Asynchronous data transmission is the simplest way of transmitting data to a receiver. Its origins lie with telegraphy (telex). Individual characters are transmitted without any additional synchronisation and in a random sequence. The bit string of a character is limited by a start bit and a stop bit. If a parity check has been activated, a parity bit is added to the string of a character. The value of a parity bit depends on whether the character bits are checked for even or odd.
String of ASCII character "A" with parity:
String of ASCII character "A" without parity:
In the case of asynchronous data transmission the signal levels follow in line with the strings of the transmitted character. This signal coding of the bits is called NRZ (non-return-to-zero).
Since the receiver has to synchronise the time of detection of each individual string, this must be coordinated with the data formats set in the sender (startbit, number of bits per character, stopbit, plus data rate).
Further information in Wikipedia: