[time-nuts] RS 232

Fri Jul 26 11:01:29 UTC 2013

Since there is repeated request for the circuit se attachment. My question  
is what can be done to get a positive voltage from some of the other unused 
pins  of the DB 9 connector. What is needed is any where from 3 to 15 volt. 
Diodes  work with 1 mA but I set Rx for 2 mA. Any thing else look at H11 
data  sheet.
Bert Kehren

In a message dated 7/26/2013 4:06:37 A.M. Eastern Daylight Time,  
rexa at sonic.net writes:

FWIW, it  was clear as mud for me too.

Bert began with, "Since joining time   nuts over four years ago I have 
not used a  single MAX
232 chip.  Two  reasons MAX do not give me isolation and do generate   noise
in critical  applications."

>From that I took that he was  doing RS232 using opto isolators. That 
implies + and - voltages to me.  Where do they come from and where are 
they relative to the isolation  boundary? If the goal is really 
isolation, how do these supplies get  isolated? If the noise is cured by 
isolation, these details seem important  as the supplies need isolation 
too. Maybe the switcher in the MAX232 is  causing the noise. Then how do 
we get a negative supply from, say, +5V  without the noise? Then, maybe 
he was saying RS232 sucks and this other  way (not RS232 compatible) 
works better.

The word picture of Bert's  solution, which provided more details, left 
me less much less than clear  too. Maybe I'm just not up on circuit 
shorthand terminology enough to  follow what sort of current limiter is 
limiting what current to what, and  what is being blocked by a diode from 
which negative level. Not really  sure if I even got the big picture of 
what he is describing. Is it an  isolated equivalent of a MAX232 
interface or something else that wouldn't  talk to an RS232 device?

So, more clarification, or possibly that  picture (~= 1k words) might 
help. Or maybe I'm just obtuse and everyone  else is getting it. (Seems 
Marki may also be in the confused  camp.)

On 7/25/2013 3:34 PM, Mark C. Stephens wrote:
>  Although your description,
> " I prefer the use of two H11 opto couplers  which
> work  perfect. On the receiving end the diode along with a  current 
limiter
> and  blocking diode for the negative level works  perfect. On the output 
side
> a  power  source is needed." Is  a perfect circuit description, I'd be 
more confident with a schematic  :)
>
>
> --marki
>
>
> -----Original  Message-----
> From: time-nuts-bounces at febo.com  [mailto:time-nuts-bounces at febo.com] On 
Behalf Of EWKehren at aol.com
>  Sent: Friday, 26 July 2013 5:32 AM
> To: time-nuts at febo.com
>  Subject: Re: [time-nuts] RS 232
>
> I do not understand your  question, I am referring to low noise 
applications  like counters for  dual mixers or other AV measurements, but also Shera 
and even  Tbolt  where external noise should be kept to a minimum. When you 
chase 1 E-14,   isolation is key and I always like to err on the cautious 
side and as I  stated  we use blue tooth or USB but in the case of USB there 
are always  H11 in the  circuit. Some still like to use RS 232 and the 
subject came  up and I have on my  boards H11's like on the counter Corby uses 
but he  ended up using an external  power source and I like to eliminate that  
requirement. David had the right  answer using the power that the RS 232  
mouse uses out of a DB 9, started looking  but I do not have one any more  
and I can not find any data.
> Bert
>    
>   
> In a message dated 7/25/2013 2:48:43 P.M.  Eastern Daylight Time, 
marks at non-stop.com.au writes:
>
> Hi   Bert,
>
> I am sure your circuit is clear in your head, but would  you mind  
attaching detail?
> You have perked my interest with the  "low-noise" keyword  ;)
>
>
> -marki
>
>  -----Original Message-----
> From:  time-nuts-bounces at febo.com  [mailto:time-nuts-bounces at febo.com] On
> Behalf Of   EWKehren at aol.com
> Sent: Thursday, 25 July 2013 11:39 PM
>  To:  time-nuts at febo.com
> Subject: [time-nuts] RS  232
>
> Since joining time  nuts over four years ago I have  not used a  single 
MAX
> 232 chip. Two  reasons MAX do not  give me isolation and do generate  
noise
> in critical   applications. I prefer the use of two H11 opto couplers 
which
>  work  perfect. On the receiving end the diode along with a current  
limiter
> and  blocking diode for the negative level works perfect.  On the output 
side
> a  power  source is needed. If one uses  an USB adapter it does have the 
+ 5
> volt which  again works  perfect.  How ever many prefer to use RS 232
> direct and that  is  why I hope to get some comments and suggestions from 
  the
> list. Corby used on our  counter circuit that he described a  separate  
power
> source.
> Present MAX circuits use a + 1.4  volt threshold but   considering legacy
> the question is what  should the voltage swing be to make  it  compatible 
for
> most  PC's and what options exist to get the necessary  voltage. In  the  
past
> other pins on the RS 232 port where an ideal power  source.  Still an  
option?
> Bert  Kehren
>  _______________________________________________
>

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