A signal between 2.4 and 5 volts is a HIGH or binary-logic 1. A signal between 0 and 0.8 volts is a LOW or binary-logic 0. Signals between 0.8 and 2.5 volts are considered invalid data. Parallel cables are limited in length, defects in individual wires over long distances could cause individual bits to arrive at different times making it impossible to interpret them as a distinct byte. An error-checking protocol.
Error detection requires that both sender and receiver use the same parity setting.
If the total of 1s is not an even number, a 1 is used for the parity-bit to make it an even number. The receiving device adds the number of 1s in each character-byte. Otherwise, an error has occurred. If so, a request is then made to send the character-byte again. PC Inside a processor, a Program Counter.