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From: hes@ecsvax.UUCP (Henry Schaffer)
Newsgroups: net.dcom,net.periphs
Subject: Re: Definition of "null modem" cable
Message-ID: <340@ecsvax.UUCP>
Date: Fri, 30-Aug-85 08:26:33 EDT
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Posted: Fri Aug 30 08:26:33 1985
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> What is the exact definition of a "null modem" cable?  ...
> 
> Will Martin
> 
  I don't believe that there is a standard wiring - but the standard
definition is that the "null modem" (also called a "crossover" or
sometimes a "modem eliminator") converts a DTE interface to a DCE
and vice-versa  (DTE-data terminal equipment, e.g. terminal.  DCE-data
communication equipment, e.g. modem.)
  To do this some pins are passed through, e.g. 1 and 7,; some are
"crossed over", e.g. 2 and 3, and some are just fed back at each
end, e.g.6 and 20 are connected together at each end and don't go
through.
  Then vendors differ as to exactly what else they do.  Glasgal
Communications also connects 4,5 and 8 at each end, while Black Box
Products connects 4 and 5 at each end and connects 8 to the 6 and 20
at each end.  Black Box also passes through 11 and 12 crossed over.
(This info is from their respective catalogs.)
  The one I use to connect a Model 100 to an IBM PC has the following
connections:    Male         Female
                 1            1
                 2            3
                 3            2
                 4            5
                 5            4
                 6,20         8
                 7            7
                 8            6,20
Someone elso made it for me, so I can't say whether this is the
minimum necessary wiring - but it does work.  Note it isn't 
exactly either one of the above vendor's schemes.  Also
note that the genders of the connectors are not standard (although
female-female is probably most common.)  You have to specify what you
want.
--henry schaffer