Character Codes

• BCD
  • Binary Coded Decimal
  • Probably should have been in the prevous section
  • Use 4 bits for each digit, encode only as decimal
  • 458 = 0100 0101 1000_BCD
  • Special codes 1100_BCD = +
  • Special codes 1101_BCD = -
• EBCDIC
  • Old IBM character code
  • Extended Binary Coded Decimal Interchange Code
  • Had holes between i-j, r-s (i++ is not j)
  • Yuck
• ASCII
  • 7 bit code, one parity bit fits a byte
  • IBM with the pc used extra bit for more characters
  • 1000001 = A, 110001 = a (add 20 hex)
  • Parity
    • Even - count the 1's and add one more if we need to make the count even.
    • Odd - count the 1s and add one more if we need to make the count odd.
• Unicode
  • 16 bit (65536 possible) code.
  • Internationalize things.
  • 000000000ASCII

Transmitting Data

• Transmitting data is less stable than storing data in memory
• Distortions due to
  • Media
  • Electronics
  • The Sun
• Timing of the signal will drift, so we need to build a clock into the signal (ie both systems don't know the lenght of a pulse)
• It actually may be hard to detect what is positive and what is negative
• Some coding forms have been developed to deal with this.
  • A simple example
    • The word Dan is 44616E (0100 0100 0110 0001 0110 1110)
    • It might be encoded as follows
    • 
    • This is called Non-Return-to-Zero, (NRZ)
      • 1 is +3 to +5 volts
      • 0 is -3 to -5 volts
      • Bits are represented directly
    • The problem is judging the lenght of each bit, no clock is carried by this signal
    • 
    • This means if the clocks get out of sync, for example the receiver counts the 6 (0110) as a (0101), ie merges the two 1 bits, we are lost. (And can't recover)
  • Non-Return-to-Zero-Invert encoding (NRZI)
    • Each time a 1 is encountered transition from low to high or high to low.
    • This provides a clock pulse on a 1.
    • 
    • We still have a problem with sync on the 0 bits, but not on the 1 bits.
  • Phase Modulation or Manchester (PM)
    • Each bit does a transistion,
    • 1 go from low to high
    • 0 go from high to low
    • There is a transition in the middle of every bit.
    • 
    • Unfortunately, it requires double the bit rate 11 -> 0101
  • Frequency Modulation (FM)
    • Like PM
    • Transition in the middle of a 1,
    • Transition at the end of a 0
  • Modified Frequency Modulation (MFM)
    • Transition in the middle of a 1, at the end of a 0
    • 
  • These are just a few examples, Hilman in 475 will be happy to help you pick up a few more.

Error Detection/Correction

• Error detection - finding that data transmissitted/stored has an error
• Error correction - fixing that error
• Error detection is much easier than correction
• Parity bits.
• CRC
  • Cyclic Redundancy Check
  • We add extra information the the message so we can calculate if the bits are correct.
  • Again, see Hillman.