Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!ukc!acorn!moncam!emmo
From: emmo@moncam.co.uk (Dave Emmerson)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Help on programming of the Serial P
Summary: Ask Maxim
Message-ID: <410@marvin.moncam.co.uk>
Date: 6 Feb 90 03:14:16 GMT
References: <8137@<25B62374> <111700197@uxa.cso.uiuc.edu>
Organization: Monotype ADG, Cambridge, UK
Lines: 23

In article <111700197@uxa.cso.uiuc.edu>, mms00786@uxa.cso.uiuc.edu writes:
> 
> Hey! We just learnt about the Max232 in class; so how does it manage to turn
> a DC +5 into +- 12? The only way I can think of is to make an oscillator, feed
> it to a transformer, and convert it back to DC. But how do you do it within a
> chip?
> 
> Milan.
> .

You don't - quite. The chip has sets of mosfets acting like switches.
First you charge up the 22uF tant straighht across the power rails,
then you unhook it, connect it's 'negative' lead to +5volts, et voila-
the positive side is at +10 volts, compared to ground. Do this repeatedly
at high frequency, add a bit of smoothing, and stack 2 more like it
to get the -10 volts, this time connecting the tant's positive side to
ground, and that's a crude 232 chip. 
The idea had been around for years, the technology to put those
super fet's all on one small chip only got here a couple of years back.
There's a new one out now which doesn't need any external tants! I
haven't got a data sheet yet, but I'm told it works fine.

Dave E.