6.3 FDD Interface and Cabling
The FDD
interface and power requirements are completely standardized, as
follows:
- Controller
-
PC-class systems used a separate FDD controller card. XT-
and AT-class systems and some early 386s used a combination HDD/FDD
controller card. Current systems use an embedded FDD controller.
These controllers differ only in their maximum data rate, which
determines the FDD types they support. Early controllers run at 250
Kb/s, which supports only 360 KB 5.25" FDDs and 720 KB 3.5" FDDs.
Later controllers run at 500 Kb/s, which supports any standard FDD,
or at 1 Mb/s, which is required for 2.88 MB 3.5" FDDs. Run
BIOS Setup to determine which FDD types a
given system supports.
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If you must install a higher capacity FDD than the controller
supports—e.g., if you must salvage data from a hard drive in a
system whose old 5.25" 360 KB FDD has failed and you have only a 3.5"
HD FDD to replace it—you have two alternatives:
Remove or disable the on-board FDD
controller, and replace it with a third-party FDD controller that
supports the higher capacity FDD. Lie to the old system about what type
of FDD you are installing. For example, install a 1.44 MB FDD, but
tell the system that it is a 360 KB or 720 KB FDD. All FDDs run at
300 RPM, except 5.25" 1.2 MB FDDs, which run at 360 RPM. That means
that any ED, HD, or DD 3.5" FDD can emulate any lower capacity 3.5"
drive, as well as the 5.25" 360 KB FDD. Use blank DD diskettes, and
format them in the new drive. Copy data to the floppies and then
attempt to read them on another system. Some systems will happily
read such oddities as a 3.5" 360 KB diskette, but others will simply
return an "unknown media type"
message. If the latter occurs, use BIOS Setup on the good system to
reconfigure the FDD temporarily to the same settings as those on the
older system.
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- Data cable
-
FDD data cables use a standard 34-pin pinout
(see Table 6-2), but connectors vary. 5.25" drives use a card edge
connector. 3.5" drives use a header pin connector. Older standard FDD
cables have at least three connectors, one for the FDD interface and
two for drives. Many FDD cables have five connectors, with redundant
header pin and card edge connectors at each of the two drive
positions, allowing any type of FDD to be connected at either
position. Because many newer systems support only one FDD, new data
cables have only two connectors, one for the FDD interface and one
for the drive itself.
Table 6-2. Floppy disk drive cable pinouts
|
Odd pins (1-33)
|
Ground
|
|
20
|
Step pulse
|
|
2, 4, and 6
|
Not Used
|
|
22
|
Write Data
|
|
8
|
Index
|
|
24
|
Write Enable
|
|
10
|
Motor Enable A
|
|
26
|
Track 0
|
|
12
|
Drive Select B
|
|
28
|
Write Protect
|
|
14
|
Drive Select A
|
|
30
|
Read Data
|
|
16
|
Motor Enable B
|
|
32
|
Select Head 1
|
|
18
|
Direction (Stepper motor)
|
|
34
|
Disk Change
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- Power
-
Obsolete 5.25" drives accept the larger Molex
power connector. Current 3.5"
drives accept the smaller Berg power connector, shown in Figure 6-1. A chassis that permits a 3.5" drive to be
installed in a 5.25" bay typically includes a Molex-to-Berg adapter.
Pinouts and voltages for both of these power connectors are covered
in Chapter 26.
The BIOS identifies the drive as
A: or B: based on how the drive is jumpered and by
the cable position to which that drive connects. Older 5.25" drives
have four drive select (DS) jumper positions,
labeled DS0 through DS3 or DS1 through DS4. Later drives have only
two settings, labeled DS0/DS1 or DS1/DS2. Many recent 3.5" drives are
permanently set to the second DS position (DS1/DS2). The BIOS
recognizes a drive set to the first DS position (DS0/DS1) as
A: and a drive set to the second
DS position (DS1/DS2) as B:,
assuming that drive is connected to the controller with a
straight-through cable.
But a standard two-drive FDD cable has wires 10 through 16 twisted
between the first (middle) drive connector and the second (end) drive
connector, which effectively reverses the jumper setting on the drive
connected to the end connector. In other words, a drive that is
jumpered as the second drive (DS1/DS2 or B:) and
connected to the end connector is seen by the system as the first
drive (DS0/DS1 or A:) because of the twist.
Because many recent chipsets support only a single FDD, many recent
FDD cables have only two connectors, one for the motherboard FDD
interface, and the second for the single FDD. This cable has a twist,
shown in Figure 6-2, which means that a drive
connected to it must be set to the second DS position if it is to be
recognized as A:.
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