The simple answer is that the noise cancels out.  I believe the
impedance of the line is a lot lower also - around 120 ohms for 485 and
2K for 232.  Any external noise will be equally induced in the same
polarity in both the 'true' and 'not-true' signals.  The lower the
impedance of the line, the harder it is to induce a noise signal to it.
Same principal for low z balanced microphones.  Another example is your
telephone.  Lots of unshielded wire running throughout your house and
yet not much hum.  Does this help?

>----------
>From:  Frans Gunawan[SMTP:f@POBOXES.COM]
>Sent:  Thursday, November 06, 1997 6:02 AM
>To:    PICLIST@MITVMA.MIT.EDU
>Subject:       [OT}balanced line
>
>I would like to ask, why with using balanced line(rs485) data can be
>transmitted longer than using unbalanced line(rs232)?
>I can't find good explanation about this. if possible, detail please...
>thanks
>www.poboxes.com/f
>f
>