eNotes: Mechatronics and Controls

21.3 DATA CHARACTERIZATION

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21.3.1 Parity

Parity - used to detect errors in data. A parity bit can be added to the data. For example older IBM PCs store data as bytes, with an extra bit for parity. This allows real-time error checking of memory.

Parity can be even or odd.

The odd parity bit is true if there are an odd number of bits on in a binary number. On the other hand the Even parity is set if there are an even number of true bits.

Convert the decimal value below to a binary byte, and then determine the odd parity bit,

21.3.2 Gray Code

A scheme to send binary numbers, but encoded to be noise resistant.

The concept is that as the binary number counts up or down, only one bit changes at a time. Thus making it easier to detect erroneous bit changes.

ASIDE: When the signal level in a wire rises or drops, it induces a magnetic pulse that excites a signal in other nearby lines. This phenomenon is known as `cross-talk'. This signal is often too small to be noticed, but several simultaneous changes, coupled with background noise could result in erroneous values.

21.3.3 Checksums

Parity bits work well when checking a small number of bits, but when the sequence becomes longer a checksum will help detect transmission errors.

Basically this is a sum of values.