Parity/Non-parity Check


Early transmission codes had a serious problem in that a bit
could be lost or gained in transmission because of an electrical
or mechanical failure/ If the loss went undetected, the character
received on the other end of the lime was incorrect.

To Prevent this from happening, a parity check system was
developed. Each character is represented by a byte consisting of
a combination of intelligence bits (seven bits in ASCII and eight
bits in EBCDIC) and an additional bit called a check or parity
bit.

Even parity codes place a check bit with each byte that
contains an uneven number of 1 bits. (Remember that a bit is
either 1 or o). Because the check bit is transmitted only with
characters composed of an uneven number of 1 bits,all characters
transmitted will have n even number of 1 bits. The check bit
is transmitted to and from the computer along with character
code. If a bit is lost (or added) in transmission, the system
will detect its loss. An uneven number of 1 bits received in a
code string composed of even bits will signal an error.

Odd Parity codes add a check bit to code combinations that
have an even number of 1 bits. Thus,all characters transmitted
have an odd number of bits. Odd and even parity care similar in
nature. They are both designed to signal an error in the even
that data are lost or added.