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-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf
Of ewkeh...@aol.com
Sent: Friday, 26 July 2013 5:32 AM
To: time-nuts@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, ma...@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-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On
Behalf Of ewkeh...@aol.com
Sent: Thursday, 25 July 2013 11:39 PM
To: time-nuts@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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