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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