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From: bugs@freebsd.netcom.com (Mark Hittinger)
Message-Id: <199607092118.QAA24002@freebsd.netcom.com>
Subject: Re: forcing a modem to hangup (fwd)
To: questions@freebsd.org
Date: Tue, 9 Jul 1996 16:18:35 -0500 (CDT)
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>terry@lambert.org writes:
> The default POST setting for DTR should be such that it is *not* asserted
> unless it has been specifically asserted.
> 
> An on-to-off transition of DTR will cause your modem to reset as if
> powered off then on.
> 
> Failure modes:
> 
> 1)	Internal modems.  Don't buy them, you can't trust them.
> 2)	Modem not set to follow DTR correctly.  Modem should reset
> 	as if powered off then back on on on-to-off DTR transition;
> 	if your modem does not do this, it is set up incorrectly.
> 3)	Modem does not signal DCD loss to computer.  Modem should
> 	on-to-off transition DCD signal when remote carrier is lost.

Another weird failure mode is that the dtr signal is not dropped for long
enough for your modem - you might see this on a faster box.  I remember 
having to patch the getty on Xenix to sleep an additional second before 
opening the terminal device - this forced dtr down at least a second 
and allowed the required hangup.

Regards,

Mark Hittinger
Netcom/Dallas
bugs@freebsd.netcom.com