The FDD interface and power requirements are completely standardized, as follows:
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-inch FDDs and 720 KB 3.5-inch 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-inch FDDs. Run BIOS Setup to determine which FDD types a given system supports.
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FDD data cables use a standard 34-pin pinout (see Table 6-2), but connectors vary. 5.25-inch drives use a card-edge connector. 3.5-inch 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.
Pin # |
Signal |
Pin # |
Signal | |
---|---|---|---|---|
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 |
Obsolete 5.25-inch drives accept the larger Molex power connector. Current 3.5-inch drives accept the smaller Berg power connector, shown in Figure 6-2. A chassis that permits a 3.5-inch drive to be installed in a 5.25-inch 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-inch 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-inch 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 the 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 6-3, which means that a drive connected to it must be set to the second DS position if it is to be recognized as A:.