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

[msokolov@xxxxxxxxxxxxxxx (Michael Sokolov)]

Hello fellow gEDA users,

I have a general hardware design question not specific to a particular
design tool, so please forgive me for OT.

Suppose I have an RS-232 output (DTR in my case) that is always
asserted.  Is it acceptable to drive it directly from the +12V supply
(through a resistor I presume) instead of using a proper RS-232 driver
with a pegged input?  The reason I want to do this is because I'm using
a quad driver chip and all 4 drivers are used for non-static signals, so
I would much rather avoid using another quad driver chip just to use
only one section of it with a pegged input to put out an asserted DTR.

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?

TIA,
MS


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