[time-nuts] GPDSO is working

Brian Alsop alsopb at nc.rr.com
Sat Jul 13 10:08:56 EDT 2013


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 <bob at 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 <alsopb at nc.rr.com>
>>> To: Bob Stewart <bob at evoria.net>; Discussion of precise time and
>> frequency measurement <time-nuts at 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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>
>
>



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