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                      Luis M. Garcia-Barrio
                     8/N/1 #4, (215) 848-5728

Think of a "null modem" as a way to fool two computers into
"thinking" they're connected to modems, when in fact they are
connected to each other through their serial ports.

The RS-232 is one standard used for serial communications. The
most common connector used for this standard is the DB-25 (male
and female) (but ATs and others use a 9-pin connector; if this
is your case, see the note at the end).

Two types of equipment are connected:

  - DTE (Data Terminal Equipment)
  - DCE (Data Communications Equipment).

DCE normally uses female connectors, but for DTE it varies. In
most cases, the computer is DTE and the modem is DCE.

The following is a list of connector pins used in the project
and their use. Normal modem communications use also pins 12 and
22, Speed Indicator (SI) and Ring Indicator (RI).

--------------------------------------------------------------
PIN  NAME                     NOTES
--------------------------------------------------------------
 1   GND   Protective Ground
           Not always used
 2   TD    Transmitted Data. DTE -> DCE
           DTE sends data to DCE through this pin
 3   RD    Received Data. DTE <- DCE
           DTE receives data from DCE through this pin
 4   RTS   Request To Send. DTE -> DCE
           DTE tells DCE if it is ready to send data
 5   CTS   Clear To Send. DTE <- DCE
           DCE tells DTE if it is ready to receive from DTE
 6   DSR   Data Set Ready. DTE <- DCE
           DCE tells DTE if it is connected to the phone line
 7   SG    Signal Ground for all pins
           Sometimes, 1 and 7 are connected
 8   CD    Carrier Detect. DTE <- DCE
           DCE tells DTE if it is connected to another modem
20   DTR   Data Terminal Ready. DTE -> DCE
           DTE tells DCE if it is on and ready to work
--------------------------------------------------------------

A "null modem" makes each computer (DTE) believe that the other
is DCE, by pairing the lines and crossing the signals used.

 - TD of computer A goes to RD of computer B and viceversa.
 - RTS of computer A goes to CTS of computer B and viceversa
 - DSR of computer A goes to CD of computer B and viceversa
 - DTR and DSR signals for each computer are combined

For the construction of the "null modem" let's assume that each
computer has a cable connected to its RS-232 serial port, and
that there is a male connector at the free end of each cable.
Since you would normally connect that end to a modem, and since
most modems (DCE) have female connectors, that should be the
case in most instances, but do not underestimate manufacturers.

1.- Buy two female DB-25 connectors of the type that uses "pin
    insertion" (AMP 205207-1 8331) or the type that requires
    soldering. (The type designed for pressing ribbon cable
    into the connector is not well suited for this case, but it
    could be used with lots of patience.)
2.- Get 10 short (about 2 inches) pieces of insulated wire,
    remove the insulation from both ends and prepare them for
    connection to the type of DB-25 connectors you are using.
3.- Note the pin numbers in the female connectors. As seen from
    the front, there are two rows: the top row has 13 pins, and
    the bottom row has 12. Pin 1 is the rightmost pin in the
    top row; pin 25 is the leftmost pint of the bottom row.
    Note that when you look at the connecter from the back, the
    position of the pins is reversed!
4.- Connect pin 1 of connector A to pin 1 of connector B
       "        2      "       A    "   3       "      B
       "        3      "       A    "   2       "      B
       "        4      "       A    "   5       "      B
       "        5      "       A    "   4       "      B
       "        6      "       A    "   8       "      B
       "        7      "       A    "   7       "      B
       "        8      "       A    "   6       "      B
       "        6      "       A    "  20       "      A
       "        6      "       B    "  20       "      B
    (Note that in the last two steps, pin 6 is connected to pin
    20 of the SAME connector)
5.- Use whatever enclosure you like best. I prefer the type of
    DB-25 hood that allows for two connectors in one hood.
6.- Connect the free ends of the two cables coming from the
    computers to the "null modem".

You have built a "full" null modem. There are other types, such
as the "standard" null modem. To build the "standard" type:

    Connect pin 1 of connector A to pin 1 of connector B
       "        2      "       A    "   3       "      B
       "        3      "       A    "   2       "      B
       "        6      "       A    "   20      "      B
       "        20             B    "   6       "      A
       "        7      "       A    "   7       "      B
       "        8      "       A    "   4/5     "      B
       "        4/5    "       A    "   8       "      B
       "        4      "       A    "   5       "      A
       "        4      "       B    "   5       "      B

    (Note that in the last two steps, you are connecting pins 4
    and 5 of the same connector)

Depending on the two computers being connected, one of the two
types indicated could be more suitable than the other. Try both
before giving up!

                      ------------------

A common use for "null modems" is the transfer of files from
one machine to another. I will cover it briefly.

Let us assume you have a TRS-80 Model 4 and a Tandy 2000, and
that you have terminal programs for both. (If we had double
sided 80 track drives in your Model 4, we could do it with
SX4, but let us assume we don't).

1.- Start the terminal programs for both machines, after they
    are connected through the "null modem".
2.- Use the same configuration parameters for both machines:
    8 bit word, No Parity, 1 Stop bit. The baud should also be
    the same. I would suggest 9600 baud if your two terminal
    programs allow it. If they allow for faster baud, try it:
    it may or may not work. Lower it if it doesn't.
3.- Turn ECHO ON and HALF DUPLEX in ONE of the two computers.
    Some programs use a different terminology. For example,
    FastTerm uses "REMOTE ON" for "ECHO ON", and "LOCAL ON" for
    "HALF DUPLEX" (correclty so!). If connections and settings
    are correct, you can type at either computer and see it
    in the screens of both, just as if you were connected to a
    Bulletin Board System.
4.- Select XMODEM Send in one and XMODEM Receive in the other.
    You could also use other transfer methods allowed by your
    terminal progams, such as "buffer capture", etc., but they
    may not be as reliable as XMODEM.
5.- Press <ENTER> in both and the transfer should start.
    Experiment with higher baud and select the highest allowed.

                         -----------------

              NOTE ON DB-9 (nine-pin) CONNECTORS
             ------------------------------------

The AT and similar computers (and the Model 1400-LT!) don't use
the DB-25 connector: they use a smaller, 9-pin version. Since
in most microcomputers only 9 of 25 pins in the DB-25 connector
are used for the RS-232, it makes a lot of (painful!) sense.

Here are the equivalent signals for the 9-pin connector:

           Pin 1 is CD, equivalent to pin 8
            "  2  " RD       "     "   "  3
            "  3  " TD       "     "   "  2
            "  4  " DTR      "     "   "  20
            "  5  " GROUND   "     "   "  7 (and maybe 1)
            "  6  " DSR      "     "   "  6
            "  7  " RTS      "     "   "  4
            "  8  " CTS      "         "  5
            "  9  " RI       "     "   "  22

           ---------------------------------