IT 2122

1
IT 212-2

• Part 5
• Chapters 17-19

2
Types of Common Ports

• Parallel
• Serial
• IDE
• USB
• SCSI

3
How a Parallel Port Works

• Usually used to connect printers
• Different signal commands travel over different
  specific lines to communicate information to the
  printer and PC
• When to prepare to accept data
• When the printer is too busy
• To acknowledge that data was sent
• When the printer runs out of paper
• Error conditions
• Printer reset
• Advance paper
• When data is complete

4
How a Parallel Port Works

• 25-36 pin connection
• PC side has 25, printer side has 36 with a
  different connector on each end
• Parallel ports can send several bits of data
  across eight parallel wires simultaneously
• Similar to a formation of soldiers
• Think of a platoon of soldiers marching 8 abreast
• When the platoon reaches a line on the ground, 8
  soldiers cross the line at one time, followed by
  the next 8 and so on
• Faster than serial ports

5
How a Serial Port Works

• Often referred to as a RS-232 port, a name given
  to it because of the protocol or language the
  cable speaks
• Sends data one bit at a time over a single
  one-way wire
• One line sends data, one receives data
• Other lines regulate how data travels over wires
  in the cable

6
How a Serial Port Works

• 9-25 pin connection
• One side has 9 pins, the other has 36 pins only
  9 are used
• Similar to parallel, specific signals travel over
  specific lines
• Pins crossover to other pins on either end e.g.
  pin 4 to pin 20 arrangement depends on the
  peripheral
• Signals always travel in the same direction on
  each cable either in the direction of the
  peripheral or in the direction of the PC
• Sends one bit at a time

7
How a Serial Port Works

• Think of soldiers in a single file line, one
  behind another
• When the soldier reaches a line on the ground, he
  and only he can cross at one time, then the next,
  the next and so on
• Different than the 8 simultaneously crossing on
  the parallel port
• Slower than parallel port

8
Universal Serial BusUSB

• USB controller inside the PC
• A set of specialized chips and connections act as
  an interface between software and hardware
• Applications, OS and drivers all send details of
  how items on the USB connection work via the USB
  Host Hub, located on the controller
• Unique pair of connections on the back of a PC
• Cable is much smaller and only contains 4 wires!

9
Universal Serial BusUSB

• Cables can connect to a USB hub capable of
  connecting to many other USB devices
• Expands the 2 ports on the PC to many more
• Each USB device can act as an extension to
  another USB device e.g. from a hub to a monitor,
  to speakers, to a keyboard and so on, up to 127
  devices

10
Universal Serial BusUSB

• USB cable contains 4 wires
• 2 for electrical power
• 2 for sending data and commands
• Two speeds of data transfer via USB cable
• High speed 12Mbs megabits a second
• Used for monitors, scanners, printers
• Low speed 1.5Mbs
• Used for keyboard or mice
• Serial ports send 100kbs
• Parallel ports send 2.5Mbs

11
Universal Serial BusUSB

• USB host controller polls all devices on the bus
  to see if they are ready to send or receive data
• This happens about 1 million times per second
• If a device is ready, a portion of the cables
  capacity is set aside for that device
• Each of the hosts messages begin with a token
  that identifies which device the data is intended
  for
• Messages go to all devices, ignored by all but
  the intended device
• Only when the USB controller back inside the PC
  is ready, will the device be allowed to send its
  data back to the PC

12
Universal Serial BusUSB

• USB can work with three types of data transfers
  and assigns bandwidth priorities according to the
  type of data that will travel on the cable
• Isochroous transfers real time data highest
  priority
• Used when there can be no interruption of the
  flow video or digital sound
• Interrupt transfers keyboard, mice, joystick
  second highest priority
• Used when the device generates an occasional
  interrupt
• Bulk transfers lowest priority
• Printers, scanners, digital still cameras

13
Integrated Drive ElectronicsIDE Connections

• Designed to overcome limitations in CMOS
  evolution
• Could not keep up with the current number of
  different hard drives
• Today virtually all PCs come with E-IDE
  Enhanced Integrated Drive Electronics controllers
• Connected from the motherboard to devices via a
  40 wire ribbon cable with two connectors on it

14
IDE Connections

• Each of the connections can connect to a hard
  drive
• E-IDE can control floppy drives, CDROMs, and tape
  drives
• Devices must be E-IDE compatible and mounted
  inside the PC
• For each pair on a single cable, one device is
  master and the other is slave
• This serves as a traffic light for signals
  destined for one or the other

15
Small Systems Computer InterfaceSCSI

• Commonly referred to as skuzzy
• Fastest and most versatile way to communicate to
  peripherals
• Single SCSI controller can handle up to 15
  devices
• Each device on the bus identified by a unique
  number

16
Small Systems Computer InterfaceSCSI

• Last device in the daisy chain must have a
  terminator to provide a ground for the port not
  used in the last device
• Cable is a flat ribbon cable with 50 lines
  inside
• Two speeds of data transfer
• Most common, 8 lines used to send data
• Capable of 10-20MB second

17
Small Systems Computer InterfaceSCSI

• Wide SCSI uses a 16 bit path, capable of 40MB
  second
• All data transfer managed by the SCSI controller
  card inside the PC
• Each wire of the ribbon cable has a specific
  purpose
• Some are for specific bits in a byte 0 bit
  parity bit
• Some are command lines that send specific
  commands like BSY, ANT, ACK

18
How a Keyboard Works

• Two types of keys used on keyboards
• Capacitive keys built around a spring that
  makes a clicking sound when depressed
• Hard-Contact keys mounted over a rubber dome
• Each transfers current in a slightly different
  manner

19
How a Keyboard Works

• When you press a key, you create a change in the
  current flowing through a circuit associated
  specifically with that key
• A microprocessor in the keyboard detects the
  increase and decrease in current from the key
  that was pressed
• Detecting both an increase and decrease, the
  processor can tell when the key was pressed and
  released

20
How a Keyboard Works

• Each key has a unique set of codes
• Processor can distinguish between the left and
  right shift keys
• To distinguish real signals from an aberrant
  current fluctuation, the keyboard is scanned
  hundreds of times per second
• Depending on which key is pressed, the processor
  will generate a scan code

21
How a Keyboard Works

• There are two scan codes for each key
• One for when it is depressed
• One for when it is released
• The keyboard processor sends an interrupt to the
  processor to tell it a scan code is waiting for
  it
• The interrupt tells the CPU to divert its
  attention to the IRQ
• The BIOS reads the scan code from the keyboard
  and tells the keyboard to dump the scan code from
  the keyboard buffer

22
How a Keyboard Works

• The BIOS translates the scan code into an ASCII
  code representing a character and sends the data
  through the CPU and onward to the screen
• If the scan code is one of the ordinary shift
  keys or special shift keys
• The BIOS will change two bytes in a special area
  of RAM
• Will maintain a record of which one was pressed,
  and what the associated modified command is

23
Computer Displays

• Super VGA
• LCD
• Digital Light Processing

24
Super VGA Display

• All signals sent to the monitor originate as
  digital signals
• But signals used by the monitor are analog
• The first step is to convert digital signals to
  the corresponding voltages for the three primary
  colors needed to generate color for a single
  pixel picture element
• Super VGA adapters can store up to 24 bits of
  data for a single pixel
• Low end is 16 bits/pixel or 16,000 colors high
  color
• High end is 24 bits/pixel or 16,777,216 colors
  true color

25
Super VGA Display

• Adapter sends signals to three electron guns
  located in the back of the monitors cathode ray
  tube CRT
• Gun shoots 3 streams of electrons
• One for each of the 3 primary colors
• Intensity controlled by the signals from the
  adapter
• Adapter also sends signals to the magnetic
  deflection yoke
• This focuses and aims the electron beams
• Signals also help determine the resolution of the
  monitor number of pixels horizontally and
  vertically and the refresh rate how frequently
  the screen is redrawn

26
Super VGA Display

• The beams pass through holes in a metal plate
  called a shadow mask
• The mask keeps the beams aligned with their
  targets on the inside of the CRTs screen
• Dot Pitch
• The measurement of how close the holes are to
  each other
• The closer the holes, the smaller the dot pitch,
  smaller the dot pitch, sharper the image

27
Super VGA Display

• The electrons from the gun strike phosphors on
  the inside of the screen
• Phosphors glow when struck by electrons
• One each for red, green and blue
• Stronger electron beam brighter color
• If each red, green, and blue phosphor is struck
  equally, the result is a white dot
• After the beam leaves a phosphor, it glows for a
  short time called persistence
• For an image to remain stable, phosphors must be
  must be reactivated repeatedly by the electron
  stream

28
Super VGA Display

• Raster Scanning
• After the beams make one sweep across the screen,
  the beam is turned off and the magnetic yoke
  refocuses the path back to the left edge of the
  screen just below the previous scan
• A complete sweep of the screen is called a field
• Upon completing a field, the beams return to the
  upper left corner to be redrawn or refreshed
• Refresh rate is normally about 60 times per second

29
Super VGA Display

• Interlacing
• When the adapter scans only every other line
  within each field
• Allows the adapter to create higher resolutions
  to be able to scan more lines with less expensive
  circuitry
• Normally creates fading of the phosphors that is
  noticeable resulting in screen flicker

30
LCD Screens

• Liquid Crystal Display
• Named for the fact that there is a layer of
  liquid crystal material sandwiched in between the
  front and back panels that make up the LCD panel
• Light emanating from a panel at the back of the
  screen spreads out in waves that vibrate in all
  directions

31
LCD Screens

• A polarizing filter at the back of the screen
  lets only light waves that are vibrating more or
  less horizontally pass through
• Liquid crystal cells make up the three primary
  colors of pixels
• The graphics adapter applies a varying charge to
  some of the cells and no charge to others

32
LCD Screens

• Where current is applied, long, rod shaped
  molecules that make up the liquid crystal
  material react to the charge by forming a spiral
• The greater the charge, the more the molecules
  twist
• The strongest charge will twist the molecules 90
  from the orientation of the molecules at the
  other end of the cell

33
LCD Screens

• Polarized light entering the cells from the rear
  is twisted along the spiral path of the molecules
• If a full charge was applied, the polarized light
  emerges vibrating at a 90 angle to its original
  alignment
• Light passing cells that have no charge emerges
  unchanged
• Cells with a partial charge twist the light some
  degree between 0-90

34
LCD Screens

• The light emerging from each liquid crystal cell
  passes through one of three color filters red,
  blue, or green arranged very close to each
  other
• The colored beams of light pass through a second
  polarizing filter that only passes waves that are
  vibrating more or less vertically
• Light that passed through a liquid crystal with a
  full charge is now oriented perfectly to pass
  through the second filter

35
LCD Screens

• Since the filter is not precise, some of the
  light waves pass through that were only partially
  charged
• Those without a charge are blocked completely
• Since there are millions of combinations of
  charged cells producing millions of combinations
  of twisted molecules, the color on the screen is
  able to represent millions of colors to our eyes

36
Digital Light ProcessingDLP

• DLP produces brighter, sharper images over a
  conventional projection system
• By shinning a light through a spinning wheel
  divided into red, blue and green sections, each
  color is produced 60 times a second
• After passing through the spinning disc, the
  light strikes a panel of 508,800 microscopic
  mirrors on the surface of a DLP chip
• The mirrors reflect the light through a lens and
  onto a screen

37
DLP

• Each mirror is attached to a flexible hinge that
  holds the mirror above a pair of electrodes and a
  circuit
• The electrodes are connected to an array of video
  memory bits that can represent a 1 or a 0
• A color pixel is turned on when a bit is written
  it video memory
• An electrical charge is sent to one of the
  electrodes
• The corner of the mirror nearest that electrode
  is pulled down

38
DLP

• Mirrors functioning as pixels turn one way to
  reflect light toward a screen
• Mirrors tilted in the opposite direction reflect
  light in the opposite direction where it is
  absorbed they represent pixels that are turned
  off
• Hues are created by how often the source of light
  is turned on and off while the three filters pass
  in front of it

39
DLP

• The color wheel and Red, Green Blue RGB data
  are synced so that the time a mirror reflects any
  one of the three primary colors is in proportion
  to the amount of that color in the mixture that
  is perceived by the human eye
• Each mirror can tilt on or off in less than 20
  milliseconds
• Each mirror can generate 16.7 million colors