Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!decwrl!sgi!vjs@rhyolite.SGI.COM
From: vjs@rhyolite.SGI.COM (Vernon Schryver)
Newsgroups: comp.mail.uucp
Subject: Re: Bidirectional Modem Lines under SunOS 4.0.1
Summary: pullups considered good
Message-ID: <30390@sgi.SGI.COM>
Date: 10 Apr 89 23:57:00 GMT
References: <160@osc.COM> <743@key.COM> <260@ladcgw.UUCP> <16910@dcatla.UUCP>
Sender: daemon@sgi.SGI.COM
Organization: Silicon Graphics, Inc., Mountain View, CA
Lines: 27

In article <16910@dcatla.UUCP>, itwaf@dcatla.UUCP (Bill Fulton [Sys Admin]) writes:
> I would think that CD should actually be pulled LOW by the
> internal hardware, requiring that it be actively tugged up in order for a
> connection to be honored, and insuring that a physical disconnect be sensed.
> Anybody out there familier enough with RS232 specs to comment?
> 
> Bill Fulton
> ..!gatech!dcatla!itwaf

I call this a feature, not a bug.  Having a 5-10K pullup on DCD and
possibly CTS and maybe DSR is good.  It makes common 3-wire cables
(RD,TD,SG) work, without hurting real modem use.  As long as the modem
follows RS-232-C, avoiding the +/- 3v dead zone, and so is not just a TTL
driver (does anyone still use such hacks?),  what's the harm?

Having a session going on an unplugged port does not sound like much a
security problem. :-)

At various employers over the last couple of decades, I've done it both
ways, with and without pullups, and now prefer having pullups.  Having to
repeatedly communicate to novices that their dumb device which uses only
TD,RD,&SG must have a connector which wraps RTS/CTS, DTR/DCD/DSR, and so on
and on is harmful to one's good humor.

Vernon Schryver
Silicon Graphics
vjs@sgi.com