[time-nuts] RS 232

[Bob Camp]

Hi

The only other thing I have seen done on opto isolated RS-232 is to add a protection diode on the "transmit" opto output transistor. There's a spec that says you have to be able to apply +/- 25 volts to any pin weather the device is powered up or not. If you want high(er) speed you can put a pair of transistors on the output of the opto to boost the drive current. Not needed at 9600 baud, probably useful  if you are trying to go > 100K baud. 

Bob

On Jul 26, 2013, at 10:38 AM, Chuck Harris <cfharris at erols.com> wrote:

> 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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