[time-nuts] RS 232

[John Miles]

Agreed, nobody should be using RS232 for anything nowadays.  USB doesn't
seem to cause noise problems in equipment that works at the -170 dBc/Hz and
below level (ahem) so it will be good enough for most other sensitive
applications, assuming good design/layout practices are followed.

USB support chip manufacturers have a lot of experience supporting customers
who need their USB devices to pass FCC, CE, and other worldwide EMI
standards.  There are a lot of good app notes and other literature out
there.  As far as what components to use, I'd suggest checking out
www.ftdichip.com if you're looking for the proverbial "path of least
resistance."   You can't get much simpler than an FTD232B.

-- john, KE5FX
Miles Design LLC


> -----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, July 25, 2013 12:32 PM
> 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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