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 <cfhar...@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.) > <DSCN2204.JPG>_______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.