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From: pwong@batcomputer.tn.cornell.edu (Patrick Wong)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: null modem cable
Message-ID: <1991Jan31.140137.25101@batcomputer.tn.cornell.edu>
Date: 31 Jan 91 14:01:37 GMT
References: <1991Jan27.123115.23732@ericsson.se> <3446@nosc.NOSC.MIL> <11184@uhccux.uhcc.Hawaii.Edu>
Organization: Cornell Theory Center
Lines: 69

In article <11184@uhccux.uhcc.Hawaii.Edu> robin@uhunix1.uhcc.Hawaii.Edu (Robin Amano) writes:
>
>  ------------------------(lines deleted).............
>
>          I'd have to side with Lars on this one.  By what you are
>          saying above, and looking at your diagram above I'd have
>          to say that your connection is invalid.  Most of the time
>          you use a null modem cable for DTE to DTE.  In the old days,
>          actually not that old, when computer ports weren't as 
>          flexible they looked like a DTE expecting a modem or 
>          communication device to be hooked up to it.  So, when hooking
>          up a terminal you would use a null modem cable to make each
>          device look like a communication device to each other, although
>          they were actually terminal devices, thus crossing 2 & 3.
>          Above you are talking about hooking up a terminal device(DTE)
>          to a communication device(DCE), but your wiring diagram
>          crosses pins two and three.  I'd say your connection is
>          indeed incorrect and invalid.  And as far as 'fake-out'
>          that's exactly what a null modem, or sometimes called modem
>          eliminator, is for to fake out the DTE into thinking there
>          is a modem or communication device connected to it.  So,
>          for those of you who haven't seen Lars' cable 
>
>                         2 -------------- 3
>                         3 -------------- 2
>                       |-4                4-|
>                       |-5                5-|
>                         7 -------------- 7
>                      |-20 -------------- 20-|
>                      |--8                8--|
>                      |--6                6--|
>  
>          I think this is what he layed out.  This is what we use.
>          So, if you don't want to use a breakout box to figure out
>          the exact pin configuration try this.  Remember DTE to DTE
>          or DCE to DCE cross 2 & 3, DTE to DCE 2 & 3 straight through.
>
>
>--
>--------------------------------------------------------
>  Robin Amano          |  robin@uhunix.uhcc.hawaii.edu
>  UHCC                 |  Data Communications Dept.
>  2565 The Mall        |  Honolulu, HI  96822

Just to add confusion to the net.  I opened up a el cheapo "RS232 NULL
MODEM" sold by CompuAdd and here is what I find:

                2 ------ 3
                3 ------ 2
              |-4        4-|
              |-5        5-|
                7 ------ 7
             |-20        20-|
             |--8        8--|
             |--6        6--|

Note that there is no interconnection between any of 6,8,20 pins on one
side to any of the same set of pins on the other side.

But, what the heck.  I have used it in connecting a pc to a serial
printer and also connecting two pc's.  It works in both cases and I
am not going to loose sleep over it.  One thing thou, if I am going
to make a NULL MODEM, I will most likely follow the cable manufacturers'
suggestion (e.g., Black Box catalog's diagram, etc.)

Have fun in searching for the ultimate null modem!

Patrick
pcw@squid.graphics.cornell.edu