<P> In synchronous communications, the stream of data to be transferred is encoded as fluctuating voltage levels in one wire (the' DATA'), and a periodic pulse of voltage on a separate wire (called the "CLOCK" or "STROBE") which tells the receiver "the current DATA bit is' valid' at this moment in time". </P> <P> Practically all parallel communications protocols use synchronous transmission . For example, in a computer, address information is transmitted synchronously--the address bits over the address bus, and the read or write' strobe's of the control bus . </P> <P> A logical one is indicated when there are two transitions in the same time frame as a zero . In the Manchester coding a transition from low to high indicates a one and a transition from high to low indicates a zero . When there are successive ones or zeros, an opposite transition is required on the edge of the time frame to prepare for the next transition and signal . </P> <P> The most common asynchronous signalling, asynchronous start - stop signalling, uses a near - constant' bit' timing (+ / - 5% local oscillator required at both end of the connection). Using this, the receiver detects the' first' edge transition...(the START BIT), then waits' half a bit duration' then reads A further delay of one' whole bit duration' is executed before the next data bit is' read' - repeating for length of the whole serial word (typically 7 / 8 - data bits). Finally an optional STOP bit is appended to identify the end of the data word . </P>

Pros and cons of synchronous and asynchronous communication