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Objectives:
- Binary codes types.
- BCD code (8421 code).
- Alphanumeric codes.
- Excess-3 and Gray code.
- Parity method for error detection.
1. Binary codes types:
Weighted codes o BCD (8421) o 6311 o 2421 o 642- o 84-2-
Non_ Weighted codes o Excess- o Gray
Alphanumeric codes. o EBCDIC o ASCII
Error detection codes (Parity). Weighted codes and non-weighted codes are used to represent the decimal numbers. Alphanumeric codes are used to represent the numeric and nonnumeric data (characters). Error detection codes are used to detect the errors during the data transmission. Weighted codes use 4 binary digits to represent (0-9) decimal numbers.
2. BCD code (8421 code)
Simplest form : each decimal digit is replaced by its binary equivalent.
Example1 : 937.25 is represented by
1001 0011 0111 0010 0101
(937.25)= (100100110111.00100101)BCD
This representation is referred to as "Binary-Coded-Decimal": BCD or more explicitly as 8-4-2-1(8421 code). Note: The result is quite different than that obtained by converting the number as a whole into binary. Example 2:
= 100001010100(BCD)
BCD is inefficient, e.g. to represent 999 and 999999 bits needed: o 10 and 20 in binary numbers o 12 and 24 for BCD code.
Decimal numbers 8421(BCD) 6311 642 - 3 0 0000 0000 0000 1 0001 0001 0101 2 0010 0011 0010 3 0011 0100 1001 4 0100 0101 0100 5 0101 0111 1011 6 0110 1000 0110 7 0111 1001 1101 8 1000 1011 1010 9 1001 1100 1111 Example 3: convert 0110100000111001(BCD) to its decimal equivalent.
Solution :
Divide the BCD number into four-bit groups and convert each to decimal:
Alphanumeric codes: ASCII and EBCDIC Codes
