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From: mcovingt@athena.cs.uga.edu (Michael A. Covington)
Newsgroups: sci.electronics
Subject: Re: RS-232 powered modems
Message-ID: <1991Jan20.232819.23502@athena.cs.uga.edu>
Date: 20 Jan 91 23:28:19 GMT
References: <19983@hydra.gatech.EDU>
Organization: University of Georgia, Athens
Lines: 11

In article <19983@hydra.gatech.EDU> jgb@prism.gatech.EDU (James G. Baker) writes:
>I've seen small 2400 baud modems that *only* have COM and phone connections.
>How do these pull power from the telephone or computer?  I need to make a 
>box with similar needs (a few LS parts) and of course they need a +5 power
>supply.  Is there a +5V line on RS-232C and is it standard with DB25 and DB9
>connectors?  Or is there a +12,etc that I could feed into a voltage regulator?
>

No, there are no power lines. What you can do is use switching diodes to
automatically give you a path to the most positive pin(s) and draw just
a _little_ power from them. It has to be micropower (on the order of 1 mA).