 Transmission operation is simpler as the timing does not have to be determined from the line state, nor is it bound to any fixed timing intervals.As soon as the sending system deposits a character in the shift register (after completion of the previous character), the UART generates a start bit, shifts the required number of data bits out to the line, generates and sends the parity bit (if used), and sends the stop bits.Since full-duplex operation requires characters to be sent and received at the same time, UARTs use two different shift registers for transmitted and received characters.