piclist 2002\10\11\183137a >
Thread: It's official: The Real-World Serial FAQ
www.piclist.com/techref/io/serials.htm?key=serial
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face BY : Russell McMahon email (remove spam text)



> > Using protection diodes to clamp pin excursions under certain conditions
can
> > be acceptable as long as abnormal circuit operation of any possible
nature
> > is acceptable during these conditions. (eg a 1 megohm resistor of
suitable
> > ratings between an input and a pin may allow full 230 VAC mains to be
> > temporarily applied to the input. Correct processor operation during
such an
> > incident would not be guaranteed.
>
> Umm, so you're saying that it's OK to do this for 230VAC, but not RS-232?
> Wow, there's a stretch.  ...........

No.

>  ......................'Splain to me, please, how the first is somehow
more
> acceptable than the second.
>
> Methinks either case falls under the same umbrella...

Both are OK provided you accept the potential consequences.
Both fall under the same umbrella.

The qualifier I used was ALL important. See above and I'll repeat it here -

   >> as long as abnormal circuit operation of any possible nature
   >> is acceptable during these conditions.

Sorry, I thought I'd made my point clear enough but obviously I  failed.
I'll try again.

Protection diodes are "PROTECTION AGAINST DESTRUCTION" diodes. NOT
"protection against improper operation" diodes. They are accordingly in the
relevant section of the spec sheet.

My example was meant to illustrate that even AC mains (let alone RS232) CAN
be protected against by the protection diodes *as long as abnormal circuit
operation of any possible nature is acceptable during [OR AFTER] these
conditions*.

Warning, AC mains detected on input: Going to third stage boost / Danger
Will Robinson ...... / You are in violation of Federal Statute 303.29b and
have 15 seconds to drop your fire arm ... /  Fire at will ... / Weapons
systems free ... / Dave, Dave, what are yyou ddddoing Dave ? / Houston , we
have a proble......

YM*W*V

Bzzzzzt.




           Russell McMahon

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