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From: howard@COS.COM (Howard C. Berkowitz)
Newsgroups: comp.unix.questions
Subject: Re: The DSR pin - what is it used for?
Message-ID: <417@cos.COM>
Date: Wed, 26-Aug-87 16:12:44 EDT
Article-I.D.: cos.417
Posted: Wed Aug 26 16:12:44 1987
Date-Received: Sat, 29-Aug-87 03:12:01 EDT
References: <304@pvab.UUCP>
Organization: Corporation for Open Systems, McLean, VA
Lines: 41
Summary: DSR usually not significant but still needed

In article <304@pvab.UUCP>, robert@pvab.UUCP (Robert Claeson) writes:
> This is maybe a dumb question, but what is the intended use of pin 6,
> the one named DSR or Data Set Ready in the RS232 standard?

There is a difference between what the standard says and what most
implementers do with it.  The standard says:

"Signals on this circuit are used to indicate the status of
 the local data set.  The ON condition on this circuit is
 presented to indicate that
    
    a) the local data communications equipment is connected to
       a communication channel (OFF HOOK in switched service)
  AND b) the local DCE is not in test (local or remote),
         talk, or dial mode
  AND c) the local DCE has completed, where applicable,
          1.  any timing functions required by the switching
              system to complete call establishment,
          2.  the transmission of any discrete answer tone..."

In practice, the signal does not mean that much on most modems;
it usually means only that the modem has power.  Carrier detect
and Clear to Send usually mean, to a much greater extent, that
you have a viable channel.

 I've seen
> that some devices (especially modems) uses it, but most ignores it (such
> as computers and terminals). Is it really needed for modem-control
> signaling or can I simply connect it to pin 20 (DTR) at the other end?
> How should it be connected in a null-modem cable when I want to connect
> two DTE devices with modem control?
  
    Be sure you connect Pin 20 of end 1 to Pin 6 of end 2 and vice
   versa, or simply connect local Pin 20 to local Pin 6 at both 
   ends.   You may also want to tie Pin 8 to pin 20 at each end.

-- 
-- howard(Howard C. Berkowitz) @cos.com
 {seismo!sundc, hadron, hqda-ai}!cos!howard
(703) 883-2812 [ofc] (703) 998-5017 [home]
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