Binary digital codes are codes which are represented in binary system with modification from the original ones. For ease of data processing and transmission, binary data is represented in various binary codes. Here all Binary digital codes are discussed. Error Detecting Codes and Error Correcting Codes, Weighted Binary Codes: 8421/2421/5211 weighted binary codes, Non-Weighted Binary Codes: Excess-3 Codes and Gray Codes, Reflective Binary Codes, Sequential Binary Codes, Alphanumeric Codes, EBCDIC Code, and ASCII Codes.
Weighted Binary Codes
Weighted binary codes are those which obey the positional weighting principles, each position of the number represents a specific weight. 8421, 2421, and 5211 are weighted binary codes.
Non-Weighted Binary Codes
Non weighted codes are codes that are not positionally weighted. That is, each position within the binary number is not assigned a fixed value. Excess-3 and Gray codes are non-weighted binary codes
A code is said to be reflective when code for 9 is complement for the code for 0, and so is for 8 and 1 codes, 7 and 2, 6 and 3, 5 and 4. Codes 2421, 5211, and excess-3 are reflective, whereas the 8421 code is not.
A code is said to be sequential when two subsequent codes, seen as numbers in binary representation, differ by one. This greatly aids mathematical manipulation of data. The 8421 and Excess-3 codes are sequential, whereas the 2421 and 5211 codes are not.
8421 Code/BCD Code
The BCD (Binary Coded Decimal) is a straight assignment of the binary equivalent. It is possible to assign weights to the binary bits according to their positions. The weights in the BCD code are 8,4,2,1.
Example: The bit assignment 0101, can be seen by its weights to represent the decimal 5 because:
0x8+1×4+0x2+1×1 = 5
This is a weighted code, its weights are 2, 4, 2 and 1. A decimal number is represented in 4-bit form and the total four bits weight is 2 + 4 + 2 + 1 = 9. Hence the 2421 code represents the decimal numbers from 0 to 9.
This is a weighted code, its weights are 5, 2, 1 and 1. A decimal number is represented in 4-bit form and the total four bits weight is 5 + 2 + 1 + 1 = 9. Hence the 5211 code represents the decimal numbers from 0 to 9.
Excess-3 is a non weighted code used to express decimal numbers. The code derives its name from the fact that each binary code is the corresponding 8421 code plus 0011(3).
Example: 0110 of 8421 = 1001 in Excess-3
The gray code belongs to a class of codes called minimum change codes, in which only one bit in the code changes when moving from one number to the next. The Gray code is non-weighted code, as the position of bit does not contain any weight. The gray code is a reflective digital code which has the special property that any two subsequent numbers codes differ by only one bit. This is also called a unit-distance code. In digital, Gray code has got a special place.
Error Detecting Codes
When data is transmitted from one point to another, like in wireless transmission, or it is just stored, like in hard disks and memories, there are chances that data may get corrupted. To detect these data errors, we use special codes, which are error detection codes.
Error Correcting Codes
Error-correcting codes not only detect errors, but also correct them. This is used normally in Satellite communication, where turn-around delay is very high as is the probability of data getting corrupt. ECC (Error correcting codes) are used also in memories, networking, Hard disk, CDROM, DVD etc. Normally in networking chips (ASIC), we have 2 Error detection bits and 1 Error correction bit.
The binary codes that can be used to represent all the letters of the alphabet, numbers and mathematical symbols, punctuation marks, are known as alphanumeric codes or character codes. These codes enable us to interface the input-output devices like the keyboard, printers, video displays with the computer.
EBCDIC stands for Extended Binary Coded Decimal Interchange. It is mainly used with large computer systems like mainframes. EBCDIC is an 8-bit code and thus accomodates up to 256 characters. An EBCDIC code is divided into two portions: 4 zone bits (on the left) and 4 numeric bits (on the right).
ASCII stands for American Standard Code for Information Interchange. It has become a world standard alphanumeric code for microcomputers and computers. It is a 7-bit code representing 27 = 128 different characters. These characters represent 26 upper case letters (A to Z), 26 lowercase letters (a to z), 10 numbers (0 to 9), 33 special characters and symbols and 33 control characters.
The 7-bit code is divided into two portions, The leftmost 3 bits portion is called zone bits and the 4-bit portion on the right is called numeric bits.
An 8-bit version of ASCII code is known as USACC-II 8 or ASCII-8. The 8-bit version can represent a maximum of 256 characters.