[time-nuts] RS 232

[Rex]

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
> _______________________________________________
>