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'[EE]: How can I put together a discrete RS-232 lev'
2001\01\21@121641 by Byron A Jeff

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I'm current using Wouter's Wloader and Woj's linwload to program 16F877 parts.

You can find the wloader homepage here:
http://www.xs4all.nl/~wf/wouter/pic/wloader

Wouter uses a discrete level translator to build a single pin interface between
the PIC and the serial port. It consists of a 330 ohn and two 10k current
limiting resistors, a 47k pullup and 5V zener that translates the -12/12V
RS-232 to 5/0V for the PIC.

Unfortunately it doesn't work with my serial port, which seems to require
the standard +3/-3V thresholds. Wouter points out that a MAX-232 interface
can be built to maintin proper RS-232 levels, but I'd prefer not to have to
wire the entire chip to maintain proper RS-232 levels.

So I'd like to get some advise about wiring a transistor switch to switch
the -12V available on DTR instead of passing 0V. So that 5V will pass 5V but
0V will turn on the transistor and pass -12V.

This question in general is how to switch a larger magnitude voltage with a
smaller magnitude control signal. I need a simular circuit for switching
Vpp with 5V.

Any suggestions will be welcome.

BAJ

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2001\01\21@123926 by Dan Michaels

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wrote:

>So I'd like to get some advise about wiring a transistor switch to switch
>the -12V available on DTR instead of passing 0V. So that 5V will pass 5V but
>0V will turn on the transistor and pass -12V.
>

Derivation of several published circuits:

http://www.oricomtech.com/appnotes.htm
AN-ECD02 - A Simple Inexpensive Network-Compatible RS-232 Interface:
an-ecd02.zip

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2001\01\21@150229 by Wojciech Zabolotny

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On Sun, Jan 21, 2001 at 12:17:37PM -0500, Byron A Jeff wrote:
> Unfortunately it doesn't work with my serial port, which seems to require
> the standard +3/-3V thresholds. Wouter points out that a MAX-232 interface
> can be built to maintin proper RS-232 levels, but I'd prefer not to have to
> wire the entire chip to maintain proper RS-232 levels.
>
Simple transistor based circuits are published at:
http://www.ise.pw.edu.pl/~wzab/picadc/RS232.png

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2001\01\21@163233 by James Newton

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www.piclist.com/techref/io/serial/ttl-rs232.htm
Lists all the methods I know... did I miss any?

James Newton, PICList Admin #3
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{Original Message removed}

2001\01\22@173045 by Byron A Jeff

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>
> www.piclist.com/techref/io/serial/ttl-rs232.htm
> Lists all the methods I know... did I miss any?

Doubtful.

Turns out I didn't need it. I did some further testing on a breadbaord and
found that if I reduced the current limited resistor after the zener from
10k to 1.2k, that it worked like a champ. I gathered the emprical evidence
by testing echo at the junction of the zener and the 1st 10k current limiting
resistor. It worked there but didn't after the second 10k. So I figured if
I lower the resistance that it may work, and it did.

It's all wired up and working. It's great to be able to build a fully
programmable board with just 2 pullup resistors along with the crystal
and caps.

I soldered the non-inverting RS-232 circuit and the DTR reset circuit on 1/2
of a Ratshack general purpose circuit board.

I'll post my next issue, getting 16F877's to respond in programming mode, in
another post.

Thanks to all for the help,

BAJ
>
> James Newton, PICList Admin #3
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>
> {Original Message removed}

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