[time-nuts] RS 232

[Chuck Harris]

I have enclosed a sketch of the usual way of using opto's to make an
RS232 like interface.  I haven't spent much time thinking, so I am sure
that I have a few things backwards, but you should get the gist of how
it is done.

It is only RS232 like because it has only a single threshold.

-Chuck Harris



Rex wrote:
> 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.)
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