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From: oeschi@netmbx.UUCP (Johann Deutinger)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: null modem cable
Message-ID: <1783@netmbx.UUCP>
Date: 30 Jan 91 00:05:49 GMT
References: <3439@nosc.NOSC.MIL> <1991Jan27.123115.23732@ericsson.se> <3446@nosc.NOSC.MIL>
Organization: netmbx, Berlin, West Germany
Lines: 60

In article <3446@nosc.NOSC.MIL> koziarz@halibut.nosc.mil (Walter A. Koziarz) writes:
>In article <1991Jan27.123115.23732@ericsson.se> etxsral@california.ericsson.se (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]
  [other connections deleted ...]
>>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 input.
>DCE -- printer; DTE -- modem in my context.  I submit that my connection is
>indeed correct and valid.

The nullmodem is a discussion item with quite a long history ;-)

If you are connecting two similar pieces of hardware (and this is the
typical reason to cross pins 2/3 and pins 4/5 and so on) you should
expect both sides to use pin 8 (DCD) for the same thing, typically as
input.

The main point in this discussion is the difference between nullmodem and
other serial connections. If you really want to create a nullmodem, that
means, a replacement of an analog connection including two modems, there
is only one solution:

2    -    
3    -    2
4,5  -    8    (activates DCD from RTS at remote side - esp. for half duplex)
8    -    4,5  (vice versa)
6,20
	  6,20

This gives the response to RTS, which is CTS and to DTR, which is DSR.
This would be normally done by the modem. If synchronous procedures
should be used, additionally the clock lines (15,17,24) have to be
connected the right way (too much info for this context)

Looking at the history of this discussion the main application of such
cables is not to replace two modems but to connect a computer to a
peripheral device. For that application there is really no standard (some
use hardware handshake, some not; some use DTR/DSR for handshake, some
use RTS/CTS, some ignore DCD, some don't etc.)

So all of the presented solutions work with some special setup. This
misleads to thinking that it might be the only one!

Hans


-- 
oeschi@netmbx.UUCP     | Johann Deutinger
voice +49 30 396 50 21 | Ferrari electronic GmbH (.. no, we don't sell cars)
fax   +49 30 396 80 20 | Beusselstr. 27  -  1000 Berlin 21  -  FRG