4.  Digital system codes are
› BCD code
› Excess-3 code
› EBCDIC code
› Error detection code
› UNI CODE
› ASCII code
› Extended ASCII code
› Gray code

5.  It should have some desirable properties
 Ease of coding
 To increase efficiency of transmission
 Ease in arithmetic operations
 Minimum use of hardware
 Error detection property
 Ability to prevent wrong output during
transitions

6. BCD – Binary Coded Decimal
 BCD is a convention for mapping binary
numbers to decimal numbers & for Decimal
to binary numbers.
 When the decimal numbers are represented
in BCD, each decimal digit is represented by
the equivalent BCD code.
 Example :BCD Representation of Decimal
6349
6 3 4 9
0110 0011 0100 1001
6

7.  0-9 decimal digits need to be
represented in a binary code which must
contain at least four bits.
› Four bits can make upto 16 different
combinations.
› Only first 10 combinations are used. (0-9)
 BCD is different from binary representation.
› 15 in binary is 1111

8.  Note: the Digit Bit pattern
following bit 0 0000
patterns are not 1 0001
used: 2 0010
3 0011
1010 4 0100
1011 5 0101
1100 6 0110
1101 7 0111
1110 8 1000
1111 9 1001

9.  Example:
lets add 56 & 98
56 0101 0110
+ 98 1001 1000
=154 1110 1110 Not in BCD
0110 0110 add 6
1 0101 0100
1 5 4

10. EXCESS 3 CODE
 The excess-3 code is obtained by adding
3 (0011) to the corresponding BCD
equivalent binary number.
10

11. EXCESS 3 CODE
Decimal BCD Excess-3
Number Number Number
0 0000 0011
1 0001 0100
2 0010 0101
3 0011 0110
4 0100 0111
5 0101 1000
6 0110 1001
7 0111 1010
8 1000 1011
9 1001 1100
11

12.  Extended BCD Interchange Code
 8-bit code
 It contains the numbers from 0 to 28-1
 Developed by IBM
 Rarely used today
 IBM mainframes only

13. We need a mechanism of correcting the
errors that occur
 It is not always possible or may prove to be
expensive
 It is necessary to know if an error occurred
 If an occurrence of error is known, data
may be retransmitted
 Data integrity is improved by encoding
 Encoding may be done for error correction.

14.  Error detection code detect errors during
transmission of data from one location to
another.
 Error rate cannot be reduced to zero
 To achieve error-detection we use a
parity bit.

15.  A parity bit is an extra bit included with a
message to the total number of 1’s
transmitted either odd or even.
 Parity bit allows us only to detect the
presence of one bit error in a group of bits.
 It does not enable us to exactly locate the
bit that changed.
 Parity bit scheme can be extended to
locate the faulty bit In a block of
information.

17.  Odd parity bit generator can be formed
by inverting the output of the Even parity
bit generator.

20.  Gray coding is an important code and is
used for its speed, it is also relatively free
from errors.
 Gray coding avoids this since only one bit
changes between subsequent numbers.
 In pure binary coding then counting from 7
(0111) to 8 (1000) requires 4 bits to be
changed simultaneously.
 Gray code is used to represent the digital data
when it is converted from analog data.

21. Decimal Binary Gray Code Decimal Binary Gray Code
0 0000 0000 8 1000 1100
1 0001 0001 9 1001 1101
2 0010 0011 10 1010 1111
3 0011 0010 11 1011 1110
4 0100 0110 12 1100 1010
5 0101 0111 13 1101 1011
6 0110 0101 14 1110 1001
7 0111 0100 15 1111 1000

22.  Scan the Gray code word from left to right
 All the bits of the binary code are the same
as those of the Gray code until the first 1 is
encountered, including the first 1
 1’s are written until the next 1 is
encountered, in which case a 0 is written.
 0’s are written until the next 1 is
encountered, in which case a 1 is written.
 Examples
Gray code : 1 1 0 1 1 0
Binary code: 1 0 0 1 0 0

23. UNICODE is a 16-bit code for
representing alphanumeric data.
 Developed by a consortia(An association or a
combination, as of businesses, financial institutions, or investors, for
the purpose of engaging in a joint venture.)
 With 16 bits, can represent 216 or 65536
different symbols.
 16 bits = 2 Bytes per character.
 UNICODE used by Web browsers and
Java these days.

24. ASCII Code
• The standard binary code for representation of
alphanumeric characters is ASCII
•ASCII (American Standard for Information
Interchange)
•It hands not only numbers but letters and
special characters
• Uses 7 bits to code 128 characters
•In ASCII, every letter, number, and
punctuation symbol has a corresponding
number, or ASCII code 24

25. This code is a popular code used to
represent information sent as character-
based data. It uses 7-bits to represent:
› 94 Graphic printing characters.
› 34 Non-printing characters
 Some non-printing characters are used for
text format (e.g. BS = Backspace, CR =
carriage return)
 Other non-printing characters are used for
record marking and flow control (e.g. STX
and ETX start and end text areas).

26. 95 Graphic codes
000 001 010 011 100 101 110 111
0000 NULL DLE 0 @ P ` p
0001 SOH DC1 ! 1 A Q a q
0010 STX DC2 " 2 B R b r
0011 ETX DC3 # 3 C S c s
0100 EDT DC4 $ 4 D T d t
0101 ENQ NAK % 5 E U e u
0110 ACK SYN & 6 F V f v
0111 BEL ETB ' 7 G W g w
1000 BS CAN ( 8 H X h x
1001 HT EM ) 9 I Y i y
1010 LF SUB * : J Z j z
1011 VT ESC + ; K [ k {
1100 FF FS , < L l |
1101 CR GS - = M ] m }
1110 SO RS . > N ^ n ~
1111 SI US / ? O _ o DEL

27. 33 Control codes
000 001 010 011 100 101 110 111
0000 NULL DLE 0 @ P ` p
0001 SOH DC1 ! 1 A Q a q
0010 STX DC2 " 2 B R b r
0011 ETX DC3 # 3 C S c s
0100 EDT DC4 $ 4 D T d t
0101 ENQ NAK % 5 E U e u
0110 ACK SYN & 6 F V f v
0111 BEL ETB ' 7 G W g w
1000 BS CAN ( 8 H X h x
1001 HT EM ) 9 I Y i y
1010 LF SUB * : J Z j z
1011 VT ESC + ; K [ k {
1100 FF FS , < L l |
1101 CR GS - = M ] m }
1110 SO RS . > N ^ n ~
1111 SI US / ? O _ o DEL

28. Alphabetic codes
000 001 010 011 100 101 110 111
0000 NULL DLE 0 @ P ` p
0001 SOH DC1 ! 1 A Q a q
0010 STX DC2 " 2 B R b r
0011 ETX DC3 # 3 C S c s
0100 EDT DC4 $ 4 D T d t
0101 ENQ NAK % 5 E U e u
0110 ACK SYN & 6 F V f v
0111 BEL ETB ' 7 G W g w
1000 BS CAN ( 8 H X h x
1001 HT EM ) 9 I Y i y
1010 LF SUB * : J Z j z
1011 VT ESC + ; K [ k {
1100 FF FS , < L l |
1101 CR GS - = M ] m }
1110 SO RS . > N ^ n ~
1111 SI US / ? O _ o DEL

29. Numeric codes
000 001 010 011 100 101 110 111
0000 NULL DLE 0 @ P ` p
0001 SOH DC1 ! 1 A Q a q
0010 STX DC2 " 2 B R b r
0011 ETX DC3 # 3 C S c s
0100 EDT DC4 $ 4 D T d t
0101 ENQ NAK % 5 E U e u
0110 ACK SYN & 6 F V f v
0111 BEL ETB ' 7 G W g w
1000 BS CAN ( 8 H X h x
1001 HT EM ) 9 I Y i y
1010 LF SUB * : J Z j z
1011 VT ESC + ; K [ k {
1100 FF FS , < L l |
1101 CR GS - = M ] m }
1110 SO RS . > N ^ n ~
1111 SI US / ? O _ o DEL

30. Punctuation, etc.
000 001 010 011 100 101 110 111
0000 NULL DLE 0 @ P ` p
0001 SOH DC1 ! 1 A Q a q
0010 STX DC2 " 2 B R b r
0011 ETX DC3 # 3 C S c s
0100 EDT DC4 $ 4 D T d t
0101 ENQ NAK % 5 E U e u
0110 ACK SYN & 6 F V f v
0111 BEL ETB ' 7 G W g w
1000 BS CAN ( 8 H X h x
1001 HT EM ) 9 I Y i y
1010 LF SUB * : J Z j z
1011 VT ESC + ; K [ k {
1100 FF FS , < L l |
1101 CR GS - = M ] m }
1110 SO RS . > N ^ n ~
1111 SI US / ? O _ o DEL

31.  Let us convert You & I, to decimal, hex and
binary using the ASCII code table :
› Y: 8910 5916 10110012
› o: 11110 6F16 11011112
› u: 11710 7516 11101012
› Space: 3210 2016 01000002
› &: 3810 2616 01001102
› Space: 3210 2016 01000002
› I: 7310 4916 10010012
› ,: 4410 2C16 01011002

32.  The term extended ASCII (or high ASCII)
describes eight-bit or larger character
encodings that include the standard
seven-bit ASCII characters as well as
others.

33. Extended ASCII