Newsgroups: sci.electronics
Path: utzoo!henry
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: RS-232 powered modems
Message-ID: <1991Jan21.025151.6524@zoo.toronto.edu>
Organization: U of Toronto Zoology
References: <19983@hydra.gatech.EDU>
Date: Mon, 21 Jan 1991 02:51:51 GMT

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? ...

If you're thinking of the really tiny portable ones, there is generally
a 9V battery tucked in somewhere, I believe.

It is possible to pull a small amount of power from the phone lines, but
this has to be done very carefully.  There have been line-powered modems,
but my impression is that they historically have not been very satisfactory.
The available power is quite small and somewhat variable.

>...s there a +5V line on RS-232C and is it standard with DB25 and DB9
>connectors?  Or is there a +12,etc ...

No.  There is no dependable source of any power voltage on an RS232
connector with 25 pins, much less one with 9.

Some very small leased-line modems draw their power from the RS232 signal
and modem-control lines, but this takes quite clever low-power design.
The total available is measured in individual milliamps.
-- 
If the Space Shuttle was the answer,   | Henry Spencer at U of Toronto Zoology
what was the question?                 |  henry@zoo.toronto.edu   utzoo!henry