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From: hull@hao.UUCP (Howard Hull)
Newsgroups: net.dcom,net.periphs
Subject: Re: Definition of "null modem" cable
Message-ID: <1734@hao.UUCP>
Date: Sat, 31-Aug-85 12:10:53 EDT
Article-I.D.: hao.1734
Posted: Sat Aug 31 12:10:53 1985
Date-Received: Mon, 2-Sep-85 04:11:17 EDT
References: <195@almsa-1>
Distribution: net
Organization: High Altitude Obs./NCAR, Boulder CO
Lines: 35
Xref: watmath net.dcom:1235 net.periphs:839

I just perused the RS232 standard and the Application Notes and did not find
the words "null modem" anywhere.  I did find "DCE(modem)" mentioned once in
the Application notes.  So "null modem" must be trash slang.  A "full" null
modem can be quite complex.  Following is a description for one I have had
miscellaneous cable collection.

DEC		TERMINAL / COMPUTER RS-232 NULL-MODEM CABLE	10-MAR-82

Cable Type:	Belden 9504	4 pair overall shielded
Outside Dia.:	0.265 inches	(6.75 millimeters)
Connectors:	DB-25
Terminal								Computer
Pin #	Signal Name		Wire Color	Signal Name		Pin #

 1	Protective ground-------Shield Drain----Protective ground	 1
 2	Transmit Data---------> red ----------> Receive Data		 3
 3	Receive Data <--------- white <-------- Transmit Data		 2
 4	Request to Send--:----> violet -------> Carrier Detect		 8
 5	Clear to Send <--: 5 jumpered to 4
 6	Data Set Ready <-:-----	green <-------- Data Terminal Ready	20
22	Ring Indicator <-: 22 jumpered to 6
 7	Signal Ground--white's blk & red's blk--Signal Ground		 7
 8	Carrier Detect <--- violet's blk <--:---Request to Send		 4
			    5 jumpered to 4 :-> Clear to Send		 5
20	Data Terminal Ready -> green's blk -:-> Data Set Ready		 6
			   22 jumpered to 6 :-> Ring Indicator		22

Notes:	This cable is wired like a DEC H312 Null Modem Card.

Judging by the above note, at least DEC seems to think it knows what a
"null modem" is.
								     Howard Hull
[If yet unproven concepts are outlawed in the range of discussion...
                   ...Then only the deranged will discuss yet unproven concepts]
        {ucbvax!hplabs | allegra!nbires | harpo!seismo } !hao!hull