magnetic field that magnetizes the particles of the disk's recording surface directly under the magnetic head. The pulse can be one of two polarities, positive or negative.">
RECORDING DIGITAL DATA ON MAGNETIC DISKS
Digital data is stored on a magnetic disk using magnetic pulses. These pulses are generated by passing a frequency modulated (FM) current through the disk drive's magnetic head. This FM current generates a magnetic field that magnetizes the particles of the disk's recording surface directly under the magnetic head. The pulse can be one of two polarities, positive or negative.
Digital data isn't just recorded onto a magnetic disk as-is. Instead, it's encoded onto the disk. Three of the most popular encoding methods are (1) frequency modulation (FM), (2) modified frequency modulation (MFM), and (3) run length limited (RLL). The following paragraphs describe each of these encoding methods.
FREQUENCY MODULATION (FM) ENCODING
The FM method of encoding digital data onto a disk uses two pulse periods to represent each bit of data (a pulse period is the time span of one pulse). The first pulse period always contains a clock pulse. The second pulse-period may, or may not, contain a data pulse. If the digital data is a "1," a data pulse will be present in the second pulse-period.
But, if the digital data is a "0," then there's no pulse present.
Figure 8-7 shows this. The clock pulse, which is always present, tells the disk drive's interface that the next pulse is a data pulse. It is used to compensate for changes in the disk's rotation speed.
Figure 8-7. - Frequency-modulation (FM) encoding.
The FM method of encoding is old, and isn't used much anymore.
You'll only see it in some of the older single-sided, single-density floppy disk drives, and in some of the older military hard disk drives.
MODIFIED FREQUENCY-MODULATION (MFM) ENCODING
The MFM method of encoding digital data onto a disk is more popular because it is more efficient and more reliable than straight FM encoding.
MFM encoding still uses two pulse periods, but uses a lot fewer pulses to store the digital data onto the disk. It does this in two ways:
It does away with the clock pulse that the FM method uses.
It stores a digital "1" by generating a no-pulse and a pulse in the two pulse periods. It stores a digital "0" as either a pulse and a no-pulse if the last bit was a "0," or as two no-pulses if the last bit was a "1." Figure 8-8 shows this.
Figure 8-8. - Modified frequency-modulation (MFM) encoding.
RUN LENGTH-LIMITED (RLL) ENCODING
The RLL method of encoding digital data onto a disk is actually a refinement of the MFM encoding method. As its name implies, RLL limits the run length (distance) between pulses (also called flux reversals) on a hard disk. The basic theory of RLL encoding is that you can store more data in less space if you reduce the number of flux reversals (or pulses) that you must record.
There are several versions of the RLL encoding method, the most popular version being the 2,7 RLL. This means that no fewer than 2 no-pulses and no more than 7 no-pulses can occur between pulses.
MAGNETIC DISK DRIVE TRANSPORTS
Magnetic disk drive transports, like magnetic tape drive transports, move the magnetic disks across the magnetic heads and protect the disks from damage. The following paragraphs will (1) introduce you to the characteristics of both floppy and hard disk drive transports, and (2) describe their preventive maintenance requirements.
FLOPPY DISK DRIVE TRANSPORTS
Floppy disk drive transports contain the electromechanical parts that (1) rotate the floppy disk, (2) write data to it, and (3) read data from it. Figure 8-9 shows a typical floppy disk drive transport. Four of the drive transport's more important parts are the
Figure 8-9. - Typical floppy disk drive transport.
drive motor/spindle assembly, head arm assembly, actuator arm assembly, and drive electronics circuit board.
Drive Motor/Spindle Assembly
The spindle in this assembly holds the floppy disk in place while it spins. The drive motor spins the spindle at 300 to 600 RPM, depending on the type of floppy disk drive. The following is a list of the types of floppy disk drives and the spinning speeds of their spindles.
The spindle of a 5-1/4" disk drive is activated and released by a small arm that's mounted on the front of the disk drive. You must turn the small arm to lock and release the floppy disk.
The spindle of a 3-1/2" disk drive is activated when the floppy disk is inserted into the disk drive. It's released by a push-button that's located on the front of the disk drive. When you push this button, the floppy disk is released and pops out of the disk drive.
Head Arm Assembly
This part of a floppy disk drive transport holds the magnetic read/write heads. There are four heads on a head arm assembly, two write heads and two read heads - one of each for each recording surface. The head arm assembly is attached to the actuator arm assembly.
Actuator Arm Assembly
The actuator arm assembly positions the magnetic heads over the recording surface of the floppy disk. It does this by using a special type of dc motor called a stepper motor. This motor, which can be moved in very small steps, allows the read/write heads to be moved from track to track as needed to write data onto and read data off of the floppy disk.
This circuit board contains the circuitry which (1) controls the electromechanical parts of the disk drive transport, (2) writes data to and reads data from the floppy disk, and (3) interfaces the floppy disk drive to the host computer.
HARD DISK DRIVE TRANSPORTS
Hard disk drive transports contain the electromechanical parts that (1) rotate the hard disk platter, (2) write data to it, and (3) read data from it.
There are two types of hard disk drive transports, fixed disk and cartridge disk. Fixed disk drive transports use non-removable hard disk platters. Cartridge-disk drive transports use removable hard disk platters that are built into protective cartridges.
These two transports serve very different purposes, but they each contain the same basic parts. Figure 8-10 shows a typical hard disk drive transport. Four of the more important parts of a hard disk drive transport are the drive motor/spindle assembly, head arm assembly, actuator arm assembly, and drive electronics circuit board.
Figure 8-10. - Typical hard disk drive transport.
Drive Motor/Spindle Assembly
This assembly holds and spins the hard disk pack. The spindle assembly holds the hard disk pack in place and the drive motor spins the spindle at 3600 RPM. On cartridge disk drives, the spindle is electronically disengaged to release the disk pack so it can be removed.
Head Arm Assembly
This part of the hard disk drive transport holds the magnetic read/write heads. There is a separate head arm assembly for each of the hard disk platters in the disk pack. Each assembly has four magnetic heads, two write heads and two read heads-one pair of heads for each surface of the hard disk platter. The head arm assembly is attached to the actuator arm assembly.
Actuator Arm Assembly
This part of the hard disk drive transport positions the magnetic heads so they can write data to and read data from the correct track of the hard disk. It does this by using either a stepper motor or a voice coil servo. A stepper motor is a special type of dc motor which can be moved in very small steps to accurately position the magnetic heads. A voice coil servo by itself cannot move the magnetic heads from track to track. Instead, it must use special signals called servo signals to make sure it's positioning the heads where they should be. The servo signals are pre-recorded signals which are stored on either the same hard disk platter as the data or on a separate hard disk platter.
The voice coil servo method of moving the magnetic heads to the correct track of the hard disk is also called embedded servo control. This type of control is becoming very popular because voice coil actuator assemblies can position the magnetic heads much quicker and more accurately than dc stepper motors.
This circuit board contains the circuitry which (1) controls the electromechanical parts of the hard disk drive transport, (2) writes data to and reads data from the hard disk, and (3) interfaces the hard disk drive to the host computer.
Q.18 What are the three most popular methods for encoding digital data onto magnetic