12.1 Serial Data Communication

Within a processor character data are generally transferred, stored, or processed as bytes, i.e., as 8 bits in parallel. Parallel transmission of character data to a peripheral device requires a bundle of lines that includes one line for each bit of the character, a strobe or clock line, and possibly some other control and status signals. For long distances, parallel transmission becomes unattractive due to the increased cost of the multiple wires as well as the increased difficulty of avoiding crosstalk and skew between the bundled signals. The alternative, serial transmission, requires only one line over which the bits that represent a character are sent one after the other. Serial transmission is also used for data communication via the commercial voice-grade telephone network, where the serial bit stream is used to modulate an analog tone frequency signal appropriate for transmission over the telephone network.

Data communication is usually bidirectional, alternately in either direction (half-duplex) or simultaneously in both directions (full-duplex). Half-duplex operation permits the use of the same line (and of the same telephone line signals) for transmission as well as reception; the special character EOT (End-of-Transmission) is used to trigger the turn-around. Full-duplex operations requires separate transmit and receive lines (and two distinct sets of telephone line signals).

Figure 12-1 shows one side of a full-duplex connection between a CPU and another device via the telephone network. The Asynchronous Communication Adapter, located on a card that plugs into an I/O slot of the PC, performs the parallel-to-serial conversion of transmitted data and the serial-to-parallel conversion of received data; the "Modem" (Modulator/Demodulator), also called "Data Set," performs the conversion between discrete voltage levels and analog tone signal representations, and vice versa. The interface between the Asynchronous Communications Adapter and the modem should follow the standards set by the Electronic Industries Association (EIA), e.g., EIA RS-232C, or the similar international standards set by the Comite Consultatif International Telephonique et Telegraphique (CCITT), e.g., CCITT V.24.