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# Codes

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• Excess-3 have a self-complementing property
• Check It
• ### Codes

1. 1.  Digital system codes are › BCD code › Excess-3 code › EBCDIC code › Error detection code › UNI CODE › ASCII code › Extended ASCII code › Gray code
2. 2.  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
3. 3. 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
4. 4.  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
5. 5.  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
6. 6.  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
7. 7. EXCESS 3 CODE The excess-3 code is obtained by adding 3 (0011) to the corresponding BCD equivalent binary number. 10
8. 8. 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
9. 9.  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
10. 10. 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.
11. 11.  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.
12. 12.  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.
13. 13.  Odd parity bit generator can be formed by inverting the output of the Even parity bit generator.
14. 14.  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.
15. 15. 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
16. 16.  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
17. 17. 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.
18. 18. ASCII Code• The standard binary code for representation ofalphanumeric characters is ASCII•ASCII (American Standard for InformationInterchange)•It hands not only numbers but letters andspecial characters• Uses 7 bits to code 128 characters•In ASCII, every letter, number, andpunctuation symbol has a correspondingnumber, or ASCII code 24
19. 19. 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).
20. 20. 95 Graphic codes 000 001 010 011 100 101 110 1110000 NULL DLE 0 @ P ` p0001 SOH DC1 ! 1 A Q a q0010 STX DC2 " 2 B R b r0011 ETX DC3 # 3 C S c s0100 EDT DC4 \$ 4 D T d t0101 ENQ NAK % 5 E U e u0110 ACK SYN & 6 F V f v0111 BEL ETB 7 G W g w1000 BS CAN ( 8 H X h x1001 HT EM ) 9 I Y i y1010 LF SUB * : J Z j z1011 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
21. 21. 33 Control codes 000 001 010 011 100 101 110 1110000 NULL DLE 0 @ P ` p0001 SOH DC1 ! 1 A Q a q0010 STX DC2 " 2 B R b r0011 ETX DC3 # 3 C S c s0100 EDT DC4 \$ 4 D T d t0101 ENQ NAK % 5 E U e u0110 ACK SYN & 6 F V f v0111 BEL ETB 7 G W g w1000 BS CAN ( 8 H X h x1001 HT EM ) 9 I Y i y1010 LF SUB * : J Z j z1011 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
22. 22. Alphabetic codes 000 001 010 011 100 101 110 1110000 NULL DLE 0 @ P ` p0001 SOH DC1 ! 1 A Q a q0010 STX DC2 " 2 B R b r0011 ETX DC3 # 3 C S c s0100 EDT DC4 \$ 4 D T d t0101 ENQ NAK % 5 E U e u0110 ACK SYN & 6 F V f v0111 BEL ETB 7 G W g w1000 BS CAN ( 8 H X h x1001 HT EM ) 9 I Y i y1010 LF SUB * : J Z j z1011 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
23. 23. Numeric codes 000 001 010 011 100 101 110 1110000 NULL DLE 0 @ P ` p0001 SOH DC1 ! 1 A Q a q0010 STX DC2 " 2 B R b r0011 ETX DC3 # 3 C S c s0100 EDT DC4 \$ 4 D T d t0101 ENQ NAK % 5 E U e u0110 ACK SYN & 6 F V f v0111 BEL ETB 7 G W g w1000 BS CAN ( 8 H X h x1001 HT EM ) 9 I Y i y1010 LF SUB * : J Z j z1011 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
24. 24. Punctuation, etc. 000 001 010 011 100 101 110 1110000 NULL DLE 0 @ P ` p0001 SOH DC1 ! 1 A Q a q0010 STX DC2 " 2 B R b r0011 ETX DC3 # 3 C S c s0100 EDT DC4 \$ 4 D T d t0101 ENQ NAK % 5 E U e u0110 ACK SYN & 6 F V f v0111 BEL ETB 7 G W g w1000 BS CAN ( 8 H X h x1001 HT EM ) 9 I Y i y1010 LF SUB * : J Z j z1011 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
25. 25.  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
26. 26.  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.
27. 27. Extended ASCII