line in the opposite direction. Engineers sometimes call this effect signal ringing. Simply put, noise and distortion can disrupt the original signal and prevent proper communications between the drive and controller. Another function of proper termination is to place the proper resistive load on the output drivers in the controller and drive.

Most older 5-1/4 inch drives use a terminating resistor in the drive plugged into the physical end of a cable. The function of this resistor is to prevent reflections or echoes of signals from reaching the end of the cable. Most removable terminating resistors used in 5-1/4 inch drives have resistance values of 150 to 330 ohms.

In a typical cabling arrangement with two 5 1/4-inch floppies, for example, the terminating resistor is installed in drive A: (at the end of the cable), and this resistor is removed from the other floppy drive on the same cable (B:). The letter to which the drive responds is not important in relation to terminator settings; the important issue is that the drive at the end of the cable has the resistor installed and functioning, and that other drives on the same cable have the resistor disabled or removed.

Most 3 1/2-inch drives have permanently installed, non-configurable terminating resistors. This is the best possible setup because you never have to remove or install them, and there are never any TR jumpers to set. Although some call this automatic termination, technically the 3 1/2-inch drives use a technique called distributed termination. With distributed termination, each 3 1/2-inch drive has a much higher value (1,000 to 1,500 ohm) terminating resistor permanently installed, and therefore carries a part of the termination load. These terminating resistors are fixed permanently to the drive and never have to be removed or adjusted.

When you mix 5 1/4-inch and 3 1/2-inch drives, you should enable or disable the terminators on the 5 1/4-inch drives appropriately, according to their position on the cable, and ignore the non-changeable settings on the 3 1/2-inch drives.

A terminating resistor usually looks like a memory chip--a 16-pin dual inline package (DIP) device. The device is actually a group of eight resistors physically wired in parallel with each other to terminate separately each of the eight data lines in the interface subsystem. Normally, this "chip" is a different color from other black chips on the drive. Orange, yellow, blue, or white are common colors for a terminating resistor. Some drives use a resistor network in a single inline pin (SIP) package, which looks like a slender device with eight or more pins in a line. IBM always labels the resistor with a T-RES sticker for easy identification on their drives. On some systems, the resistor is a built-in device enabled or disabled by a jumper or series of switches (often labeled TM or TR).


CAUTION: Be aware that not all drives use the same type of terminating resistor, however, and it might be physically located in different places on different manufacturer's drive models. The OEM manual for the drive comes in handy in this situation because it shows the location, physical appearance, enabling and disabling instructions, and even the precise value required for the resistors.

Do not lose the terminator if you remove it from a drive; you might need to reinstall it later if you relocate the drive to a different position in a system or even to a different system.


Figure 13.6 shows the location and appearance of the terminating resistor or switches on a typical floppy drive. Because most 3 1/2-inch drives have a form of automatic termination, there is no termination to configure.

You don't have to worry about the controller end of the cable because a terminating resistor network is built into the controller to properly terminate that end of the bus.

Note that in many cases, even if the termination is improper a system seems to work fine, although the likelihood of read and write errors may be increased. In older systems with only 5-1/2 inch drives, the drives do not work properly at all unless termination is properly configured.

Diskette Changeline

The standard PC floppy controller and drive use a special signal on pin 34 called Diskette Changeline to determine whether the disk has been changed, or more accurately, whether the same disk loaded during the previous disk access is still in the drive. Disk Change is a pulsed signal that changes a status register in the controller to let the system know that a disk has been either inserted or ejected. This register is set to indicate that a disk has been inserted or removed (changed) by default. The register is cleared when the controller sends a step pulse to the drive and the drive responds, acknowledging that the heads have moved. At this point, the system knows that a specific disk is in the drive. If the disk change signal is not received before the next access, the system can assume that the same disk is still in the drive. Any information read into memory during the previous access can therefore be reused without rereading the disk.

FIG. 13.6  A typical floppy drive terminating resistor, or termination switches.

Because of this process, systems can buffer or cache the contents of the file allocation table (FAT) or directory structure of a disk in the system's memory. By eliminating unnecessary rereads of these areas of the disk, the apparent speed of the drive is increased. If you move the door lever or eject button on a drive that supports the disk change signal, the DC pulse is sent to the controller, thus resetting the register and indicating that the disk has been changed. This procedure causes the system to purge buffered or cached data that had been read from the disk because the system then cannot be sure that the same disk is still in the drive.

AT-class systems use the DC signal to increase significantly the speed of the floppy interface. Because the AT can detect whether you have changed the disk, the AT can keep a copy of the disk's directory and FAT information in RAM buffers. On every subsequent disk access, the operations are much faster because the information does not have to be reread from the disk in every individual access. If the DC signal has been reset (has a value of 1), the AT knows that the disk has been changed and appropriately rereads the information from the disk.

You can observe the effects of the DC signal by trying a simple experiment. Boot DOS on an AT-class system and place a formatted floppy disk with data on it in drive A:. Drive A: can be any type of drive except 5 1/4-inch double-density, although the disk you use can be anything the drive can read, including a double-density 360K disk, if you want. Then type the following command: DIR A: The disk drive lights up, and the directory is displayed. Note the amount of f time spent reading the disk before the directory is displayed on-screen. Without touching the drive, enter the DIR A: command again, and watch the drive-access light and screen. Note again the amount of time that passes before the directory is displayed. The drive A: directory should appear almost instantly the second time because virtually no time is spent actually reading the disk. The directory information was simply read back from RAM buffers or cache rather than read again from the disk. Now eject and re-insert the disk. Type the DIR A: command again. The disk again takes some time reading the directory before displaying anything because the system "thinks" that you changed the disk.

Older PC and XT low-density controllers (and systems) are not affected by the status of the DC signal. These systems "don't care" about signals on pin 34. The PC and XT systems always operate under the assumption that the disk is changed before every access, and they reread the disk directory and FAT each time--one reason why these systems are slower in using the floppy disk drives.

One interesting problem can occur when certain drives are installed in a 16-bit or greater system. As mentioned, some drives use pin 34 for a "Ready" (RDY) signal. The RDY signal is sent whenever a disk is installed and rotating in the drive. If you install a drive that has pin 34 set to send RDY, the system "thinks" that it is continuously receiving a disk change signal, which causes problems. Usually the drive fails with a Drive not ready error and is inoperable. The only reason that the RDY signal exists on some drives is that it happens to be a part of the standard Shugart SA-400 disk interface; however, it has never been used in PC systems.

The biggest problem occurs if the drive is not sending the DC signal on pin 34, and it should. If a system is told (through CMOS setup) that the drive is any other type than a 360K (which cannot ever send the DC signal), the system expects the drive to send DC whenever a disk has been ejected. If the drive is not configured properly to send the signal, the system never recognizes that a disk has been changed. Therefore, even if you do change the disk, the AT still acts as though the first disk is in the drive and holds the first disk's directory and FAT information in RAM. This can be dangerous because the FAT and directory information from the first disk can be partially written to any subsequent disks written to in the drive.


CAUTION: If you ever have seen an AT-class system with a floppy drive that shows "phantom directories" of the previously installed disk, even after you have changed or removed it, you have experienced this problem firsthand. The negative side effect is that all disks after the first one you place in this system are in extreme danger. You likely will overwrite the directories and FATs of many disks with information from the first disk.

If even possible at all, data recovery from such a catastrophe can require quite a bit of work with utility programs such as Norton Utilities. These problems with Disk Change most often are traced to an incorrectly configured drive. This problem will be covered in more detail in the section "Phantom Directory (Disk Change) Problems" later in this chapter.


If the drive is a 5 1/4-inch 360K drive, set the status of pin 34 to Open (disconnected) regardless of the type of system in which you are installing the drive. The only other option normally found for pin 34 on 360K drives is RDY, which is incorrect. If you are using only a low-density controller, as in a PC or XT, pin 34 is ignored no matter what is sent on it.

If the drive you are installing is a 5 1/4-inch 1.2M or 3 1/2-inch 720K, 1.44M, or 2.88M drive, be sure to set pin 34 to send the Disk Change (DC) signal. The basic rule is simple: For 360K drives only, pin 34 = Open (disconnected) For any other drive, pin 34 = Disk Change

Media Sensor

This configuration item exists only on the 3 1/2-inch 1.44M or 2.88M drives. The jumper selection, called the media sensor (MS) jumper, must be set to enable a special media sensor in the disk drive, which senses a media sensor hole found only in the 1.44M HD and the 2.88M ED floppy disks. The labeling of this jumper (or jumpers) varies greatly between different drives. In many drives, the jumpers are permanently set (enabled) and cannot be changed.

The three types of configurations with regards to media sensing are as follows:

  • No media sense (sensor disabled or no sensor present)

  • Passive media sense (sensor enabled)

  • Active or intelligent media sense (sensor supported by Controller/BIOS)

Most systems use a passive media sensor arrangement. The passive media sensor setup enables the drive to determine the level of recording strength to use and is required for most installations of these drives, because of a bug in the design of the Western Digital hard disk and floppy controllers used by IBM in the AT systems. This bug prevents the controller from properly instructing the drive to switch to double-density mode when you write or format DD disks. With the media sensor enabled, the drive no longer depends on the controller for density mode switching and relies only on the drive's media sensor. Unless you are sure that your disk controller does not have this flaw, make sure that your HD drive includes a media sensor (some older or manufacturer-specific drives do not), and that it is properly enabled.

The 2.88M drives universally rely on media sensors to determine the proper mode of operation. The 2.88M drives, in fact, have two separate media sensors because the ED disks include a media sensor hole in a different position than the HD disks.

With only a few exceptions, HD 3 1/2-inch drives installed in most PC-compatible systems do not operate properly in double-density mode unless the drive has control over the write current (recording level) via an installed and enabled media sensor. Exceptions are found primarily in systems with floppy controllers integrated on the motherboard, including most older IBM PS/2 and Compaq systems as well as most laptop or notebook systems from other manufacturers. These systems have floppy controllers without the bug referred to earlier, and can correctly switch the mode of the drive without the aid of the media sensor.

In these systems, it technically does not matter whether you enable the media sensor. If the media sensor is enabled, the drive mode is controlled by the disk you insert, as is the case with most PC-compatible systems. If the media sensor is not enabled, the drive mode is controlled by the floppy controller, which in turn is controlled by DOS.

If a disk is already formatted correctly, DOS reads the volume boot sector to determine the current disk format, and the controller then switches the drive to the appropriate mode. If the disk has not been formatted yet, DOS has no idea what type of disk it is, and the drive remains in its native HD or ED mode.

When you format a disk in systems without an enabled media sensor (such as most PS/2s), the mode of the drive depends entirely on the FORMAT command issued by the user, regardless of the type of disk inserted. For example, if you insert a DD disk into an HD drive in an IBM PS/2 Model 70 and format the disk by entering FORMAT A:, the disk is formatted as though it is an HD disk because you did not issue the correct parameters (/F:720) to cause the FORMAT command to specify a DD format. On a system with the media sensor enabled, this type of incorrect format fails, and you see the Invalid media or Track 0 bad error message from FORMAT. In this case, the media sensor prevents an incorrect format from occurring on the disk, a safety feature most older IBM PS/2 systems lack.

Most of the newer PS/2 systems--including all those that come standard with the 2.88M drives--have what is called an active or intelligent media sensor setup. This means that the sensor not only detects what type of disk is in the drive and changes modes appropriately, but also the drive informs the controller (and the BIOS) about what type of disk is in the drive. Systems with an intelligent media sensor do not need to use the disk type parameters in the FORMAT command. In these systems, the FORMAT command automatically "knows" what type of disk is in the drive and formats it properly. With an intelligent media sensor, you never have to know what the correct format parameters are for a particular type of disk; the system figures it out for you automatically. Many high-end systems such as the newer PS/2 systems as well as high-end Hewlett-Packard PCs have this type of intelligent media sensor arrangement.

The Floppy Disk Controller

The floppy disk controller consists of the circuitry either on a separate adapter card or integrated on the motherboard, which acts as the interface between the floppy drives and the system. Most PC- and XT-class systems use a separate controller card that occupied a slot in the system. The AT systems normally have the floppy controller and hard disk controller built into the same adapter card and also plugged into a slot. In most of the more modern systems built since then, the controller is integrated on the motherboard. In any case, the electrical interface to the drives has remained largely static, with only a few exceptions.

The original IBM PC and XT system floppy controller was a 3/4-length card that could drive as many as four floppy disk drives. Two drives could be connected to a cable plugged into a 34-pin edge connector on the card, and two more drives could be plugged into a cable connected to the 37-pin connector on the bracket of this card. Figures 13.7 and 13.8 show these connectors and the pinouts for the controller.

FIG. 13.7  A PC and XT floppy controller internal connector.

The AT used a board made by Western Digital, which included both the floppy and hard disk controllers in a single adapter. The connector location and pinout for the floppy controller portion of this card is shown in Figure 13.9.

IBM used two variations of this controller during the life of the AT system. The first one was a full 4.8 inches high, which used all the vertical height possible in the AT case. This board was a variation of the Western Digital WD1002-WA2 controller sold through distributors and dealers. The second-generation card was only 4.2 inches high, which enabled it to fit into the shorter case of the XT-286 as well as the taller AT cases. This card was equivalent to the Western Digital WD1003-WA2, also sold on the open market.

FIG. 13.8  A PC and XT floppy controller external connector.

Disk Physical Specifications and Operation

PC-compatible systems now use one of as many as five standard types of floppy drives. Also, five types of disks can be used in the drives. This section examines the physical specifications and operations of these drives and disks.

Drives and disks are divided into two classes: 5 1/4-inch and 3 1/2-inch. The physical dimensions and components of a typical 5 1/4-inch disk and a 3 1/2-inch disk are shown later in this chapter.

FIG. 13.9  An AT floppy controller connector.

The physical operation of a disk drive is fairly simple to describe. The disk rotates in the drive at either 300 or 360 RPM. Most drives spin at 300 RPM; only the 5 1/4-inch 1.2M drives spin at 360 RPM (even when reading or writing 360K disks). With the disk spinning, the heads can move in and out approximately 1 inch and write either 40 or 80 tracks. The tracks are written on both sides of the disk and therefore sometimes are called cylinders. A single cylinder comprises the tracks on the top and bottom of the disk. The heads record by using a tunnel-erase procedure in which a track is written to a specified width, and then the edges of the track are erased to prevent interference with any adjacent tracks.

The tracks are recorded at different widths for different drives. Table 13.1 shows the track widths in both millimeters and inches for the five types of floppy drives supported in PC systems.

Table 13.1  Floppy Drive Track-Width Specifications

Drive Type No. of Tracks Track Width
5 1/4-inch 360K 40 per side 0.300 mm 0.0118 in.
5 1/4-inch 1.2M 80 per side 0.155 mm 0.0061 in.
3 1/2-inch 720K 80 per side 0.115 mm 0.0045 in.
3 1/2-inch 1.44M 80 per side 0.115 mm 0.0045 in.
3 1/2-inch 2.88M 80 per side 0.115 mm 0.0045 in.

The differences in recorded track width can result in data-exchange problems between 5 1/4-inch drives. The 5 1/4-inch drives are affected because the DD drives record a track width nearly twice that of the HD drives. A problem occurs, therefore, if an HD drive is used to update a DD disk with previously recorded data on it.

Even in 360K mode, the HD drive cannot completely overwrite the track left by an actual 360K drive. A problem occurs when the disk is returned to the person with the 360K drive: That drive reads the new data as embedded within the remains of the previously written track. Because the drive cannot distinguish either signal, an Abort, Retry, Ignore error message appears on-screen. The problem does not occur if a new disk (one that never has had data recorded on it) is first formatted in a 1.2M drive with the /4 option, which formats the disk as a 360K disk.


NOTE: You also can format a brand new 360K disk in a 1.2M drive with the /N:9, /T:40, or /F:360 options, depending on the DOS version. The 1.2M drive can then be used to fill the brand new and newly formatted 360K disk to its capacity, and every file will be readable on the 40-track, 360K drive.


NOTE: I use this technique all the time to exchange data disks between AT systems that have only the 1.2M drive and XT or PC systems that have only the 360K drive. The key is to start with either a new disk or one wiped clean magnetically by a bulk eraser or degaussing tool. Just reformatting the disk does not work by itself because formatting does not actually erase a disk; instead it records data across the entire disk.

Disk Magnetic Properties

A subtle problem with the way a disk drive works magnetically is that the recording volume varies depending on the type of format you are tr