I was a bit careless about my words. I should certainly
have said "unconnected ports" not "unterminated lines" - termination
in this case being a bit of a vague word.
Most RS-232 circuitry seems to include some sort of low pass
filter - with *very* old equipement, if it was rated at
"up to 110 baud" or "up to 300 baud" or "up to 1200 baud",
it was pretty commonplace to find filtering to get
rid of higher speed "noise". I can remember people modifying
used teletype modems (only rated at up to 110 baud) for operation
at 300 baud by diddling with the filters. An appropriate combination
of filters and drivers could easily provide a high impedance
imput and still be quite immune to many forms of line noise.
Most modern equipment uses MC1488's & 1489's - unfortunately I don't
happen to have anything handy on these chips.
Modern rs-232 cabling of any great length is (or should be)
shielded, and only in very short connections do people sometimes
resort to kluges such as ribbon cabling. A sufficiently long & poor
cable might well pick up enough noise such as Jim Rees describes,
which could conceivably result in a heavy serial port load and odd
system performance for the reasons I described - in practice,
I have not seen this happen very often, and esp. not for
the usual short 3-5' cables common in most machine rooms and offices.
But certainly, this should be a consideration for any cable that runs
between more than one room, so I'm not surprised to hear about such
problems. Besides shielding, it should also be helpful to route it
away from high voltage wiring and flourescent lighting fixtures, if
that's possible.
At one point, it was popular to do RS-232 wiring installations with
modular jacks, patch panels, & "telephone style" flat cable - I don't
know how good that stuff was about noise. Hmm - it can't be too bad
-- it occurs to me my office has 96 of these things plugged into an
ancient LSI-11, but there are only 11 real connections to it - the rest
are just dangling unterminated in people's offices. The runs
inbetween aren't flat cable however - they seem to be thick
round grey cable, probably up to 50' runs.
-Marcus Watts
UM ITD RS Umich Systems Group
