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DISCUSSION OF PARITY
Parity is a system that helps to detect errors in transmitted
data. Three "flavors" are possible; even, odd, and none. Even
and odd parity require that data be represented by 7 data bits.
"No parity" can be used with 7 or 8 data bits, although it is
almost universally used with 8 data bits.
For even parity, the number of 1s in the binary seven-bit
representation of a character is counted. If the answer is
odd, a 1 is transmited as the eighth bit; if the answer is even,
a 0 is transmitted as the eighth bit. Either way, the total number
of 1s transmitted in eight bits is even.
For example, the 8 bit ASCII code for "C" is 01000011. Dropping the
first zero leaves 1000011. There are three ones in this, so we replace
the deleted zero with one, and transmit 11000011 for "C". If the other
end is also set for even parity, it will read this as "C". If the other
end is an IBM PC and is set for no parity, it will read this as "|".
Odd parity would be the same, except "C" would be transmitted as 01000011.
In general, if you are setting up a connection and don't know what
kind of parity to use, use none. If about half the characters transmitted
to you appear as garbage, switch to even parity (and seven data bits).
See Also:
data bits
duplex
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