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000263_news@columbia.edu _Tue May 2 10:37:08 2000.msg
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From: Peter Easthope <peter_easthope@gulfnet.pinc.com>
Subject: Re: MS-DOS null cable
Date: Tue, 02 May 2000 14:10:35 GMT
Organization: Deja.com - Before you buy.
Message-ID: <8emnkc$1ei$1@nnrp1.deja.com>
To: kermit.misc@columbia.edu
In <3905B873.3F230E52@home.com> posted at 2000 Apr 25,
Andre Cormeau <NOacorSPAM@home.com> wrote,
ac> I am having no luck with a null modem connection ...
Most documentation, EIA-RS-232 for example, assumes the 25
pin connector. In an e-mail Mark Sapiro reported this
correspondence between the two connectors.
DB-9 pin signal DB-25 pin
1 Carrier Detect 8
2 Receive Data 3
3 Transmit Data 2
4 Data Terminal Ready 20
5 Signal Ground 7
6 Data Set (Modem) Ready 6
7 Request to Send 4
8 Clear to Send 5
9 Ring Indicator 22
Signal directions as well as nomenclature are necessary to
understanding serial ports and RS-232 is not a
particularly good exposition. The book _Microcomputer
Buses and Links_, pp. 336-338, was helpful to me. Sorry
not to have the name of the author or ISBN; I found it in
the UBC Library, Science Reference. _Books in Print_
might help.
Serial ports do not implement RS-232 fully. The specs for
serial ports on Macs and on modems are not hard to find
but I have never seen the specs for the IBM serial
port--they must exist somewhere.
There is more than one wiring diagram for a null modem.
The book shows a few and you will find others. The
ambiguity is possible because the various "ready" signals
have redundancy. Also, some null modems short circuit
some "ready" signals on a port rather than take them from
the other machine. Obviously a practice to avoid where
possible.
I have used this wiring for Kermit transfers between pc's
for years. It is not one of the wirings specified in the
book above; that is deliberate. The Mac serial port lacks
some of the pins which the IBM pc has. This wiring works
between any of the 3 possible combinations of Mac and IBM
machines. A Mac modem cable connects between the null
modem and the Mac. The null modem cable plugs into the
IBM PC serial port directly.
1 1 chassis ground
2->3
3<-2
4->8
5<-20
6 6
7--7
8<-4
20->5
25--25
Yes, 1 1 is not connected but it probably will not hurt; I
don't understand the ground and shielding issues well
enough to say. Yes, 6 6 is not connected. I don't recall
where the 25--25 connection came from. Probably it does
nothing except make the assembly stronger--not a trivial
matter.
I spent many hours experimenting and finding documentation
to obtain a workable result. Information on null modems
might be a worthwhile addition to the Kermit Web site
(hint).
Regards, peter_easthope@gulfnet.pinc.com
Sent via Deja.com http://www.deja.com/
Before you buy.