Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!netnews.upenn.edu!dsl.cis.upenn.edu!touch
From: touch@dsl.cis.upenn.edu (Joe Touch)
Newsgroups: sci.electronics
Subject: Re: Help Needed: TTL<->RS232 without -12vdc
Keywords: RS232, TTL, level shifting
Message-ID: <28835@netnews.upenn.edu>
Date: 26 Aug 90 05:15:20 GMT
References: <7574@scolex.sco.COM> <1990Aug25.232127.4269@hayes.fai.alaska.edu>
Sender: news@netnews.upenn.edu
Reply-To: touch@dsl.cis.upenn.edu (Joe Touch)
Organization: University of Pennsylvania
Lines: 26

In article <7574@scolex.sco.COM>, deanr@sco.COM (Dean Reece) writes...
>
>Hi, I need some help with RS232 drivers/receivers
>I'm building a project that needs a standard RS232 port, but only has
>available supplies of +5, +12, and +24 volts.  I thought that I
>might be able to use the 0(GND), +12, and +24 kinda like the +/- 12v supply
>usually called for.  I couldn't think of any trivial way to do this,

How about the following...

Remember that an RS232 needs only a 6V level shift, 3V on either
side of ground (lower than -3V is a logical '1', greater than +3V
is a logical '1').

First convert the signals to CMOS levels, i.e. 0V and 5V for 0 and 1.
Then use variable voltage followers to set up a 4.5V and 9V references.
Send the 0-5V signals into a CMOS driver powered by the 9V circuit.
The output will be signals will a shift of 0-9V.

Connect the output of the CMOS driver (high current) to the RS232
data line.  Connect the signal ground of the RS232 to the 4.5 V ref.

I think this is a quick way to solve the problem, and uses only
2 cmos high-current drivers and 2 voltage references.  

Joe