Error Detecting Codes

·         Disturbances during data transmissions between units. The principle used to detect errors in codes is the introduction of an extra bit along with each character code.

·        A common method is the use of parity check bit. A parity check bit is appended to the 7 bits of the code of each character in such a way that the total number of 1s in each character code is even.

·      For example, the ASCII code of the letter E is 1000101. The number of 1s in the string is odd. A parity check  bit 1 is appended to this string to obtain a code which is now 8 bits long and has an even number of 1s in it. If the ASCII code of a character has already an even number of 1 s, then the parity check bit appended is 0.

·        Whenever a character is read from storage or received from a remote location, the 1s in its code are counted.  It has to be even, if it is odd, then at least one bit is wrong. A single error in any of the 8 bits of the code will definitely be detected.

·         Two errors cannot be detected in this scheme. But as the probability of more than 1 error occurring in practice  is small, this scheme is commonly accepted and is adequate.

•  There are other error-detecting codes devised which can detect more than one errors, and also correct   those  errors.