rs-232(5)                                                     rs-232(5)

  rs-232

  NAME

    rs-232 - serial line connections

  DESCRIPTION

    The lines for a serial connection are described by Electronic Industries
    Association (EIA) Standard EIA-232-D.

    When connecting the computer to a modem, a straight-through rs-232 cable
    should be used. No remapping or crossing of lines should be required.

    When connecting a terminal (such as a vt100) to the computer, you must
    have a null modem between the terminal and the computer for data
    communications to work correctly.

    There are number of choices you must make when deciding to use a null
    modem. You can use a commercial, off-the-shelf (COTS) null modem cable or
    COTS null modem adapter.

    When purchasing COTS, check the line mappings as described below. Most
    COTS null modems will correctly make a distributed computing environment
    (DCE) appear as a data terminal equipment (DTE), or a DTE appear as a DCE.
    Avoid COTS null modems that describe crossing lines other than those
    described later in this topic. (For more information about see the EIA
    Standard EIA-232-D).

    You can also make your own null modem cable or adaptor. A quality
    electronics store will have all of the parts you require. Unless you
    purchase in large quantities, however, a COTS product will likely be
    cheaper.

    A common remapping of the RS-232 lines for 25 lines is as follows:

    line           line

    1        _     1

    2        _     3

    3        _     2

    4        _     8

    5        _     8

    6        _     20

    7        _     7

    8        _     4

    8        _     5

    20       _     6

    case     _     case

    Where line numbers match the following functionality for 25 lines:

    DB-25     Signal                                                DB-9

    1         frame ground                                          -

    2         TD (transmit data)                                    3

    3         RD (receive data)                                     2

    4         RTS (request to send)                                 7

    5         CTS (clear to send)                                   8

    6         DSR (data set ready)                                  6

    7         SG (signal ground)                                    5

    8         DCD (data carrier detect (also known as DC, RLSD)     1

    9         POS (reserved; usually positive DC test voltage)

    10        NEG (reserved; usually negative DC test voltage)

    11        (unassigned)

    12        SDCD (secondary data carrier detect)

    13        SCTS (secondary clear to send)

    14        STD (secondary transmit data)

    15        TC (transmit clock)

    16        SRD (secondary receive data)

    17        RC (receive clock)

    18        LL (local loopback)

    19        SRTS (secondary request to send)

    20        DTR (data terminal ready)                             4

    21        SQ (signal quality detect)

    22        RI (ring indicator)                                   9

    23        - (data rate selector)

    24        SCTE (external clock transmit)

    25        TM (test mode; often used as BUSY)

    For nine-pin connections, use a DB9 to DB25 connector before using the
    above mappings.

  FILES

    The serial lines are accessible through the files /dev/tty[01][0-9a-f].
    The file /dev/tty00 is equivalent to Windows COM1:.

  SEE ALSO

    igetty(1)