Upgrading and Repairing PCs

Drive Components

All floppy disk 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 functions (see Figure 11.1).

Read/Write Heads

A floppy disk drive usually has two read/write heads—one for each side of the disk, with both heads being used for reading and writing on their respective disk sides (see Figure 11.2). 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.

Note Many people do not realize that Head 0, or the first head on a floppy disk drive, is the bottom one. Single-sided drives, in fact, used only the bottom head; the top head has been replaced by a felt pressure pad. Another bit of disk trivia is that the top head (Head 1) is not positioned directly over the bottom head (Head 0). The top head is instead offset by either four or eight tracks inward from the bottom head, depending on the drive type.

A motor called a head actuator moves the head mechanism. The heads can move in and out over the surface of the disk in a straight line to position themselves over various tracks. On a floppy drive, the heads move in and out tangentially to the tracks they record on the disk. This is different from hard disks, where the heads move on a rotating arm similar to the tone-arm of a record player. Because the top and bottom heads are mounted on the same rack, or mechanism, they move in unison and can't move independently of each other. The upper and lower heads each define tracks on their respective sides of the disk medium, whereas at any given head position, the tracks under the top and bottom head simultaneously are called a cylinder. Most floppy disks are recorded with 80 tracks on each side (160 tracks total), which is 80 cylinders.

The heads themselves 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 11.3).

Floppy disk drives use a recording method called tunnel erasure. As the drive writes to a track, the trailing tunnel-erase heads erase the outer bands of the track, trimming it cleanly on the disk. The heads force the data into a specified narrow "tunnel" on each track. This process prevents the signal from one track from being confused with the signals from adjacent tracks, which would happen if the signal were allowed to naturally "taper off" to each side. 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. However, this is usually not practical for the end user because one calibrated analog alignment disk can cost more than a new drive.

The floppy disk drive's two heads are spring-loaded and physically grip the disk with a small amount of pressure, which means they are in direct contact with the disk surface while reading from and writing to the disk. Because floppy disk drives spin at only 300rpm or 360rpm, 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 disk, a buildup of the magnetic material from the disk eventually forms on the heads. The buildup should periodically be cleaned off the heads as part of a preventive maintenance or normal service program. Most manufacturers recommend cleaning the heads after every 40 hours of drive operation, which—considering how often people use these drives today—could be a lifetime.

To read and write to the disk properly, the heads must be in direct contact with the magnetic medium. Very small particles of loose oxide, dust, dirt, smoke, fingerprints, or hair can cause problems with reading and writing the disk. Disk and drive manufacturers' tests have found that a spacing as little as .000032'' (32 millionths of an inch) between the heads and medium 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 medium in any way. The rigid jacket and protective shutter for the head access aperture on 3 1/2'' disks is excellent for preventing problems caused by contamination. Disks that are 5 1/4'' do not have the same protective elements, which is perhaps one reason they have fallen into disuse. If you still use 5 1/4'' floppy disks, you should exercise extra care in their handling. I recommend copying any archival data over to CD-R or DVD media if your situation doesn't require keeping the data on the original media.

The Head Actuator

The head actuator for a floppy disk drive is what moves the heads across the disk and is driven by a special kind of motor, called a stepper motor (see Figure 11.4). This type of motor does not spin around continuously; rather, the motor turns a precise specified distance and stops. Stepper motors are not infinitely variable in their positioning; they move in fixed increments—or detents—and must stop at a particular detent position. This is ideal for disk drives because the location of each track on the disk can then be defined by moving one or more increments of the motor's motion. The disk controller can instruct the motor to position itself any number of steps within the range of its travel. To position the heads at cylinder 25, for example, the controller instructs the motor to go to the 25th detent position or step from Cylinder 0.

The stepper motor can be linked to the head rack in one of two ways. In the first, the link is 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, however, use a worm-gear arrangement rather than a band. In this type of drive, the head assembly rests on a worm gear driven directly off the stepper motor shaft. Because this arrangement is more compact, you usually find worm-gear actuators on the smaller 3 1/2'' drives.

Most stepper motors used in floppy disk drives can step in specific increments that relate to the track spacing on the disk. Older 48-track-per-inch (TPI) drives have a motor that steps in increments of 3.6°. This means that each 3.6° of stepper motor rotation moves the heads from one track to the next. Most 96 or 135 TPI drives have stepper motors that move in 1.8° increments, which is exactly half of what the 48 TPI drives use. Sometimes you see this information actually printed or stamped on the stepper motor itself, which is useful if you are trying to figure out which type of drive you have. The 5 1/4'' 360KB drives are the only 48 TPI drives that use the 3.6° increment stepper motor; all other drive types typically 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 one fifth of a second—about 200ms. On average, a one-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 is often 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 is what spins the disk. The normal speed of rotation is either 300rpm or 360rpm, depending on the type of drive. The 5 1/4'' high-density (HD) drive is the only drive that spins at 360rpm. All others, including the 5 1/4'' double-density (DD), 3 1/2'' DD, 3 1/2'' HD, and 3 1/2'' extra-high density (ED) drives, spin at 300rpm. This is a slow speed when compared to a hard disk drive, which helps explain why floppy disk drives have much lower data transfer rates. However, this slow speed also enables the drive heads to be in physical contact with the disk while it is spinning, without causing friction damage.

Many earlier drives used a mechanism by 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, on the other hand, use an automatic torque-compensation capability that sets the disk-rotation speed to a fixed 300rpm or 360rpm and compensates with additional torque for high-friction disks or less torque for more slippery ones. Besides compensating for varying amounts of friction, this arrangement eliminates the need to adjust the rotational speed of the drive—something that was frequently required on older drives.

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 other components on the drive. The logic board implements the drive's interface to the controller board in the system unit.

The standard interface that all PC floppy disk drives use is termed the Shugart Associates SA400 interface, was invented in the 1970s, and is based on the NEC 765 controller chip. All modern floppy controllers contain circuits that are compatible with the original NEC 765 chip. This industry-standard interface is why you can purchase drives from almost any manufacturer and they will all be compatible.

The Controller

At one time, the controller for a computer's floppy disk drives took the form of a dedicated expansion card installed in an Industry Standard Architecture (ISA) bus slot. Later implementations used a multifunction card that provided the IDE/ATA, parallel, and serial port interfaces in addition to the floppy disk drive controller. Today's PCs have the floppy controller integrated into the motherboard, usually in the form of a Super I/O chip that also includes the serial and parallel interfaces, among other things. Even though the floppy controller can be found in the Super I/O chip on the motherboard, it is still interfaced to the system via the ISA bus and functions exactly as if it were a card installed in an ISA slot. These built-in controllers are typically configured via the system BIOS Setup routines and can be disabled if an actual floppy controller card is going to be installed.

Whether it is built in or not, each primary floppy controller uses a standard set of system resources:

• IRQ 6 (interrupt request)

• DMA 2 (direct memory address)

• I/O ports 3F0–3F5, 3F7 (input/output)

These system resources are standardized and generally not changeable. This usually does not present a problem because no other devices will try to use these resources (which would result in a conflict).

Unlike the IDE interface, the floppy disk controller has not changed much over the years. Virtually the only thing that has changed is the controller's maximum speed. As the data density of floppy disks (and their capacity) has increased over the years, the controller speed has had to increase, as well. Nearly all floppy disk controllers in computers today support speeds of up to 1 megabit per second (Mbps), which supports all the standard floppy disk drives. 500 kilobits per second (Kbps) controllers can support all floppy disk drives except the 2.88MB extra high-density models. Older computers used 250Kbps controllers that could support only 360KB 5 1/4'' and 720KB 3 1/2'' drives. To install a standard 1.44MB 3 1/2'' drive in an older machine, you might have to replace the floppy controller with a faster model.

Tip The best way to determine the speed of the floppy disk drive controller in your computer is to examine the floppy disk drive options provided by the system BIOS.

Even if you do not intend to use a 2.88MB floppy disk drive, you still might want to ensure that your computer has the fastest possible controller. Some of the older tape drives on the market use the floppy disk interface to connect to the system, and in this case, the controller has a profound effect on the overall throughput of the tape drive. Although traditional floppy controller cards and multi I/O cards have provisions for two floppy drives—A: and B:—many recent systems that integrate Super I/O features into the South Bridge chip on the motherboard support only a single floppy drive.

The Faceplate

The faceplate, or bezel, is the plastic piece that comprises the front of the drive. This piece, usually removable, comes in various colors and configurations.

Most floppy drive manufacturers offer drives with matching faceplates in gray, beige, or black and with a choice of red, green, or yellow activity LEDs as well. This enables a system builder to better match the drive to the aesthetics of the case for a seamless, integrated, and more professional look.

Connectors

Nearly all floppy disk drives have 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 4-pin inline connector (called Mate-N-Lock by AMP) in both large and small styles is used for power (see Figure 11.5), and a 34-pin connector in both edge and pin header designs is used for the data and control signals. Typically, 5 1/4'' drives use the large-style power connector and the 34-pin edge-type connector, whereas most 3 1/2'' 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 on the Vendor List on this book's DVD.

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. Note that the pin-to-signal designations on the small connector are the opposite of those on the large connector.

One common problem with installing 3 1/2'' drives in older systems, or in some cases adding a second drive to newer systems, is that the power supply has only one or none of the small-style power connectors used by the smaller drives. An adapter cable that converts the large-style peripheral power connector to the proper small style connector used on most 3 1/2'' drives is available from Dalco (www.dalco.com) under p/n 47425 and from other sources.

Most standard PCs use 3 1/2'' 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'' drives installed in a 5 1/4'' frame assembly with a special adapter built in that enables you to use the larger power connector and older edge-type signal connectors. External floppy drives are available in USB or parallel interfaces. For more information on these interfaces, see Chapter 17.

The Floppy Disk Controller Cable

The 34-pin connector on a floppy disk drive takes the form of either an edge connector (on 5 1/4'' drives) or a pin connector (on 3 1/2'' drives). The pinouts for the floppy controller connector are shown in Table 11.1.

Table 11.1. Floppy Disk Drive Controller Connector Pinout

Pin	Signal	Pin	Signal
1	Ground	2	DD/HD Density Select
3	Key[1]	4	Reserved (Unused)
5	Key[1]	6	ED Density Select[2]
7	Ground	8	Index
9	Ground	10	Motor-On 0 (A:)
11	Ground	12	Drive Select 1 (B:)
13	Ground	14	Drive Select 0 (A:)
15	Ground	16	Motor-On 1 (B:)
17	Ground	18	Direction (Stepper motor)
19	Ground	20	Step Pulse
21	Ground	22	Write Data
23	Ground	24	Write Enable
25	Ground	26	Track 0
27	Ground	28	Write Protect
29	Ground	30	Read Data
31	Ground	32	Head Select
33	Ground	34	Disk Change

^[1] Controllers and drives can use one, both, or no key (missing) pins.

^[2] For controllers supporting ED (Extra-high Density 2.88M) drives only; otherwise unused.

The cable used to connect the floppy disk drive(s) to the controller on the motherboard is quite strange. To support various drive configurations, the cable might have up to five connectors on it—two edge connectors and two pin connectors to attach to the drives and one pin connector to connect to the controller. The cable has redundant connectors for each of the two drives (A and B) supported by the standard floppy disk drive controller, so you can install any combination of 5 1/4'' and 3 1/2'' drives (see Figure 11.6).

In addition to the connectors, the cable has a special twist that inverts the signals of wires 10–16. These are the wires carrying the Drive Select (DS) and Motor Enable signals for each of the two drives. Older floppy disk drives have DS jumpers designed to enable you to select whether a given drive should be recognized as A or B (really old ones allow a third and fourth setting as well).

You might not even know that these jumpers exist because the twist in the cable prevents you from having to adjust them, and as such they have been eliminated from most newer drives. When installing two floppy disk drives in one system (admittedly a rarity nowadays), the cable electrically changes the DS configuration of the drive that is plugged in after the twist. Thus, the 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). The adoption of this cable has enabled the use of a standard jumper configuration for all floppy disk drives, regardless of whether you install one or two drives in a computer.

If you install only a single floppy disk drive, you use the connector after the twist, which causes the drive to be recognized as drive A. Although it's seldom necessary today, many computer BIOS setup programs have an option enabling you to swap drives A: and B: without adjusting drive cables. If you have a computer with the old 5 1/4'' floppy as B: for use with old software and need to use it as A:, you can use this BIOS option, if present, to change the drive setup without opening your system.

Secrets of the Cable Twist

The original Shugart SA400 floppy interface was designed to support up to four drives on a single cable. However, IBM modified the controller pinout so it could make a less-expensive controller that would work with only two drives and at the same time eliminate the need to change drive select jumpers on the drives.

If you check the documentation for older floppy drives and controllers using the industry-standard Shugart SA400 interface, you will see that they incorporate four separate drive select signals (DS0/DS1/DS2/DS3 on pins 10/12/14/6, respectively) and that pin 16 is the Motor-On (also called Motor Enable) signal shared by all drives (see Table 11.2).

Table 11.2. Industry-Standard Shugart Floppy Drive Interface

Pin	Signal
10	Drive Select 0 (A:)
12	Drive Select 1 (B:)
14	Drive Select 2 (C:)
16	Motor-On (all drives)

Drive Select 3 (D:) would be pin 6, but I am showing only the pins that will be involved in the cable twist because PCs support only up to two drives anyway and the twist is where the confusion lies.

When the floppy controller wants to talk to a drive, it raises the drive select line (DS0 for A: and DS1 for B:) as well as the Motor-On line. Only the selected drive responds and turns on its spindle motor. The controller then uses the other signals to seek, read, write, and so on; only the selected drive responds.

IBM, however, did not use the industry-standard Shugart floppy interface exactly as it was. Instead, the IBM floppy controller provides only two Drive Select signals and puts DS0 (A:) where DS2 (C:) was originally supposed to be. IBM also provided separate Motor-On signals for drives 0 and 1 (A: and B:) on pins 10 and 16, respectively (see Table 11.3). Note that pin 10 was originally supposed to be used for DS0, and not a Motor-On signal for drive 0 (A:). Also, pin 16 was supposed to be used as a common Motor-On line for all drives, not a private Motor-On line for drive 1 (B:).

Table 11.3. IBM's Modified Shugart Floppy Controller Interface

Pin	Signal
10	Motor-On 0 (A:)
12	Drive Select 1 (B:)
14	Drive Select 0 (A:)
16	Motor-On 1 (B:)

As you can see, the pinout of the IBM floppy disk controller does not match the pinout of the standard floppy drives! Then, how does it work?

Drive B: works fine as is: The DS1 (Drive Select B:) signal comes from the controller on pin 12 and is expected at the drive on pin 12. The Motor-On 1 (Motor B:) signal comes from the controller on pin 16 and is expected at the drive on pin 16. This is why Drive B: works with a straight-through cable. The only concern is that the drive must be jumpered as DS1, which is how all PC floppy drives are set up by default.

What about Drive A:? This is where the twist comes into play, along with the fact that Drive A: is also supposed to be jumpered as DS1 (same as Drive B:).

Here's how it works: The twist in the cable swaps pins 10 and 16, as well as pins 12 and 14. When accessing Drive A:, the DS0 (Drive Select A:) signal comes from the controller on pin 14 and is swapped by the twist to pin 12 at the drive, which is DS1 (Drive Select B:). This works because the A: drive is also jumpered to respond to DS1, the same as the B: drive. The Motor-On 0 (Motor A:) signal comes from the controller on pin 10 and is swapped by the twist to pin 16 at the drive, which is the Motor-On signal for all drives. As you can see, Drive A: would then work fine.

The interface between the original IBM floppy controller and the standard floppy drive does not quite match. The twisted cable combined with setting both drives to DS1 allows things to work properly. This controller and cable twist design is standard in all PCs since the IBM original.