DTR is the Data Terminal Ready signal. Some telecom equipment use the
acronym DTR to mean DialTone and Ringing. This is a circuit that provides
the dialtone and ringing signals over an analog phone line. 

This is not a function of an RS-232 port. There are no tones on RS-232
lines. Just binary signals.


> -----Original Message-----
> From: Gan Uesli Starling [mailto:alias@starling.us]
> Sent: Friday, December 06, 2002 7:08 AM
> To: netbsd-help@netbsd.org
> Subject: Re: Detecting a closed circuit
> 
> 
> James K. Lowden writes: 
> 
> > I think this would work for two machines.  In terms of 
> RS-232, assuming
> > these machines are within 100 feet of each other, I don't 
> see any reason
> > you couldn't tie their DTRs together and their CDs 
> together, and bridge
> > them both with the same close-on-battery curcuit.   
> > 
> > It looks easy from here, but I never did any such thing.  
> And, see, no
> > relay. :) 
> > 
> > --jkl
> 
> This seems is a good idea...except for one tiny thing...
> the part about multiple machines on a single contact. 
> 
> I am a little fuzzy on this...but I do not think you should
> wire a pair of DTR lines together. If memory serves, DTR
> supplies a tone, yes? 
> 
> So...they are little generators. What are the chances
> that any two would be in phase? I think they might oppose
> one another. And if they chanced to be 180 degrees out of
> phase...that could be bad, yes? 
> 
> You might be able to get away with one DTR fanning out to
> two or more CD inputs. But there is a limit on fan-out...I
> do not know what that limit is. 
> 
> Or you could just do like James said with the CD and DTR
> lines, but separately, not joined. Then get somebody to
> multiply in parallel the contact closure with opto-isolators
> from Radio Shack. (Chances are the contact closure in the
> UPS is just power opto-isolator anyway.) 
> 
> The opto-isolator circuit would require some power. But not
> much...a trifle. But that too could be gotten from Radio
> Shack. Any tech in possession of a soldering iron ought to
> be able to puzzle that one out. 
> 
> Once you have it going. Send me the specs and I will happily
> post it, schematic and all, for others to follow. Send that to
> gan@starling.us if you so choose. 
> 
> Enjoy, 
> 
> Gan Starling 
> 
> PS - Hi James. 
> 
>  --
> Mistera Sturno - Rarest Extinct Bird 
> 
> <(+)__        Gan Uesli Starling
>  ((__/)=-    Kalamazoo, MI, USA
>   `||`
>    ++        http://starling.us 
>