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DPRG: Real good riddle.

Subject: DPRG: Real good riddle.
From: Kevin Ross kevinro at nwlink.com
Date: Sun Jul 27 20:55:18 CDT 1997

- ----------
> From: Kipton Moravec <kmoravec at airmail.net>
> To: dprglist at dprg.org
>
> So how do you convert the microcontroller's TTL signal to RS-232?  That
> is where the MAX232 chip comes in.  It is very inexpensive <$2 I think.
> (I have not purchased one.)
>

The MAX232 is a single supply (+5v) RS-232 transciever. Its primary
function is to convert between TTL and RS-232. To do this, it uses an
internal charge pump to double the +5 volts to +10 volts. Then, it uses
another circuit to convert the +10 volts to -10 volts, which is actually
what the RS-232 standard calls for. A 0 is transmitted by putting +5 to
+15volts on the transmit line, and a 1 is transmitted by putting -5 to -15
volts on the line. This is why you can just hook up a TTL level device,
because it cannot in general handle these types of voltage levels.

The MAX232 is a 18 pin DIP that requires 5 external capacitors, +5v, and
GND. You can wire them up in a simple circuit. (There are variations, such
as the MAX233 which don't need external capacitors, but cost a few bucks
more). I use the MAX232ACPE, which only needs .1uF caps. Others require
10uF caps, which are bigger and cost more. 

A simple circuit for this can be seen at 

http://kevinro/srs/encoder/apr97/pcbug11.html
 
Kevin

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