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From: ftpam1@acad3.alaska.edu (MUNTS PHILLIP A)
Newsgroups: sci.electronics
Subject: Re: RS-232 powered modems
Message-ID: <1991Jan21.234914.25788@ims.alaska.edu>
Date: 21 Jan 91 23:49:14 GMT
References: <19983@hydra.gatech.EDU> <1991Jan21.025151.6524@zoo.toronto.edu> <6128@healey>
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In article <6128@healey>, grayt@Software.Mitel.COM (Tom Gray) writes...
>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.

    FCC Part 68 specifies this for the United States.  I believe the on-hook
impedance must be greater than 10 megohms.  I have designed products that
incorporate "line sniffers" that detect loss of the -48V on-hook voltage.  The
strategy is to slowly charge a low leakage capacitor thru a 10 megohm resistor
and discharge it rapidly thru a coupling transformer or optocoupler.  The
whole cycle takes a few seconds.  Interestingly, this all falls apart when the
phone line is shared; the off-hook voltage is not enough to detect reliably.
So we added special loop current relays to detect the off-hook condition.

Philip Munts N7AHL
NRA Extremist, etc.
University of Alaska, Fairbanks