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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: Help Needed: TTL<->RS232 without -12vdc
Summary: Charge pump for -12V generation
Keywords: RS232, TTL, level shifting
Message-ID: <6743@milton.u.washington.edu>
Date: 26 Aug 90 07:36:42 GMT
References: <7574@scolex.sco.COM>
Organization: University of Washington, Seattle
Lines: 32

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. 

	Uhh... no, the third-wire ground on most machines will cause your
12V supply to burn up if you actually connect +12 to the RS-232
(or any other) grounded point.  Since you aren't starting from +5
only, the MAX232 is overkill; just make a 555 oscillator with a charge
pump rectifier like so:


         +12
          |
       -------
      |       |     C1
      |  555  +----| |----+-----|<|-----+----   -12V output
      |       |           |             |
      --------            -             -  C2
          |               V             -
         GND              -             |
                          |            GND
                         GND

	With a frequency of 50 kHz on the '555 (I've omitted the timing
components), and C1= C2= 1uF, this should easily supply several
1488's with their negative power.

		John Whitmore