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From: squishy@casbah.acns.nwu.edu (Shishin Yamada)
Newsgroups: sci.electronics
Subject: Re: RS-232 powered modems
Summary: How are they powered
Keywords: RS-232, Self-powered
Message-ID: <2820@casbah.acns.nwu.edu>
Date: 21 Jan 91 02:36:47 GMT
References: <19983@hydra.gatech.EDU>
Organization: Northwestern University
Lines: 34

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?

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.

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.

In the MacIntosh world (my niche), the RS-422 interface (a better and
compatable system to RS-232) does not support power lines. This is because
the Mac Plus was VERY prone to power overload with its small power supply.
In the example of the Practical Peripherals Pocket Modem, the Mac version
needs a 9 Volt battery to make it work. This might also be necessary with
any portable computer (pocket modems chief end-user).

Well, I hope that helps :)

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Shishin Yamada			Northwestern University
Electrical Engineering		Class of 1991
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