[time-nuts] RS 232

paul swed paulswedb at gmail.com
Thu Jul 25 16:40:10 EDT 2013


Bert
I will add back in here. The max232 has the internal switching oscillator
and that makes the noise. But you know that. So an external +/-supply
linear is cheap and simple. Full wave bridge and 2 caps. No regulation
needed. Using your opto isolator provides complete noise and electrical
isolation. Computers are nasty beasts especially when you grab +/- power
from the rs232 port. Some one re-confirmed my thinking that the rs 232 spec
was +/-3V for noise margin. I think thats about all there is.
Sorry if I am misunderstanding your need.
Regards
Paul
WB8TSL


On Thu, Jul 25, 2013 at 4:34 PM, <EWKehren at aol.com> wrote:

> Bill
> Thank you I am only looking for a solution that is simple in Corby's case
> we used a separate power source, but the question is, is it necessary. That
> is  why I brought it up to the list. Shopping for ideas.
> Bert
>
>
> In a message dated 7/25/2013 4:06:03 P.M. Eastern Daylight Time,
> wb6bnq at cox.net writes:
>
> Hi  Bert,
>
> My attitude is if you are going to provide an RS-232 port then do  it
> properly (min of +/- 3 volts & max of +/- 25 volts).  RS-232  is still a
> popular item in the hobby controller world, although USB is  gaining
> ground.  In so far as stability goes, RS-232 is much cleaner  and easier
> to handle in the micro coding compared to USB.  With that  said, there is
> nothing wrong with providing both, it just makes the coding  a bit
> harder.  If you need long lines and want common mode  suppression then
> utilize RS-485, a derivative of RS-232.
>
> You could  also use TOSLINK, the fiber optic lines used in the audio
> field.  The  components are fairly cheap compared to other forms of
> plastic or glass  (more expensive) fiber products.  Most products will
> handle up to 5  MHz at reasonable distances.  If you are careful in
> selecting the TX  and RX port parts it can go up to 15 MHz at the top
> end, especially at  short distances.
>
> Irrespective of all that, the power source has no  relationship to the
> RS-232 or USB for that  matter.
>
> 73....Bill....WB6BNQ
>
> EWKehren at aol.com  wrote:
>
> >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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