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'[OT}balanced line'
1997\11\06@060915 by

I would like to ask, why with using balanced line(rs485) data can be
transmitted longer than using unbalanced line(rs232)?
thanks
http://www.poboxes.com/f
f

> From: Frans Gunawan
> I would like to ask, why with using balanced line(rs485) data can be
> transmitted longer than using unbalanced line(rs232)?

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
ImageLogicibm.net

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?

{Quote hidden}

Hi Frans:

1. Balanced lines (with RS422/485 differential drivers and receivers)
rejects common mode noise. That's why TV, radio and recording studios
use balanced lines for all routings for high quality sound (mostly
reducing AC 60 Hz and florescent light hum).

2. RS422/485 line impedance is usually used with 50 or 120 ohms
termination, lower than RS232's 1 K unterminated - so signal reflections
are much less. This gives higher bandwidth to a given cable and allows
longer signal lines (until the wire's series resistance gets large
compared to the termination).

3. RS422/485 outputs are lower voltage swing, slew-rate controlled,
which helps crosstalk and reflections.

See this URL for more details:
http://www.maxim-ic.com/AppNotes.htm

-Wayne
{Quote hidden}

1997\11\06@134346 by
The less than simple answer is that RS-485 has a much better common mode
rejection ratio.  If one observes the same signal in both transmission
wires, the error can be corrected very easily.  Since RS-485 carries both
wires as signal wires with some impedance, the common-mode noise can be
eliminated.  This is true for RS-232, but to a much lesser extent since the
ground side of the common mode signal will usually be substantially less
than the signal side.

{Original Message removed}
Don't foget to use a twisted pair of signal wires

They have their own special properties for rejecting
interference to and from a balanced line

Peter

petercousens.her.forthnet.gr

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