Err... I hate to admit this but that is my fault... Yes it is untested and
quite probably would not work. I was trying more to express an idea ("My
thinking is that...") rather than a completed design. I'll edit the page to
make that more clear. Also, I don't really have time to correct it, so if
you have ideas on how to better accomplish the design goal, please let me
know.

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James Newton (PICList Admin #3)
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-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Jeff Frohwein
Sent: Monday, May 14, 2001 06:56
To: PICLIST@MITVMA.MIT.EDU
Subject: [PIC]: Piclist TTL to RS232 design bug?


Hi,

 I've got a question about the TTL to RS232 design that is posted
in the piclist.com web site. Here is the exact page:

  http://www.piclist.com/techref/io/serial/ttl-rs232.htm

 If you look at the circuit diagram right below Dan Michaels
app note and right above Wojciech Zabolotny's comments there
is shown a circuit that looks to be a derivative of Dan's work
but it isn't clear to me who might be the author of this
derivative work. Anyways, just looking at the design it doesn't
appear to me that it would work. It appears to me that the
transistor would always be on assuming that the voltage on pin 4
of the DB25 is higher than pic VCC + ~1.2 volts. (So a 5 volt pic
and 12 volts on pin 4 wouldn't work.) This being due to the fact
the the TX pin (assuming it connects to a pic or scenix) would
always provide VCC + ~0.6 volts due to an internal diode clamp
from vcc to this i/o pin.

 Am I missing something obvious here? If not, any suggestions
for simple changes to get this design to work?

 Thanks,

 Jeff

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