> From: Frans Gunawan
> 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...

Balanced line (RS-422 & RS-485) operates by
comparing two lines relative to ground.  If line
one is at a higher potential than line two you
have a 1.  If line one is at a lower potential
than line two, you have a 0.

RS-232 takes the absolute potential on a
single line relative to ground.  Without
checking I think the formal RS-232C
specification calls for a transition region
between -3 and +3 volts, with the region
above +3 volts (but below 25 volts) being
1.

Remember the "RS" in RS-232 stands for
"Recommended Standard."  So if all you
have is a +5V power supply you can work
out a "practical" protocol that uses a narrow
transition region around +3 volts and calls
anything below +2 a 0 and everything above
+4 a 1.

Clearly this "practical" protocol is awfully
vulnerable to anything that changes the
absolute value of the signal.  That's why
it is cable length (resistance) sensitive.

On the other hand, each of the RS-422/485
balanced lines are equally effected by line
impedance so the differential can still be
detected.  That's why it is less sensitive
to cable length.

Cheers,

Win Wiencke
Image Logic Corporation
ImageLogic@ibm.net