Re: gEDA-user: Driving a constant RS-232 output directly from +12V

[Tomaz Solc <tomaz.solc@xxxxxxxxxx>]

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Hi

> So is there anything terribly bad/spec-violating/unsafe about driving
> an always-asserted RS-232 output directly from the +12V supply through a
> resistor, or is that perfectly OK?  What should the resistor value be?
> Or should I use something more fancy like a PTC resistor acting as a
> resettable fuse protecting against external shorts?

I currently have a circuit in front of me that uses PNP transistors as
switches to connect a supply voltage directly to the TX line of a RS-232
output. I believe this is pretty much identical to shorting that line to
 supply voltage.

So I would say that it would also be safe to directly drive lines in
your case. I've also once seen 4.7k resistors in series with RS-232
lines. It is probably a good idea to limit the current that way in case
of a short circuit.

I can't say if that violates any standards. I certainly wouldn't bet on
the manufacturer of this board to stick to specs if it that meant saving
a few components :)

Best regards
Tomaz
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