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 CONNECTING TWO 25 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 25 pin to 25 pin null modem cable looks like this:

     Frame               01-----01            Frame

     TD                  02-----03            RD

     RD                  03-----02            TD

     RTS                 04-----05            CTS

     CTS                 05-----04            RTS

     DSR           ------06     06------      DSR
                   |                   |
     ground        |     07-----07     |      ground
                   |                   |
     CD            ------08-----20     |      DTR
                   |                   |
     TC            |  ---15     15---  |      TC
                   |  |             |  |
     RC            |  ---17-----24  |  |      (TC)
                   |                |  |
     (TC)          |     24-----17---  |      RC
                   |                   |
     DTR           |     20-----08-----|      CD
                   |                   |
     SQ            ------21     21------      SQ



 Notice that pins 6 and 8 and 21 are jumpered together on each end, and
 15 and 17 are jumpered together on each end.

 This is a reasonably classy version of the null modem cable.
 You can often get by without connecting pins 15, 17 and 24 at
 all.  The jumper to pin 21 is seldom required.  And the simplest version
 of the null modem consists only of the pin 2, 3, and 7 connections.