Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!gatech!dcatla!itwaf
From: itwaf@dcatla.UUCP (Bill Fulton [Sys Admin])
Newsgroups: comp.mail.uucp
Subject: Re: Bidirectional Modem Lines under SunOS 4.0.1
Summary: CD must be actively pulled low on some/all Sun machines.
Message-ID: <16910@dcatla.UUCP>
Date: 10 Apr 89 19:38:03 GMT
References: <160@osc.COM> <743@key.COM> <260@ladcgw.UUCP>
Reply-To: dcatla!itwaf@gatech.UUCP (Bill Fulton [Sys Admin])
Organization: DCA Inc., Alpharetta, GA
Lines: 37

In article <260@ladcgw.UUCP> frank@ladcgw.UUCP (Frank Mayhar) writes:
>The problem I found with bidirectional modem lines on my Sun 386i/150 running
>4.0.1 was that, even when I made the kernel fix (a change to the serial port
>flags), getty was still able to open the line, EVEN WITH THE CABLE UNPLUGGED!
>Apparently either the flag is ignored (bug!) or the connection has a short.
>-- 
>Frank Mayhar       <known-world>!uunet!ladcgw!frank

Great! Two months after I wrestled with this problem, along come some 
great articles about how to make it work.  :-)

Here's something I found out, relevant to the above follow-up, that may be of
interest. I felt real dumb after I figured it out, but now I know there's
at least one other guy who missed it, too. (Sorry, Frank!)

On both a 3/50 Sun workstation, and a 3/280 Sun server (and so, I guess, on
all/most Sun machines), the CD signal must be *ACTIVELY* pulled low. It seems
that the Sun hardware actually pulls CD a little bit *HIGH*. This means
that, even if you made all the necessary software/configuration changes
(an experience in itself), if you just unplug the RS232 cable, the session/
application on that port will NOT sense a CD transition, and the session
will stay up! (Can you say "security hole"?) I had a user who's application
was never getting the SIGHUP when a *MODEM* hung up!!!

Given that I have somewhat limited experience - I was still pretty surprised
at this behaviour. I tried to contact the Sun hotline about this (just to get
thier opinion/statement), but they said it was a (all together now) "hardware
problem". 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?

Anyway, hope this helps some - it sure would have saved me at least one late
night - and a lot of forehead slapping, after the "problem" was recognized!

Bill Fulton
..!gatech!dcatla!itwaf