Path: utzoo!attcan!uunet!iscuva!jimc
From: jimc@iscuva.ISCS.COM (Jim Cathey)
Newsgroups: sci.electronics
Subject: Re: Powering circuits from serial port
Message-ID: <1712@iscuva.ISCS.COM>
Date: 5 Jul 88 20:15:48 GMT
References: <1988Jul1.124545.14929@gpu.utcs.toronto.edu>
Organization: ISC Systems Corporation, Spokane, WA
Lines: 28

In article <1988Jul1.124545.14929@gpu.utcs.toronto.edu> lharris@gpu.utcs.toronto.edu (Leonard Harris) writes:
>
>I have a small circuit (3 ttl) that hangs off a rs232 serial port.  Rather
>than add a power supply, is it possible to power this directly by stealing
>the CTS/DSR signals and converting to 5V with a zener diode?  Also, other
>than using the MAX232 level converter, is there an easier (and cheaper) way
>of converting ttl to rs232 levels - again without using an external power
>source.  (the distance signals are travelling are less than 3 feet)

You can steal power from DTR/RTS/DSR etc, but you can't draw too much.
Say a few mA from each line (it would help to know the exact number of
the driver chip(s) involved).  If you use 4000 family CMOS you don't
even need to regulate what you get!  Watch out for -12V on those lines
when negated.  So far as transmission goes, so long as you're only
going a few feet a TTL output can probably feed directly the RS-232
receivers (though you might need a pull-up if you're using real TTL and
not CMOS).  Most of those things need a few volts (like 1-2) to read
as positive, and less than that as negative.  It does depend on what 
you're driving, but 'violating' the spec often works.  

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