[time-nuts] GPDSO is working

EWKehren at aol.com EWKehren at aol.com
Sat Jul 13 15:29:55 UTC 2013


Most MAX have two sets and one could use one as an inverter if one does not 
 want to add an extra IC. 
Bert Kehren
 
 
In a message dated 7/13/2013 10:09:12 A.M. Eastern Daylight Time,  
alsopb at nc.rr.com writes:

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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