Newsgroups: sci.electronics
Path: utzoo!henry
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: RS-232 powered modems
Message-ID: <1991Jan27.030250.23175@zoo.toronto.edu>
Organization: U of Toronto Zoology
References: <19983@hydra.gatech.EDU> <2820@casbah.acns.nwu.edu> <2663@krafla.rhi.hi.is> <1010@eplunix.UUCP>
Date: Sun, 27 Jan 1991 03:02:50 GMT

In article <1010@eplunix.UUCP> raoul@eplunix.UUCP (Nico Garcia) writes:
>Ummm, I'm looking at a listing of the EIA signals from a Maxwell modem right
>here. It says pin 9 is +Testing Voltage, Pin 10 is -Testing Voltage. Now, if
>the *terminal* is wired appropriately, there could indeed be a voltage source
>there...

Those test voltages are usually run through something like a 1k resistor so
you can't accidentally fry things by misconnecting them.  This also severely
limits their usefulness as power sources, assuming your device generates them
at all.  (Many [most?] professional-grade modems do, but I doubt that many
terminals do.  PC-market modems are anyone's guess.)

As I said before, but not emphatically enough:  there is *no* pin on a
standard RS232 connector which is guaranteed to be able to supply a serious
amount of power.  Some devices do put power pins on RS232 connectors; this
is useful if you've got it, but it's not dependably available on a random
RS232 connector.  The standard RS232 pins are all signal pins except for
those two test voltages.
-- 
If the Space Shuttle was the answer,   | Henry Spencer at U of Toronto Zoology
what was the question?                 |  henry@zoo.toronto.edu   utzoo!henry