Anatomy of a Floppy Disk Drive

Inside a Floppy Disk Drive

In Figure 3, you can see the main floppy disk drive components.

Figure 3: Main floppy disk drive components.

The floppy disk drive has four sensors:

• Disk Type: 1.44 MB floppy disks have an extra hole over 720 KB disks, so the drive can “feel” if the disk is a 720 KB or a 1.44 MB floppy.
• Disk Presence: This sensor is used for the drive to know if there is a floppy disk inside it or not.
• Write-protect: To enable or disable writing data to the disk, depending if you closed or opened the write-protect hole on the floppy disk.
• Track zero: When the head set triggers this sensor it means that it reached track zero, which is the first track on the disk.

It is very interesting to note that the motor that moves the heads is an open loop system – contrary to hard disk drives, which use a closed loop system –, i.e., it doesn’t know if it positioned the heads on the right location! When the floppy disk drive is first accessed, the heads are retracted down to track zero, so the controller (located on the motherboard, not the one inside the drive) knows that the heads are in the correct place. When the controller wants to move the heads to track 40, for example, it sends 40 commands “move 1 track ahead” to the drive and keep the record that the heads are positioned over track 40 inside its internal register. So who keeps track where the heads are located is the controller on the motherboard, not the floppy disk drive.

The motor used to move the heads is a step motor. A step motor is a motor that rotates a fixed angle, so when it is connected to power, it will only move a little bit. In the case of a floppy disk drive, this angle (or “little bit”) corresponds to the distance between each track of the floppy disk. So, giving a power pulse on this motor will make it jump ahead or back one track.

Floppy disk drives have two motors, one for moving the heads (the step motor) and another one for rotating the disk at a fixed speed (spindle motor, which is a servo motor). On a floppy disk drive the spindle motor rotates at 360 rpm, far slower than a hard disk drive. That’s why floppy disks are far slower than hard disks and also why floppy disk drives don’t need to be a sealed system like hard disk drives: a particle of dust on a floppy disk won’t do anything to its surface, due to its very low rotating speed.

In Figure 4, you can see the back view of the floppy disk drive.

Figure 4: Inside a floppy disk drive, back view.