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From: squishy@casbah.acns.nwu.edu (Shishin Yamada)
Newsgroups: sci.electronics
Subject: Re: RS-232 powered modems
Summary: Where's the Power?
Keywords: RS-232, Self-powered
Message-ID: <2846@casbah.acns.nwu.edu>
Date: 22 Jan 91 06:41:19 GMT
References: <2820@casbah.acns.nwu.edu> <1991Jan21.050956.26328@athena.cs.uga.edu> <1991Jan21.142804.5726@uhura.neoucom.EDU>
Organization: Northwestern University
Lines: 51


	I might be wrong about that RS-232 and power supply. I remember
working on the IBM way back when (and frying a homebrew periapheral
interface adapter). At that time it was on a big IBm with a 25 pin RS-232
port. I do not think the 9 pin RS-232 port has the same thing.

	I could very well be wrong (and thinking of the IBM's parallel port
since I worked with that too). It is hard because I remember how tough it
was to come up with spec sheets for the RS-232 pinouts at the time.
Anyways, now I am a dedicated Mac User with its RS-422 ports that are
downward compatable with RS-232.

	In more enlightening (and accurate) news, I just picked up a
Practical Peripherals promo sheet about their new modems. In particular the
Practical Pocket Modem (2400PPM) says the following:

	"...Thanks to its remakable design, no-fail power comes directly
from a RS232 port and the telephone line; you never have to find a wall
outlet or change batteries.... Mac users will need a separate 9V battery
adaptor included in the optional Macintosh package."

	I know for SURE that Mac doesn't support any power lines since the
Mac Plus. The Mac Plus and earlier supported +5V on their 9 pin serial
lines (very similar to RS-232 nine pin standard, and I believe the same
pinout too). More recent macs drop this because of third party use that
damaged many weak Mac Plus power supplies. 

	Having thought about storing power on such a small modem poses
problems, as a storage device has to be extremely small. Also, if taken
from the phone line when "off-hook" (a DC voltage is assumed), you'd have
to look out for the 80-100 Volts AC that occur during ringing. If it was
drawn from the RS-232 data/hankshake lines, it would need a method of
tapping, filtering, and stabilizing the power. It would seem to me that
tapping from the DTR line would be the BEST bet (if there's no power line)
since it could be turned on/off at will from the computer and is not used
in modems without flow control. In this case, the 2400PPM seems to be a
standard 2400 bps without data compression/error control, which means it
probably doesn't require DTR handshaking. Please note, I am only guessing...

	Anyways, I do know of a more probable and better source of
information. You could try "GO PPIFORUM" on Compuserve or even the
"comp.dcom.modems" news group.

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Shishin "Squish" Yamada		Squishy@Casbah.Acns.Nwu.Edu
Northwestern University		Electrical Engineering Class of 1991
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PS: If anyone has a text of RS-232 standards (25/9 pin), I would appreciate
an e-mail. For sake of conscience, I'd like to see again what was on those
specs.