Purpose of a start bit in the RS-232 serial communication protocol is

CONCEPT

In asynchronous serial communication, the transmitter and receiver do not share a continuous bit clock. Each independently framed character therefore needs an observable boundary from which the receiver can schedule samples near the centres of the incoming data bits.

An RS-232 line remains in the MARK state while idle. A transition to the SPACE state begins a character frame, and the later stop interval restores the idle state and provides timing margin before another frame.

APPLICATION

1. Before transmission, the receiver observes the idle MARK level.

2. The transmitter drives the line to SPACE for one bit interval; this is the start bit and its leading edge marks the frame boundary.

3. After detecting that edge, the receiver uses its local baud clock to sample the data-bit centres, beginning about one and a half bit periods after the edge for the first data bit.

4. A byte or character is framed separately with its own start bit and stop interval, so this timing reference is renewed for each byte rather than once for an arbitrary sequence.

CROSS-CHECK AND CONTRAST

• A whole sequence can span many independent character frames; using one timing reference for the entire sequence would allow oscillator mismatch to accumulate.

• A parity bit is an optional error-detection field derived from the data bits; it is not the frame-start timing signal.

• The last byte has the same physical character framing as earlier bytes; message-final identification belongs to a higher protocol layer.

Therefore, the start bit synchronizes the receiver for receiving every byte.