Quite the oposite: They allow the receiver to properly identify data bits. So, the start and stop bits don’t need to be distinguished from data bits. The stop bit is not necessary, it’s there just for enhancing the reliability (both receiver and transmitter must use the same frequency). Imagine sending zero byte without parity: The line would just stay in 0 state all the time. Without the start bit, the receiver would not now when to start reading data bits. Then it waits for the stop bit and then it begins to start waiting for a new start bit. When the start bit arrives, it starts an internal timer and on every tick it reads the value from the line, until all data and parity bits are read. The receiver is waiting for the start bit (both start and stop bits are always ‘1’). In the beginning, there’s no data being transmitted - let´s say there is ‘0’ on the line for some time. The start bit simply marks the beginning of the data chunk (typically one byte with or without parity bit), and the stop bit marks the end of data chunk. When you want to send data over serial line, you need to synchronize transmitter and receiver.
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