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
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