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From: gws@cbnap.UUCP (Gary W. Sanders (N8EMR))
Newsgroups: net.dcom,net.periphs
Subject: Re: Definition of "null modem" cable
Message-ID: <55@cbnap.UUCP>
Date: Fri, 30-Aug-85 12:00:19 EDT
Article-I.D.: cbnap.55
Posted: Fri Aug 30 12:00:19 1985
Date-Received: Sat, 31-Aug-85 08:58:24 EDT
References: <195@almsa-1> <3209@amdcad.UUCP>
Reply-To: gws@cbnap.UUCP (Gary W. Sanders (N8EMR)
Distribution: net
Organization: AT&T Bell Labs Columbus,Ohio
Lines: 55
Xref: watmath net.dcom:1229 net.periphs:836

In article <3209@amdcad.UUCP> phil@amdcad.UUCP (Phil Ngai) writes:
>In article <195@almsa-1> control@almsa-1 (William Martin) writes:
>>What is the exact definition of a "null modem" cable? Does the term
>>*only* mean
>
>
>>The way I look at it, RS232 is not a standard, it is just a way of saying
>>you are going to try to make two devices talk together. With whatever
>>pins work. (ever try to hook up an NEC Spinwriter to a computer?)
>
>>But in answer to your question, 3) is certainly right, 1) works for many
>>DEC devices and 2) is sometimes needed. If you need 2), the most general
>>case is as follows:

	You can just cross pins 2 and 3, then pins 4 and 5 are connected
	togeter on each side of the connector. Pins 6,8 and 20 are connected
	together on each side of the connector. This way you get the null
	modem cable you need. However you loose a little also, first no
	hardware flow control, second if you used this cable with a modem
	the computer would never see someone hang up the phone and the
	process attached to that modem would never die. A "generic" better
	may is illistrated below. Keep in mind RS232/DB25 may be a standrd,
	but many companys dont fully implement it or they add the own extra
	features. 

	Generic NULL modem
	------------------

	2__________3
	3__________2
	4__________5
	        !__6
	5__________4
	6__!
	7__________7
	8__________20
	20_________8

	You may or not need the pin 5 to pin 6 strap, If you are using
	AT&T terminals to and AT&T datakit LAN you will need the strap.

	What connectors go on each side depends on your enviorment, Typicaly
	a female on each end works out great, In a DEC only world all you need
	is female to female null modem cables. In an AT&T env. you might
	want to have male to female null modems.
_______________________________________________________________________________ 
[                                                                             ]
[	Gary W. Sanders (N8EMR)						      ]
[	AT&T Bell Labs (Columbus,Ohio)					      ]
[	614-860-5965 (w)	614-457-4595 (h)			      ]
[	72277,1325 Compuserve.						      ]
[                                                                             ]
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