Most serial devices support five parity settings. Only two of the settings can be used to perform a parity check.
- Odd
- Even
- Space
- Mark
- None
Odd Parity
Odd parity is the original parity checking system first put into use by IBM. Odd parity attempts to maintain an odd number of ones in each byte. When a byte has an odd number of ones, the value of the parity bit is zero. When the number of ones in the byte is even, a single bit is added with the value of one, making the number of ones, odd.
Even Parity
Even parity, a variation on odd parity, also provides a means for detecting errors. Even parity attempts to maintain an even number of ones in each byte. When there are an even number of 1-bits, the value of the parity bit is zero. When there are an odd number of 1-bits, the value of the parity bit is 1, making the number of bits an even count of 1-bits.
Space Parity
Space parity sets the parity bit to zero for all bytes/words, effectively disabling any parity checking mechanism in use. Space parity is not actually a parity checking method, it is a method used to disable parity checking.
Mark Parity
Systems utilizing mark parity set the parity bit to one for all bytes/words, effectively disabling the parity checking. Mark parity is not actually a parity checking method, it is a method used to disable parity checking.