10.7. RS-232 and Modem Control Most terminals and modems are connected via asynchronous RS-232 serial ports. This type of connection supports several lines, in addition to those that transmit and receive data. The system typically supports only a few of these lines. The most commonly used lines are those showing that the equipment on each end is ready for data transfer. The RS-232 electrical specification is asymmetrical: Each line is driven by one of the two devices connected and is sampled by the other device. Thus, one end in any normal connection must be wired as data-terminal equipment (DTE), such as a terminal, and the other as data-communications equipment (DCE), such as a modem. Note that terminal in DTE means endpoint: A terminal on which people type is a DTE, and a computer also is a DTE. The data-terminal-ready (DTR) line is the output of the DTE end that serves as a ready indicator. In the other direction, the data-carrier-detect (DCD) line indicates that the DCE device is ready for data transfer. Historically, VAX terminal interfaces were all wired as DTE (they may be connected directly to modems or connected to local terminals with null modem cables). The terminology used in the FreeBSD terminal drivers and commands reflects this orientation, even though many computers incorrectly use the opposite convention. When terminal devices are opened, the DTR output is asserted so that the connected modem or other equipment may begin operation. If modem control is supported on a line, the open does not complete unless the O_NONBLOCK option was specified or the CLOCAL control flag is set for the line, and no data are transferred until the DCD input carrier is detected or the CLOCAL flag is set. Thus, an open on a line connected to a modem will block until a connection is made; the connection commonly occurs when a call is received from a remote modem. Data then can be transferred for as long as carrier remains on. If the modem loses the connection, the DCD line is turned off and later reads and writes fail. Ports that are used with local terminals or other DTE equipment are connected with a null-modem cable that connects DTR on each end to DCD on the other end. Alternatively, the DTR output on the host port can be looped back to the DCD input. If the cable or device does not support modem control, the system will ignore the state of the modem control signals when the CLOCAL control flag is set for the line. Finally, some drivers may be configured to ignore modem-control inputs. |
