Path: utzoo!utgpu!cunews!software.mitel.com!grayt
From: grayt@Software.Mitel.COM (Tom Gray)
Newsgroups: sci.electronics
Subject: Re: RS-232 powered modems
Message-ID: <6128@healey>
Date: 21 Jan 91 16:17:58 GMT
References: <19983@hydra.gatech.EDU> <1991Jan21.025151.6524@zoo.toronto.edu>
Organization: Mitel. Kanata (Ontario). Canada.
Lines: 23

In article <1991Jan21.025151.6524@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes:
: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.
:

The telephone loop supplies power at a nominal voltage of 48V (max 56, min 42).
The DC resistance of the loop will vary from 0 to 1800 ohms.

During an active call (line off hook) you may draw up to 40mA from the loop
at whatever voltage the loop is going to give you (remember the loop resistance).
When the line is inactive, you may not draw more than 3 or 4mA from the loop
Drawing more than this will falsely start a new call.