This section by Russ Gilbert.

Q:  How do I connect two computers using a null modem cable?

A:  You need a term program and RS-232 ports on both
    computers.  The RS-232 ports need to be connected
    together using a 'null modem cable'.

    For up to 4800 baud, 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. 25 pin #2 goes to Atari #4 (XMT to RCV), 25 pin
    #3 goes to #3 on Atari (RCV to XMT) and #5 of 850 goes
    to #7 of 25 pin (GND to GND).

    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
    to the left 1 to 13 then #14 is under #1 and left again
    so that #25 is under #13.

    Most term programs allow a null connection, without a
    carrier detect.  Notably, '850 Express!' does not. I have
    only used 'Procomm 2.4.3' (the last shareware version of
    Procomm) on the PC and BobTerm on the Atari, but other
    term programs may work.

    To check your null modem connection, start both PC and
    Atari term programs, set baud to 2400 or 4800 on both
    computers. No parity, 8 data bits, 1 stop bit on the PC.
    Be sure to use the correct COM port on the PC.  Go to
    'terminal' mode and you should now be able to type on
    either computer and see it on the other screen. To
    accomplish a file transfer, use Y-modem probably from
    BobTerm, rather than X-modem. X-modem will often append
    bytes to a file transfer, an undesirable event. There is
    also a very nice Z-modem receive program for the Atari,
    called ATAR-Z-MODEM by Larry Black for the Atari.

    A convenient way to make a null modem cable, up to about
    30 feet long, is to use two female DB25 connectors
    (Radio Shack) some three or more conductor cable. Using
    the two DB25 female connectors allows unplugging your
    modems and plugging in the null modem cable into the two
    modem cables.  This also avoids the confusion of
    variations in the computer ports. Most computers connect
    into the modem end via a standard RS-232 DB25 connection.
    With this both ends 25 pin cable, you would cross pins 2
    and 3 and connect the #7s together to make a null modem
    cable.

    The SIO port on the Atari cannot be used directly. An
    850, P:R: Connection, MIO, Black Box or similar device
    that provides an RS-232 port must be used.


    Following are pin assignments for a DB25 pin RS-232-C
    port.
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

   For higher speed connections, above 4800 or 9600, you
   need the flow control lines and Atari term software that
   has flow control built in. You also need an MIO or Black
   Box, which uses the PBI (parallel bus). A high speed
   cable would need not only XMT, RCV, and GND, but also
   flow control lines.  I suggest a commercial null modem
   from computer store to ensure correct lines.  A null
   modem is a small adapter with the correct lines already
   crossed. I don't know how to correctly connect the CTS,
   RTS, DTR, DSR, CRX lines for a high speed null modem.
   With a null modem, you just plug it into the 25 pin
   connectors of the two modem cables you might already
   have connected to your Atari and PC or Mac. You may need
   a straight thru 25 pin gender changer also.

   Following is in this FAQ elsewhere, but I summarize here:
   (Figure out or look for pin numbers on the ports.) Note
   that these are pin assignments, and NOT null modem
   connections with the XMT, RCV crossed and GND straight
   thru.

   Atari 8-bit  PC AT 25   PC AT 9 pin
   -------------------------------------
    1. DTR          20          4*
    2. CRX           8          1*
    3. XMT           2          3
    4. RCV           3          2*
    5. GND           7          5
    6. DSR           6          6
    7. RTS           4          7
    8. CTS           5          8
    9. No connect?   shield     RI
                  22 RI

Note: * above indicates the difference between an AT 9 pin
and a Atari 8-bit 9 pin cable connector. eg. If you check
continuity from pin 3 of 25 pin end and it goes to pin
4 of nine pin end, you have an Atari serial cable. If pin
3 of 25 pin goes to pin 2 of 9 pin end, you have a PC
serial cable.
(updated 3/1/99)
    (DTE = Data Terminal Equipment, i.e., your computer.
     DCE = Data Communications Equipment, i.e., your modem.)