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 CONNECTING TWO 9 PIN SERIAL PORTS

 Sometimes you wish to connect two computers together.  One of
 them has to pretend to be a modem (DCE) and one has to pretend
 to be a terminal (DTE).  Sometimes equipment is flexible and
 sometimes they refuse to go in drag.  When you have two pieces
 of equipment both steadfastly insisting on being terminals or
 two pieces both steadfastly insisting on being modems, what do
 you do?

 For two steadfast terminals, (the most common case) you build a
 special null modem cable that logically looks like two modems
 back to back over a very short phone line.


 A typical 9 pin to 9 pin null modem cable looks like this:



     TD                  03-----02            RD

     RD                  02-----03            TD

     RTS                 07-----08            CTS

     CTS                 08-----07            RTS

     DSR           ------06     06------      DSR
                   |                   |
     ground        |     05-----05     |      ground
                   |                   |
     CD            ------01-----04     |      DTR
                                       |
     DTR                 04-----01------      CD

 Note that pins 6 and 1 are jumpered together on each end.

 This is a classy version; the minimal version would only
 include the connections involving pins 3, 2, and 5.