One of the basic encodings used for asynchronous serial communications
is illustrated in Figure 5. Every character is transmitted in a “frame” which
begins with a (low) start bit followed by eight data bits and ends with a (high)
stop bit. The data bits are encoded as high or low signals for (1) and (0),
respectively. Between frames, an idle condition is signaled by transmitting a
continuous high signal. Thus, every frame is guaranteed to begin with a highlow
transition and to contain at least one low-high transition. Alternatives
to this basic frame structure include different numbers of data bits (e.g. 9),
a parity bit following the last data bit to enable error detection, and longer
stop conditions.