[time-nuts] GPDSO is working

Bob Camp lists at rtty.us
Sat Jul 13 12:05:42 EDT 2013


Hi

Hooking the ~ +/- 10V output of a MAX 232 to one of it's CMOS logic inputs probably isn't a real good idea. Two resistors and a cheap transistor make a fine inverter in this case. 

Bob

On Jul 13, 2013, at 11:29 AM, EWKehren at aol.com wrote:

> 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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