11.4) How do I transfer files using a null modem cable?

This section by mailto:cb541@cleveland.Freenet.Edu (James R. Gilbert)

A:  Simply put, you need a terminal program and an RS 232 port on
    each computer.  The RS 232 ports need to be connected
    together using a 'null modem cable'.

    For up to 4800 bps, no flow control lines need be
    connected.  Just cross the transmit and receive lines
    and join the grounds together.  (Transmit is pin #2,
    receive is pin #3 and ground is pin #7 on the 25-pin
    port.)

    The right hand pin on the 'long' side of a female 'D'
    connector is #1.  There are 13 holes on this 'long'
    side, 12 holes on the 'short' side.  The numbers go from
    #1 on the right to #13 on the left on the 'long' side
    and from #14 to #25 from right to left on the 'short'
    side. #25 is closest to being under #13.  A male
    connector is the mirror image of this.

    Most terminal programs allow a null connection, without a
    carrier detect.  Notably, '850 Express!' does not.

    A convenient way to make a null modem cable, up to about
    30 feet long, is to use two female DB25 connectors and
    some three or more conductor cable.  Using the two DB25
    female connectors allows unplugging your modem and
    plugging in the null modem cable.  This also avoids the
    confusion of the wide variety of serial port jacks on
    different computers.  Almost all computers connect into
    the modem via a DB25 connection.

    The SIO port on the Atari cannot be used directly.  An
    850 Interface Module, P:R:Connection, Multi I/O, Black
    Box (by Computer Software Services) or similar device
    that provides an RS232 port must be used.

    On the Atari, the port is a female DB9.  So you need a
    male DB9 to male DB25 modem cable to connect to your
    modem. So why not use this cable as your null modem
    cable as well?

    For higher speed connections, above about 9600 bps on the
    8-bit, you need the flow control lines.  You also need
    a Multi I/O or Black Box, which use the PBI (parallel bus),
    then you can go higher than 9600 bps.

    Following are pin assignments for a DB25 pin RS 232 C
    port.
                 13                       1
                  o o o o o o o o o o o o o
                   o o o o o o o o o o o o
                  25                     14

          (Above is female, male is mirror image.)

1.  Protective Ground        12.  Select Alternate Rate
2.  Transmit Data            15.  Transmit Clock (sync)
3.  Receive Data             17.  Receive clock (sync)
4.  RTS (Request to Send)    20.  Data Terminal Ready
5.  CTS (Clear to Send)      22.  Ring indicator
6.  Data Set Ready           23.  Select Alternate Rate
7.  Signal Ground            24.  Transmit Clock
8.  Carrier Detect

    A high speed cable would need not only pins 2 and 3 crossed
    but also pins 4 and 5 as well as 6 and 8.  Or better
    yet, make a true 25 wire, straight through cable and use
    a commercial null modem.  A commercial null modem is
    just a small device with the correct lines already
    crossed.

    (DTE = Data Terminal Equipment, i.e., your computer.
     DCE = Data Communications Equipment, i.e., your modem.)