6.2 DIGITAL SYSTEM COMPONENTS 313most common disks have 11-in diameters and 200 tracks (concentric rings of data) per surface,
numbered from 0 to 199, starting with the outside perimeter of the disk, with a typical track
packing density of 4000 bits per inch. Disks are mounted on a common spindle, and all disks
rotate at a typical speed of 3600 revolutions per minute (rpm). A typical disk has 17 sectors of
fixed size per track and 512 bytes (1 byte=8 bits) of information per sector. Any desired sector
can be quickly accessed.
Floppy disks, also known asflexible disks,are the low-cost, medium-capacity, nonvolatile
memory devices made of soft flexible mylar plastic with magnetically sensitive iron-oxide coating.
The original 8-in standard floppy is no longer in popular use. The 5^1 / 4 -in minifloppy has a disk
and a disk jacket covering the mylar media for protection, along with a write-protect notch and
index hole. The present-day minifloppy disks are either double-sided/double-density (DS/DD)
with 9 sectors per track and 40 tracks per side or double-sided/quad-density (DS/QD) with 9 or
15 sectors per track and 80 tracks per side. In a DS/DD minifloppy disk, about 720 kbytes of data
can be stored; whereas in a DS/QD mini-floppy disk, about 1–2 Mbytes of data can be stored.
The 3^1 / 2 -in microfloppy disk, also known as a microdiskette, is enclosed in a rigid protective
case and is provided with a write-protect notch. Microdiskettes are recorded in quad-density
format with a capacity of 2 Mbytes; 4- to 16-Mbyte 3^1 / 2 -in diskettes are being developed. Also,
2-in diskettes are introduced in electronic cameras and portable personal computers.
Magnetic tapesare ideal devices for storing vast quantities of information inexpensively.
However, the access time is slow because the entire tape must be read sequentially. The most
commonly used tapes are ½-in wide, 2400 or 3600 ft long, and contained in a long 10½-in reel.
Tape densities of 200, 556, 800, 1600, 6250, and 12,500 bits per inch (BPI) are standard.
In addition to these magnetic storage devices, two newer types of secondary storage have
come into use:winchester disksandvideodisks(also known asoptical disks). The former are sealed
modules that contain both the disk and the read/write mechanism, requiring little maintenance and
allowing higher-density recording. The latter have been introduced recently, with high reliability
and durability and a storage capacity of 1 Gbyte of data (equivalent to almost 400,000 typewritten
pages of information). A typical 14-in optical disk has 40,000 tracks and 25 sectors per track,
with each sector holding up to 1 kbyte of information. While a write-once optical-disk drive is
currently available, a read-and-write drive is being developed.
Display Devices
Display devices can be categorized as on/off indicators, numeric, alphanumeric, or graphical
displays. They may also be classified as active and passive devices. Active display devices emit
light, such as light-emitting diodes (LEDs), whereas passive display devices, such as liquid-crystal
displays (LCDs), reflect or absorb light.
LIGHT-EMITTINGDIODE(LED)
This is a reliable, rugged, and inexpensive semiconductor display device requiring about 10 mA
of current flow for full illumination. An LED is shown in Figure 6.2.17. It is available in red,
yellow, or green color, in common sizes of T-1 and T-1^3 / 4 , where the number after the T denotes
the diameter of the lamp in units of eighths of an inch. Two-color LEDs are also available.
LIQUID-CRYSTALDISPLAY(LCD)
This display needs only microwatts of power (over a thousand times less than a LED) and is used
in such devices as electronic wristwatches. With the application of an electric field the molecules