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From: jeq@laidbak.UUCP (Jonathan E. Quist)
Newsgroups: net.dcom,net.periphs
Subject: Re: Definition of "null modem" cable
Message-ID: <195@laidbak.UUCP>
Date: Sun, 1-Sep-85 14:26:36 EDT
Article-I.D.: laidbak.195
Posted: Sun Sep  1 14:26:36 1985
Date-Received: Mon, 2-Sep-85 04:35:04 EDT
References: <195@almsa-1> <1734@hao.UUCP>
Reply-To: jeq@laidbak.UUCP (Jonathan E. Quist)
Distribution: net
Organization: LAI Chicago
Lines: 55
Xref: watmath net.dcom:1236 net.periphs:840
Summary: 

In article <1734@hao.UUCP> hull@hao.UUCP (Howard Hull) writes:
>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
DEC obviously hasn't used equipment that toggles Request to Send
wired to a machine as above.  Most all the UNIX terminal drivers
I've seen would have a fit if the state of Carrier Detect
was constantly changing.  Wiring Data Terminal Ready on one end
to Carrier Detect on the other end seems to nearly always work.
(Data Set Ready can be substituted if equipment demands.)
Request to Send should go to Clear to Send on the other end,
and vice verse.  If you're going to use it, it may as well
be right.  If equipment on one end doesn't support it,
jumper them on *that* end.  (A lot of equipment doesn't bother with
them anyway; I've only seen it on printers that couldn't
generate ^S/^Q (*gasp*) for flow control.)
Safety Ground (pin 1) should NOT be carried all the way through,
lest you develop ground loops.  If you have shielded cable,
by all means use the shield, though.  More often than not,
the best ground will be at the computer end.  (This assumes
a proper installation, all disks, power supplies, and
cpu's grounding to a single point.)

Jonathan E. Quist
ihnp4!laidbak!jeq
``I deny this is a disclaimer.''