Over time the transmitting and receiving clocks will tend to drift apart, requiring resynchronization.
[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.
Synchronous Data Link Control (SDLC) specifies that a station continue transmitting a sequence of '1' bits on an idle line.
[4] Data to be transmitted on an idle line is prefixed with a special bit sequence '01111110'b, called a flag.
SDLC was the first bit-oriented protocol developed, and it was later adopted by the International Organization for Standardization (ISO) as High-Level Data Link Control (HDLC).