Re: RS485 is bidirectional does it mean it is fullduplex?

"kunil" <kunilkuda@xxxxxxxxx> wrote:
>May I summary this ?
>
>I think RS485 problem is 2 :
> *) Noise over long distance cable and
> *) Faulty data because of different node voltage reference
>
>To prevent noise over long distance cable, we can earth one side of the
>cable shield (refer to Ott, Henry, Noise Reduction Techiques in
>Electronic Systems).

On a "long distance cable", you'd better ground *both* ends.

First, understand that a "shield" has virtually no effect at 60
Hz. The reduction in induced signal at 1000 Hz is about 3 dB
when a shield is added. At 60 Hz the difference is about 0.04
dB! (Which says, we don't put a shield on the cable to
necessarily reduce noise in the obvious way! It has other
effects, if used correctly.)

A "ground loop" is caused by having a *common* ground path for
two signals. Hence if the "ground" for a cable shield is
provided by attaching it to the equipment, and most particularly
if it is attached in a way such that from the connection to some
other point there is a shared path with the signal, current in
the cable shield will affect the signal to the degree that it
can cause a voltage drop across the distance of that common
connection. That can be significant at higher impedances.

That is the *wrong* way to ground a long cable. I've emphasized
a separate ground cable is required, and that buildings require
a single point ground system, just to avoid said ground loops.

Here it is graphically. This is an incorrectly grounded cable
shield, causing a ground loop with each equipment. All currents
induced into the cable shield share the common connection to
ground *through* the equipment.

+-------+ +-------+
| | >--------- tx wire/pair ---------> | |
| EQUIP | <--------- rx wire/pair ---------< | EQUIP |
| | ========= cable shield ========= | |
+-------+ | | +-------+
| | | | | |
| +---+ +---+ |
o o
| |
----- Earth ----- Earth
--- Ground --- Ground
- -

By the expedient of removing the ground at one end, several things
are accomplished. One is the removal of the ground loop. It also,
however, removes common mode DC equalization, and it reduces the
current flow in the cable shield, which happens to have a negative
effect, as I'll show.

Note that this is *very* appropriate for use with cable existing
within a single building. The benefit is the same, but the
negatives are of negligible effect.


+-------+ +-------+
| | >--------- tx wire/pair ---------> | |
| EQUIP | <--------- rx wire/pair ---------< | EQUIP |
| | ========= cable shield ========= | |
+-------+ | +-------+
| | | |
| +---+ |
o o
| |
----- Earth ----- Earth
--- Ground --- Ground
- -

However, if the cable is a long run, and particularly if there
is exposure to power lines, if the ground potential is different
at the two ends, or if there are any other sources of induced
noise in the cable, this arrangement has the best effect:

+-------+ +-------+
| | >--------- tx wire/pair ---------> | |
| EQUIP | <--------- rx wire/pair ---------< | EQUIP |
| | ========= cable shield ========= | |
+-------+ | | +-------+
| | | |
| | | |
o------+ +------o
| |
----- Earth ----- Earth
--- Ground --- Ground
- -

Note the minimum common path to ground. If correctly sized
there will be no significant voltage drop across that small
section. (Which is to say, that should probably be copper strap
between a copper terminal plate and the actual ground system
connection.)

Hence, there is no "ground loop" effect. However, the two
grounds are connected electrically and the voltage is equalized
between then. The second benefit is that voltages induced into
the cable by exposure to electro magnetic fields will have a low
impedance circuit path, and will therefore conduct current.

Just as it does in a transformer, current changes in one
direction cause a opposite voltage to be induced into a coupled
conductor. Hence, we have the external field causing a voltage
in the shield and the pairs which is identical. The current
that flow is the shield causes an exactly *opposite* voltage to
be induced into the cable pairs. The externally induced voltage
and the shield induced voltage cancel to some degree in the
cable pairs, thus reducing external noise induction into signal
pairs. This effect *requires* both ends be grounded (with
quality connections presenting a relatively low impedance to the
noise current).

>To prevent faulty data because of different node voltage, we can use
>common line. But since RS485 is a differential mode protocol, we can
>use either A or B line as our common line.
>
>However, if we connect using this fashion, when the master is in the
>idle mode, there will be floating voltage between A or B line (since
>nobody is driving the bus). Therefore, we connect pull-up/pull-down
>resistors in the A and B line (to give at least definite voltage level
>when nobody's driving the bus).
>
>To make this "definite voltage level" same at the receiver /
>transceiver point, we need to earth their voltage reference node at the
>both side.
>
>I think everyone is correct here. Just the naming convention that makes
>confusion.

That is indeed a significant part of the problem.

The other part is just not being exposed to the full expanse of
what is involved in data transmission over longer lengths of
twisted pair cables.

--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@xxxxxxxxxx
.

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