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Synchronous serial communication
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<p>Synchronous serial communication describes a <a href="/facts/Serial_communication/r5xP7P1c">serial communication</a> <a href="/facts/Communications_protocol/5uady7cB">protocol</a> in which "data is sent in a continuous stream at constant rate." </p><p>Synchronous communication requires that the <a href="/facts/Clock_signal/9UfTzGHx">clocks</a> in the transmitting and receiving devices are <i>synchronized</i> – running at the same rate – so the receiver can sample the signal at the same time intervals used by the transmitter. No start or stop bits are required. For this reason "synchronous communication permits more information to be passed over a circuit per unit time" than <a href="/facts/Asynchronous_serial_communication/amrIkBQR">asynchronous serial communication</a>. Over time the transmitting and receiving clocks will tend to drift apart, requiring <i>resynchronization</i>. </p><p>Synchronous RS-232 used additional pins on the DB-25 cable: the DCE (generally the modem or other peripheral) provided two clock signals to the DTE (generally the host computer or terminal), transmitter clock (pin 15, TCK) and receiver clock (pin 17, RCK). Some systems supported an alternative mode of operation in which the transmitter clock signal was provided by the DTE instead, called transmitter timing (pin 24, TT). Note the smaller DE-9 connector commonly adopted in later systems does not have these additional signal lines, and hence cannot be used with synchronous RS-232. </p>