However, as a practical consideration, you can expect that if there is a
volt or 2 of "noise" on the rs-232 line, it won't be significant.
On the other hand, if there is 10 volts peak-to-peak, you're guaranteed to
have problems, since the peaks will be into the mark and space regions.
That's only 3.6 volts rms. You're not likely to see this on a completely
disconnected rs-232 line, but if there is even a short piece of cable
attached, with nothing on the other end, it's entirely possible. I have
personally measured four times this amount of noise on a 15 foot
unterminated rs-232 line.
But as a general rule, you can expect that at the very least, there will
be appropriate filtering and/or high resistant pullups or pulldowns or
something, so that unterminated lines will go to one state or the other
and stay there.
You can expect no such thing. The minimum impedance of an rs-232
termination is 3 K, as defined in the spec, and higher values are more
typical. That's not low enough to sink noise if there is no termination at
the other end. Professional audio is terminated at 600 ohms, and video at
75 ohms, and even at those impedances you still have plenty of noise on an
unterminated line.
In theory you could put a 60 Hz notch filter on a rs-232 input, but I've
never seen any design that included this, and I know the rs6k console port
does not include this.
