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From: adamd@rhi.hi.is (Adam David)
Newsgroups: sci.electronics
Subject: Re: RS-232 powered modems
Keywords: RS-232, Self-powered
Message-ID: <2663@krafla.rhi.hi.is>
Date: 22 Jan 91 17:07:15 GMT
References: <19983@hydra.gatech.EDU> <2820@casbah.acns.nwu.edu>
Organization: University of Iceland
Lines: 49

In <2820@casbah.acns.nwu.edu> squishy@casbah.acns.nwu.edu (Shishin Yamada) 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?  I need to make a 
>>box with similar needs (a few LS parts) and of course they need a +5 power
>>supply.  Is there a +5V line on RS-232C and is it standard with DB25 and DB9
>>connectors?  Or is there a +12,etc that I could feed into a voltage
>regulator?

CMOS parts and clever power circuitry are needed here to draw power from the
RS-232C and/or the phone line.

>The phone line supplies a small amount of DC bias typically when off-hook.
>I know in certain areas it also supplies a small bias even when on-hook.
>However, these voltages are NOT meant for use, and should not be used for
>driving external circuits.

However some products use the phone line for power. The phone company would not
be happy with any large load on the wires, and indeed the equipment which
attempted to load the wires too much would be unreliable or simply wouldn't
work. There are probably recommendations for maximum permissible loading of the
phone lines.

>On the other hand however, the RS-232 standard has connectors for +5V on
>its line. It should in serious cases have a +12V line. The modem most
>likely makes its connection here. Certain computers (notably non-ibms) and
>possibly portables do not support these lines because it in these machines
>their power supplies could overheat and breakdown. This is probably no
>longer a huge problem since IBM keeps building huge wattage power supplies
>and newer circuit boards make better use of less power.

Some "RS-232C" ports have power connections but the vast majority don't. On some
newer systems which use RTS/CTS handshaking (as opposed to DTR/CTS), it may be
possible to draw power from DTR depending on current drive capabilities of that
signal. DTR is in these cases at a steady positive level, either all the time
the computer is switched on or whenever the serial port is open.

>needs a 9 Volt battery to make it work. This might also be necessary with
>any portable computer (pocket modems chief end-user).

I have used a portable modem and use portable computers. I don't intend to use
the portable modem much on batteries because the modem fails instananeously as
soon as the batteries hit a certain voltage. What I'd really like to use is a
modem (2400 bps or more) that draws all its power from the phone lines (at least
while online), using a high level at DTR to switch the modem on. Does anyone
know about such creatures?
--
Adam David.  adamd@rhi.hi.is