Page 78 - A Practical Guide from Design Planning to Manufacturing

54 Chapter Two

built-in controllers, so the earliest version of ATA is usually referred to
by the name Integrated Drive Electronics (IDE). Later increases
in bandwidth were called Enhanced IDE (EIDE) and Ultra-ATA. The
most common alternative to ATA is Small Computer System Interface
(SCSI pronounced “scuzzy”). More commonly used in high performance
PC servers than desktops, SCSI drives are also often used with Macintosh
computers. Increasing the performance of the fastest ATA or SCSI bus
standards becomes difficult because of the need to synchronize all the
data bits on the bus and the electromagnetic interference between the
different signals.
Beginning in 2004, a competing solution is Serial ATA (SATA), which
transmits data only a single bit at a time but at vastly higher clock fre-
quencies, allowing higher overall bandwidth. To help keep sender and
receiver synchronized at such high frequencies the data is encoded to
guarantee at least a single voltage transition for every 5 bits. This
means that in the worst case only 8 of every 10 bits transmitted represent
real data. The SATA standard is physically and electrically completely
different from the original ATA standards, but it is designed to be soft-
ware compatible.
Although most commonly used with hard drives, any of these stan-
dards can also be used with high-density floppy drives, tape drives, or
optical CD or DVD drives. Floppy disks and tape drives store data mag-
netically just as hard drives do but use flexible media. This limits the
data density but makes them much more affordable as removable media.
Tapes store vastly more than disks by allowing the media to wrap upon
itself, at the cost of only being able to efficiently access the data serially.
Optical drives store information as pits in a reflective surface that are
read with a laser. As the disc spins beneath a laser beam, the reflection
flashes on and off and is read by a photodetector like a naval signal light.
CDs and DVDs use the same mechanism, with DVDs using smaller,
more tightly packed pits. This density requires DVDs to use a shorter-
wavelength laser light to accurately read the smaller pits.
A variety of writable optical formats are now available. The CD-R
and DVD-R standards allow a disc to be written only once by heating a dye
in the disc with a high-intensity laser to make the needed nonreflective
dots. The CD-RW and DVD-RW standards allow discs to be rewritten
by using a phase change media. A high-intensity laser pulse heats a spot
on the disc that is then either allowed to rapidly cool or is repeatedly
heated at lower intensity causing the spot to cool gradually. The phase
change media will freeze into a highly reflective or a nonreflective form
depending on the rate it cools. Magneto-optic (MO) discs store information
magnetically but read it optically. Spots on the disc reflect light with a
different polarization depending on the direction of the magnetic field.
This field is very stable and can’t be changed at room temperature, but