Guys,
The PIC in question was knowingly programmed "upside down" with the N
option so it could talk directly to the computer without an RS232
converter. (input side suitably protected from -voltage levels)
This works of most PC's which in actuality use 3.3 Volt logic in their
RS232 port and input clamp highs/lows to be within the logic family
limits.
There are two serial port choices for a PIC in the PICAXE/BS2 compilers
N and T.
From the PICAXE manual.
"N idles low and T idles high. When using a simple resistor interface
use N (inverted) When using a MAX232 type interface use T"
The bottom line is depending upon what your device is putting out and
what you are talking to you may or may not need an inverter for use with
the MAX232.
Regards,
Brian
On 7/13/2013 03:10, Chris Albertson wrote:
You have it 100% correct. The UT+ uses "positive" logic are the logic 1 is
5-volts but the RS-232 standard uses "negative" logic. I think the MAX232
does the conversion correctly EXCEPT if you read the RS-232 standards they
use positive logic for the control signals.
On Fri, Jul 12, 2013 at 7:34 PM, Bob Stewart <b...@evoria.net> wrote:
Hi Brian,
That's just strange. There are a whole lot of these MAX232 and MAX3232
devices being sold. Hmm, I'm looking at the UT+ User's Guide, and it lists
the voltage levels as follows. These would imply that an inverter is
necessary, right? Could it be that someone programmed your PIC upside down
- i.e. using negative logic?
TTL
0 V to 0.8 V = logic 0
2.4 V to 5.0 V = logic 1
RS-232 (reordered from manual to put logic 0 on top)
5 V to 15 V = logic 0
-5 V to -15 V = logic 1
Bob - AE6RV
________________________________
From: Brian Alsop <als...@nc.rr.com>
To: Bob Stewart <b...@evoria.net>; Discussion of precise time and
frequency measurement <time-nuts@febo.com>
Sent: Friday, July 12, 2013 9:09 PM
Subject: Re: [time-nuts] GPDSO is working
Hi Bob,
Here is my experience. I had a PIC that output RS232 at 0-5 volt
levels. It actually worked with my computer directly. When I added a
MAX 232 to make the levels something like -10/+10 volts. It didn't
work. That's because the MAX232 inverts the polarity. Look at the data
sheet, the level converters are clearly inverters.
The fix in my case was to invert the RS232 stream output by the PIC and
all was fine.
I'm not sure exactly what you have but a scope sorts it out quickly.
73 de Brian/K3KO
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