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From: jm9t+@andrew.cmu.edu (Josh Brian Mastronarde)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: null modem cable
Message-ID: <kbdcADy00WBL06H5YY@andrew.cmu.edu>
Date: 30 Jan 91 08:25:51 GMT
References: <8Fy4V2w163w@cybrspc> <3439@nosc.NOSC.MIL> <1991Jan27.123115.23732@ericsson.se>,
	<3446@nosc.NOSC.MIL>
Organization: Carnegie Mellon, Pittsburgh, PA
Lines: 56
In-Reply-To: <3446@nosc.NOSC.MIL>


Excerpts From Captions of netnews.comp.sys.ibm.pc.hardware:
28-Jan-91  Re: null modem cable           Walter A. Koziarz@halibu (1410)   
>In article <1991Jan27.123115.23732@ericsson.se> etxsral@california.ericsson.s
>e (Lars Nilsson) writes:
>>In article <3439@nosc.NOSC.MIL> koziarz@halibut.nosc.mil (Walter A. Koziarz)
> writes:
>>>It's never a good move to try to 'fake-out' hardware handshaking!!!!!  Don'
>t do
>>>it!!  Presented below is *the RIGHT way* to make a null modem cable --
>>>[1]------chassis GND--------[1]
>>>[2]-------------------------[3]
>>>[3]-------------------------[2]
>>>[4]-------------------------[5]
>>>[5]-------------------------[4]
>>>[6]------------------------[20]
>>>[7]-----signal GND----------[7]
>>>[8]-------------------------[8]
>>>[20]------------------------[6] 
>>>This is the correct wat to make a FULL-HANDSHAKING null modem; throw away
>>>ANYTHING that tells you to use the 'other', fake-out connection.
>>
>>Uhmm.  Your solution seems good except for the pin 8 to pin 8 connection
>>,connecting an input to an input doesnt work so good.
>>I usually connect pin 8 -- 6 to 20 in the other end.
>>/Lars Nilsson
> 
> 
>Ah, but I'm *not* connecting an input to an input.  A null modem is used when
>'I've got an available DCE port and need to connect a DTE device' or vice
>versa.  Examination of pin 8 [DCD] for a DCE device port (female on the
>computer) shows it to be an output.  And for a DTE device port; it is an inpu
>t.
>DCE -- printer; DTE -- modem in my context.  I submit that my connection is
>indeed correct and valid.
> 
>Walt K.

Bzzt.  DTE stands for Data Terminal Equipment, such as a computer or
VT100 terminal, etc.  i.e.  It transmits on pin 2 and receives on pin 3.
 DCE stands for Data Communications Equipment, such as a modem.  DCE
transmits on pin 3 and receives on pin 2 (those might be reversed, I'm
not sure, but that's not important).  Many (most?) serial printers are
set up as DTE, god knows why (so you can connect your printer to your
modem, perhaps :-).  When making a null modem cable, almost all of the
pins have a corresponding input/output pin (transmit-receive, RTS-CTS,
DTR-DSR).  However, DCD (data carrier detect) doesn't have such a
partner.  It is always an input from the DTE's point of view.  So,
connecting the pin 8's together does indeed connect an input to an
input.  Fortunately, the concept of carrier detect is lost when
connecting two computers, so you should connect DCD to pin 20 (DTR).

Hope this clears things up.

-Josh Mastronarde
-jm9t+@andrew.cmu.edu