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From: phil@amdcad.UUCP (Phil Ngai)
Newsgroups: net.bugs.4bsd
Subject: Re: Login Problem (250' RS-232 Lines)
Message-ID: <3683@amdcad.UUCP>
Date: Sun, 8-Sep-85 20:43:11 EDT
Article-I.D.: amdcad.3683
Posted: Sun Sep  8 20:43:11 1985
Date-Received: Mon, 9-Sep-85 04:11:04 EDT
References: <132@ndsuvax.UUCP>
Reply-To: phil@amdcad.UUCP (Phil Ngai)
Organization: AMD, Sunnyvale, California
Lines: 33


This doesn't really belong in net.bugs.4bsd but there are two things you
can do. One is to make getty ignore its input for a couple of seconds
after it sends out its banner and then flush input. This will have the
side-effect of screwing your users who type quickly but they will learn
to wait before trying to type their name.

The second one is more drastic and is needed if you run two or more terminals
over a single cable, such as inside telephone 25 pair cable and some of
the lines are left open or connected to turned off terminals. This is
a hardware hack. What you need to do is pull the computer's receive line
far enough below its logic threshold that crosstalk from the adjacent
lines will not be seen as input. If you can find the right voltage on the
DMF, then you just need a (perhaps 10K) resistor per tty port. If you don't
have access to anything (our situation) you can fake it with a diode, a
resistor, and a cap. Connect the cathode of the diode to the computer's 
transmit line. Connect the anode to a .1uF cap. Connect a 10K resistor
between the cap and the receive line of the computer.

This is a gross and ugly kludge but it does work. The diode and cap
implement a -12V source for the pulldown resistor without loading the
transmit line in a noticeable way.

Hope one of these works for you.


-- 
 A hacker is someone who orders Sweet and Sour Bitter Melon just because
 it is "an impossible combination".

 Phil Ngai (408) 749-5720
 UUCP: {ucbvax,decwrl,ihnp4,allegra}!amdcad!phil
 ARPA: amdcad!phil@decwrl.ARPA