Today Unicode is becoming popular
because it is backwards compatible with ASCII and allows the encoding of more complex alphabets, such as
those used for Russian, Chinese, and other languages. We will use ASCII to illustrate the idea of character
encoding, since it is still widely used, and it is simpler to describe than Unicode.
In ASCII each character is assigned a 7-bit integer value. For instance, ???A??™ = 65 (1000001), ???B??™ = 66
(1000010), ???C??™ = 67 (1000011), etc. The 8th bit in a character byte is intended to be used as a parity bit, which
allows for a simple error detection scheme.
If parity is used, the 8th or parity bit is used to force the sum of the bits in the character to be an
even number (even parity) or an odd number (odd parity). Thus, the 8 bits for the character ???B??™ could take
these forms:
01000010 even parity
11000010 odd parity
01000010 no parity
If parity is being used, and a noisy signal causes one of the bits of the character to be misinterpreted, the
communication device will see that the parity of the character no longer checks. The data transfer can then be
retried, or an error announced. This topic is more properly discussed under the heading of data communications,
but since we had to mention the 8th bit of the ASCII code, we didn??™t want you to be left completely in the dark
about what parity bits and parity checking are.
Pages:
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100