Synchronous serial communication

Synchronous serial communication describes a serial communication protocol , "In synchronous transmission, groups of bits are combined into frames,and frames are sent continuously with or without data to be transmitted."^[1]

Synchronous communication requires that the clocks in the transmitting and receiving devices are synchronized – 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"^[2] than asynchronous serial communication. Over time the transmitting and receiving clocks will tend to drift apart, requiring resynchronization.

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).^[3] 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.