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Re: gEDA-user: Driving a constant RS-232 output directly from +12V
Tomaz Solc <tomaz.solc@xxxxxxxxxx> said:
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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 :)
The last reference I saw to the actual EIA RS-232 standard said something to
the effect that 'Any line can be shorted to any other line, or ground, for
an indefinite period of time, without causing damage to the port'. How you
implement this is up to you. Hooking directly to a power supply without
some kind of current limiting might violate this if the power supply does
not shut down in case of a short. On the other hand, it's probably not
desired behaviour to have your whole box shut down if someone happens to cut
a communications cable. I would recommend an approach that limits the
current to, say, about 10mA. Since this is a static signal, you can
probably simply connect it up via a resistor as you suggest.
--
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Mike Jarabek
FPGA/ASIC Designer
http://www.istop.com/~mjarabek
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