The RS-232 standard defines 25 signals used to transmit data and control the communications channel. Seldom do we see so many actually used, which is one source of grief with connecting serial devices - it seems each device requires a different number of the signals. The following table lists the most common signals in PC and embedded work, both for the 25 and 9 pin connector configurations. With some clever electronics you can even feed both data directions onto a single phone line pair, permitting world-wide communications over the existing phone system. There's only one problem.
Computer busses are wide .
Even a small computer moves data around in 8 bit chunks. Either the processor must painstakingly convert each character to and from a bit stream, or we have to add hardware to do the work automatically. In fact, virtually every system does use hardware. Synchronous transmission is relative to an external clocking source, one that is often transmitted on an additional wire. UARTs were some of the earliest integrated peripherals.