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From: mangler@cit-vax.Caltech.Edu (System Mangler)
Newsgroups: net.unix-wizards,net.unix
Subject: Re: Arcane modem configuration (was Re: Modems on Ultrix)
Message-ID: <926@cit-vax.Caltech.Edu>
Date: Mon, 25-Aug-86 00:18:51 EDT
Article-I.D.: cit-vax.926
Posted: Mon Aug 25 00:18:51 1986
Date-Received: Mon, 25-Aug-86 03:04:20 EDT
References: <282@cirl.UUCP> <3030@umcp-cs.UUCP>
Distribution: net
Organization: California Institute of Technology
Lines: 30
Summary: DEC null modem cables incompatible with tty boards
Xref: mnetor net.unix-wizards:7677 net.unix:5278

In article <3030@umcp-cs.UUCP>, chris@umcp-cs.UUCP (Chris Torek)
advises configuring all ports for modem control (flags=0x0) and
using null modems for directly-connected terminals.

Chris, as usual, has exactly the right idea (if we ignore DMF's).
The problem is all those improperly wired cables out there.

For instance, the BC22D null modem cable that DEC used to sell for
the DZ11 connects DTR to DSR, leaving CXR (carrier) open.  But the
DZ11 wants CXR and doesn't even have a wire connecting to DSR.
Most tty interfaces are similar.

To clarify:
DTR (pin 20)	Tells the modem that the terminal/computer is alive.
DSR (pin 6)	Tells the terminal/computer that the modem is alive.
CXR (pin 8)	Tells the terminal/computer that carrier is present.

What Unix cares about is Carrier (CXR), i.e. whether a connection is
present.  How many of your cables have pin 8 connected to anything?
Oh, all your cables have only 3 wires (like ours)?

If you can connect CXR to DTR, even if it just loops back in the
connector, you will most likely still save yourself all this grief
with carrier detect.  Just one jumper from pin 20 to pin 8 - that
isn't too hard.

(DMF's have to use carrier flags no matter what, because the wires
just aren't there to jumper).

Don Speck	speck@vlsi.caltech.edu	seismo!cit-vax!speck