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